MAX16814_13 MAXIM | Alldatasheet
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Technical content
Features
S 4-Channel Linear LED Current Sinks with Internal MOSFETs Full-Scale LED Current Adjustable from 20mA to 150mA Drives One to Four LED Strings S Boost, SEPIC, or Coupled-Inductor Boost-Buck Current-Mode DC-DC Controller 200kHz to 2MHz Programmable Switching Frequency External Switching Frequency Synchronization S Adaptive Output-Voltage Optimization to Minimize Power Dissipation S 4.75V to 40V Operating Input Voltage Range S Less than 40µA Shutdown Current S 5000:1 PWM Dimming at 200Hz (MAX16814A _ _ and MAX16814U_ _ Only) S Open-Drain Fault Indicator Output S Open-LED and LED Short Detection and Protection S Overtemperature Protection S Thermally Enhanced, 20-Pin TQFN, QFND, and TSSOP Packages Typical Operating Circuit and Pin Configurations appear at end of data sheet. 19-4722; Rev 7; 1/13 +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. /V Denotes an automotive qualified part, (SW) = side wettable.
Ordering Information
PART TEMP RANGE PIN-PACKAGE MAX16814ATP+ -40°C to +125°C 20 TQFN-EP* MAX16814ATP/V+ -40°C to +125°C 20 TQFN-EP* MAX16814AGP/VY+ -40°C to +125°C 20 QFND-EP* (SW) MAX16814AUP+ -40°C to +125°C 20 TSSOP-EP* MAX16814AUP/V+ -40°C to +125°C 20 TSSOP-EP* MAX16814BETP+ -40°C to +85°C 20 TQFN-EP* MAX16814BEUP+ -40°C to +85°C 20 TSSOP-EP* MAX16814BUTP+ 0°C to +85°C 20 TQFN-EP* MAX16814BUUP+ 0°C to +85°C 20 TSSOP-EP* MAX16814UTP+ 0°C to +85°C 20 TQFN-EP* MAX16814UUP+ 0°C to +85°C 20 TSSOP-EP* For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Continuous Power Dissipation (TA = +70NC) (Note 1) 20-Pin Side-Wettable QFND Operating Temperature Range
ELECTRICAL CHARACTERISTICS
(VIN = VEN = 12V, RRT = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = V CS = V LEDGND = V PGND = V SGND = 0V, T A = T J = -40 NC to +125NC for MAX16814A_ _, TA = -40 NC to +85NC for MAX16814BE_ _, and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) ABSOLUTE MAXIMUM RATINGS Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer board. For detailed information on package thermal considerations, refer to http://www.maximintegrated.com/thermal-tutorial. PACKAGE THERMAL CHARACTERISTICS (Note 1)
20 TQFN/QFND
20 TSSOP
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Operating Voltage Range VIN 4.75 40 V Active Supply Current IIN MAX16814A_ _ and MAX16814U_ _ 2.5 5 mAMAX16814B_ _ _ only 2.75 5.5 Standby Supply Current VEN = 0V 15 40 µA IN Undervoltage Lockout VIN rising 3.975 4.3 4.625 V IN UVLO Hysteresis 170 mV VCC REGULATOR Regulator Output Voltage VCC 6.5V < VIN < 10V, 1mA < ILOAD < 50mA 4.75 5.0 5.25 V10V < VIN < 40V, 1mA < ILOAD < 10mA Dropout Voltage VIN - VCC, VIN = 4.75V, ILOAD = 50mA 200 500 mV Short-Circuit Current Limit VCC shorted to SGND 100 mA VCC Undervoltage Lockout Threshold VCC rising 4 V VCC UVLO Hysteresis 100 mV RT OSCILLATOR Switching Frequency Range fSW 200 2000 kHz
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 ELECTRICAL CHARACTERISTICS (continued) (VIN = VEN = 12V, RRT = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = V CS = V LEDGND = V PGND = V SGND = 0V, T A = T J = -40 NC to +125NC for MAX16814A_ _, TA = -40 NC to +85NC for MAX16814BE_ _, and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Maximum Duty Cycle fSW = 200kHz to 600kHz, MAX16814A_ _ and MAX16814U_ _ 85 89 93 %fSW = 600kHz to 2000kHz, MAX16814A_ _ and MAX16814U_ _ 82 86 90 fSW = 200kHz to 600kHz, MAX16814B_ _ 90 94 98 fSW = 600kHz to 2000kHz, MAX16814B _ _ _ 86 90 94 Oscillator Frequency Accuracy fSW = 200kHz to 2MHz, MAX16814A_ _ and MAX16814U_ _ -7.5 +7.5 % fSW = 200kHz to 2MHz, MAX16814B_ _ _ -7 +7 Sync Rising Threshold 4 V Minimum Sync Frequency 1.1fSW Hz PWM COMPARATOR PWM Comparator Leading-Edge Blanking Time 60 ns PWM to NDRV Propagation Delay Including leading-edge blanking time 90 ns SLOPE COMPENSATION Peak Slope Compensation Current Ramp Magnitude Current ramp added to the CS input, MAX16814A_ _ only 44 49 54 µA x fSWCurrent ramp added to the CS input, MAX16814U_ _ and MAX16814B_ _ _ 45 50 55 CS LIMIT COMPARATOR Current-Limit Threshold (Note 3) 396 416 437 mV CS Limit Comparator to NDRV Propagation Delay 10mV overdrive, excluding leading-edge blanking time 10 ns ERROR AMPLIFIER OUT_ Regulation Voltage 1 V Transconductance gM 340 600 880 µS No-Load Gain (Note 4) 75 dB COMP Sink Current VOUT_ = 5V, VCOMP = 2.5V 160 375 800 µA COMP Source Current VOUT_ = 0V, VCOMP = 2.5V 160 375 800 µA MOSFET DRIVER NDRV On-Resistance ISINK = 100mA (nMOS) 0.9 ωISOURCE = 100mA (pMOS) 1.1 Peak Sink Current VNDRV = 5V 2.0 A Peak Source Current VNDRV = 0V 2.0 A Rise Time CLOAD = 1nF 6 ns Fall Time CLOAD = 1nF 6 ns LED CURRENT SOURCES OUT_ Current-Sink Range VOUT_ = VREF 20 150 mA Channel-to-Channel Matching IOUT_ = 100mA ±2 %IOUT_ = 100mA, all channels on ±1.5
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Note 2: All MAX16814A_ _ are 100% tested at T A = +125NC, while all MAX16814U_ _ and MAX16814B _ _ _ are 100% tested at TA = +25°C. All limits overtemperature are guaranteed by design , not production tested. Note 3: CS threshold includes slope compensation ramp magnitude. Note 4: Gain = δVCOMP/δVCS, 0.05V < VCS < 0.15V. ELECTRICAL CHARACTERISTICS (continued) (VIN = VEN = 12V, RRT = 12.25kI, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, VRSDT = VDIM = VCC, VOVP = V CS = V LEDGND = V PGND = V SGND = 0V, T A = T J = -40 NC to +125NC for MAX16814A_ _, TA = -40 NC to +85NC for MAX16814BE_ _, and TA = TJ = 0NC to +85NC for MAX16814U_ _ and MAX16814BU_ _, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Output Current Accuracy IOUT_ = 100mA TA = +125°C, MAX16814A_ _ only ±3 TA = -40°C to +125°C, MAX16814A_ _ only ±5 IOUT_ = 50mA to 150mA TA = +25°C, MAX16814U_ _ and MAX16814B_ _ _ ±2.75 TA = 0°C to +85°C, MAX16814U_ _ and MAX16814BU _ _ ±4 TA = -40°C to +85°C for MAX16814BE _ _ ±4 OUT_ Leakage Current VDIM = 0V, VOUT_ = 40V 1 µA LOGIC INPUTS/OUTPUTS EN Reference Voltage VEN rising, MAX16814A_ _ only 1.125 1.23 1.335 VVEN rising, MAX16814U_ _ and MAX16814B_ _ _ 1.144 1.23 1.316 EN Hysteresis 50 mV EN Input Current VEN = 40V ±600 nA DIM Input High Voltage 2.1 V DIM Input Low Voltage 0.8 V DIM Hysteresis 250 mV DIM Input Current ±2 µA DIM to LED Turn-On Delay DIM rising edge to 10% rise in I OUT_ 100 ns DIM to LED Turn-Off Delay DIM falling edge to 10% fall in I OUT_ 100 ns IOUT_ Rise and Fall Times 200 ns FLT Output Low Voltage VIN = 4.75V and ISINK = 5mA 0.4 V FLT Output Leakage Current VFLT = 5.5V 1.0 µA LED Short Detection Threshold Gain = 3V 1.75 2.0 2.25 V Short Detection Comparator Delay 6.5 µs RSDT Leakage Current ±600 nA OVP Trip Threshold Output rising 1.19 1.228 1.266 V OVP Hysteresis 70 mV OVP Leakage Current VOVP = 1.25V ±200 nA Thermal-Shutdown Threshold Temperature rising 165 °C Thermal-Shutdown Hysteresis 15 °C
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Typical Operating Characteristics (VIN = VEN = 12V, fSW = 300kHz, RSETI = 15kI, CVCC = 1FF, VCC = VDRV, NDRV = COMP = OUT_ = unconnected, V OVP = VCS = VLEDGND = VDIM = VPGND = VSGND = 0V, load = 4 strings of 7 white LEDs, TA = +25NC, unless otherwise noted.) SUPPLY CURRENT vs. SWITCHING FREQUENCY MAX16814 toc03 fSW (kHz) IIN (mA) 1600140012001000800600400 3.2 3.4 3.6 3.8 4.0 4.2 4.4 3.0 200 1800 2000 CNDRV = 13pF VSETI vs. PROGRAMMED CURRENT MAX16814 toc06 LED STRING CURRENT (mA) VSETI (V) 124987246 1.229 1.230 1.231 1.232 1.233 1.234 1.228 20 150 SWITCHING WAVEFORM AT 5kHz (50% DUTY CYCLE) DIMMING MAX16814 toc01 IOUT1 100mA/div VOUT 10V/div VLX 10V/div 40Fs/div FIGURE 2 SWITCHING FREQUENCY vs. TEMPERATURE MAX16814 toc04 TEMPERATURE (NC) SWITCHING FREQUENCY (kHz) 1007525 500-25 292 294 296 298 300 302 304 306 308 310 290 -50 125 EN THRESHOLD VOLTAGE vs. TEMPERATURE MAX16814 toc07 TEMPERATURE (NC) EN THRESHOLD VOLTAGE (V) 1007550250-25 1.15 1.20 1.25 1.30 1.10 -50 125 VEN RISING VEN FALLING SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX16814 toc02 VIN (V) IIN (mA) 40353025201510 2.6 2.8 3.0 3.2 3.4 3.6 3.8 2.4 54 5 CNDRV = 13pF TA = +125NC TA = +25NC TA = -40NC VSETI vs. TEMPERATURE MAX16814 toc05 TEMPERATURE (NC) VSETI (V) 1007550250-25 1.224 1.228 1.232 1.236 1.240 1.220 -50 125 EN LEAKAGE CURRENT vs. TEMPERATURE MAX16814 toc08 TEMPERATURE (NC) EN LEAKAGE CURRENT (nA) 1007550250-25 120 150 -50 125 VEN = 2.5V
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Pin Description PIN NAME FUNCTIONTQFN/ QFND TSSOP 1 4 IN Bias Supply Input. Connect a 4.75V to 40V supply to IN. Bypass IN to SGND with a ceramic capacitor. 2 5 EN Enable Input. Connect EN to logic-low to shut down the device. Connect EN to logic-high or IN for normal operation. The EN logic threshold is internally set to 1.23V. 3 6 COMP Switching Converter Compensation Input. Connect the compensation network from COMP to SGND for current-mode control (see the Feedback Compensation section). 4 7 RT Oscillator Timing Resistor Connection. Connect a timing resistor (RT) from RT to SGND to program the switching frequency according to the formula RT = 7.350 x 109/fsw (for the MAX16814A_ _ and the MAX16814U_ _) or to the formula RT = 7.72 x 109/fsw (for the MAX16814B_ _ _). Apply an AC-coupled external clock at RT to synchronize the switching frequency with an external clock. 5 8 FLT Open-Drain Fault Output. FLT asserts low when an open LED, short LED, or thermal shutdown is detected. Connect a 10kω pullup resistor from FLT to VCC. 6 9 OVP Overvoltage Threshold Adjust Input. Connect a resistor-divider from the switching converter output to OVP and SGND. The OVP comparator reference is internally set to 1.23V. 7 10 SETI LED Current Adjust Input. Connect a resistor (R SETI) from SETI to SGND to set the current through each LED string (ILED) according to the formula ILED = 1500/RSETI. 8 11 RSDT LED Short Detection Threshold Adjust Input. Connect a resistive divider from VCC to RSDT and SGND to program the LED short detection threshold. Connect RSDT directly to VCC to disable LED short detection. The LED short detection comparator is internally referenced to 2V. 9 12 SGND Signal Ground. SGND is the current return path connection for the low-noise analog signals. Connect SGND, LEDGND, and PGND at a single point. 10 13 DIM Digital PWM Dimming Input. Apply a PWM signal to DIM for LED dimming control. Connect DIM to VCC if dimming control is not used. 11 14 OUT1 LED String Cathode Connection 1. OUT1 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT1. OUT1 sinks up to 150mA. If unused, connect OUT1 to LEDGND.
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Pin Description (continued) PIN NAME FUNCTIONTQFN/ QFND TSSOP 12 15 OUT2 LED String Cathode Connection 2. OUT2 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT2. OUT2 sinks up to 150mA. If unused, connect OUT2 to LEDGND. 13 16 LEDGND LED Ground. LEDGND is the return path connection for the linear current sinks. Connect SGND, LEDGND, and PGND at a single point. 14 17 OUT3 LED String Cathode Connection 3. OUT3 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT3. OUT3 sinks up to 150mA. If unused, connect OUT3 to LEDGND. 15 18 OUT4 LED String Cathode Connection 4. OUT4 is the open-drain output of the linear current sink that controls the current through the LED string connected to OUT4. OUT4 sinks up to 150mA. If unused, connect OUT4 to LEDGND. 16 19 CS Current-Sense Input. CS is the current-sense input for the switching regulator. A sense resistor connected from the source of the external power MOSFET to PGND sets the switching current limit. A resistor connected between the source of the power MOSFET and CS sets the slope compensation ramp rate (see the Slope Compensation section). 17 20 PGND Power Ground. PGND is the switching current return path connection. Connect SGND, LEDGND, and PGND at a single point. 18 1 NDRV Switching n-MOSFET Gate-Driver Output. Connect NDRV to the gate of the external switching power MOSFET. 19 2 DRV MOSFET Gate-Driver Supply Input. Connect a resistor between V CC and DRV to power the MOSFET driver with the internal 5V regulator. Bypass DRV to PGND with a minimum of 0.1µF ceramic capacitor. 20 3 VCC 5V Regulator Output. Bypass VCC to SGND with a minimum of 1µF ceramic capacitor as close as possible to the device. — — EP Exposed Pad. Connect EP to a large-area contiguous copper ground plane for effective power dissipation. Do not use as the main IC ground connection. EP must be connected to SGND.
Figure 1. Simplified Functional Diagram
Figure 2. Circuit Used for Typical Operating Characteristics
7 HBLEDS
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Detailed Description The MAX16814 high-efficiency HB LED driver inte - grates all the necessary features to implement a high- performance backlight driver to power LEDs in small to medium-sized displays for automotive as well as general applications. The device provides load-dump voltage protection up to 40V in automotive applications. The MAX16814 incorporates two major blocks: a DC-DC controller with peak current-mode control to implement a boost, coupled-inductor boost-buck, or a SEPIC-type switched-mode power supply and a 4-channel LED driv- er with 20mA to 150mA constant current-sink capability per channel. Figure 1 is the simplified functional diagram and Figure 2 shows the circuit used for typical operating characteristics. The MAX16814 features a constant-frequency peak current-mode control with programmable slope com - pensation to control the duty cycle of the PWM control - ler. The high-current FET driver can provide up to 2A of current to the external n-channel MOSFET. The DC-DC converter implemented using the controller generates the required supply voltage for the LED strings from a wide input supply range. Connect LED strings from the DC-DC converter output to the 4-channel constant current-sink drivers that control the current through the LED strings. A single resistor connected from the SETI input to ground adjusts the forward current through all four LED strings. The MAX16814 features adaptive voltage control that adjusts the converter output voltage depending on the forward voltage of the LED strings. This feature mini - mizes the voltage drop across the constant current-sink drivers and reduces power dissipation in the device. A logic input (EN) shuts down the device when pulled low. The device includes an internal 5V LDO capable of powering additional external circuitry. All the versions of the MAX16814 include PWM dimming. The MAX16814A_ and the MAX16814U_ versions, in par- ticular, provide very wide (5000:1) PWM dimming range where a dimming pulse as narrow as 1µs is possible at a 200Hz dimming frequency. This is made possible by a unique feature that detects short PWM dimming input pulses and adjusts the converter feedback accordingly. Advanced features include detection and string-dis - connect for open-LED strings, partial or fully shorted strings and unused strings. Overvoltage protection clamps the converter output voltage to the programmed OVP threshold in the event of an open-LED condition. Shorted LED string detection and overvoltage protec - tion thresholds are programmable using RSDT and OVP inputs, respectively. An open-drain FLT signal asserts to indicate open-LED, shorted LED, and overtemperature conditions. Disable individual current-sink channels by connecting the corresponding OUT_ to LEDGND. In this case, FLT does not assert indicating an open-LED con - dition for the disabled channel. The device also features an overtemperature protection that shuts down the con - troller if the die temperature exceeds +165 NC. Current-Mode DC-DC Controller The peak current-mode controller allows boost, coupled- inductor buck-boost, or SEPIC-type converters to gener- ate the required bias voltage for the LED strings. The switching frequency can be programmed over the 200kHz to 2MHz range using a resistor connected from RT to SGND. Programmable slope compensation is available to compensate for subharmonic oscillations that occur at above 50% duty cycles in continuous conduction mode. The external MOSFET is turned on at the beginning of every switching cycle. The inductor current ramps up linearly until it is turned off at the peak current level set by the feedback loop. The peak inductor current is sensed from the voltage across the current-sense resistor R CS connected from the source of the external MOSFET to PGND. The MAX16814 features leading-edge blanking to suppress the external MOSFET switching noise. A PWM comparator compares the current-sense voltage plus the slope compensation signal with the output of the transcon- ductance error amplifier. The controller turns off the exter- nal MOSFET when the voltage at CS exceeds the error amplifier’s output voltage. This process repeats every switching cycle to achieve peak current-mode control. Error Amplifier The internal error amplifier compares an internal feed - back (FB) with an internal reference (REF) and regulates its output to adjust the inductor current. An internal minimum string detector measures the minimum current- sink voltage with respect to SGND out of the 4 constant- current-sink channels. During normal operation, this minimum OUT_ voltage is regulated to 1V through feedback. The error amplifier takes 1V as the REF and the minimum OUT_ voltage as the FB input. The ampli - fied error at the COMP output controls the inductor peak current to regulate the minimum OUT_ voltage at 1V. The resulting DC-DC converter output voltage is the highest LED string voltage plus 1V.
the DC-DC converter continues switching. VIN and VCC exceed their respective UVLO thresholds. is 4V when VCC is rising and 3.9V when V CC is falling. network (see the Typical Operating Circuit). where the new minimum OUT_ voltage is 1V. (for the MAX16814A_ _ and the MAX16814U_ _). Figure 3. Setting the MAX16814 Undervoltage Lockout
turn-on and turn-off of high gate-charge MOSFETs. MOSFET (IDRV) if there are no additional loads on V CC. together to drive the string as shown in Figure 4. and a logic-low signal disables them. that the minimum voltage at OUT_ is 1V. Figure 4. Configuration for Higher LED String Current
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 flag output ( FLT) goes low when an open-LED string is detected, a shorted LED string is detected, and during thermal shutdown. FLT is cleared when the fault condi - tion is removed during thermal shutdown and shorted LEDs. FLT is latched low for an open-LED condition and can be reset by cycling power or toggling the EN pin. The thermal shutdown threshold is +165 NC and has 15NC hysteresis. Open-LED Management and Overvoltage Protection On power-up, the MAX16814 detects and disconnects any unused current-sink channels before entering soft- start. Disable the unused current-sink channels by connecting the corresponding OUT_ to LEDGND. This avoids asserting the FLT output for the unused chan - nels. After soft-start, the MAX16814 detects open LED and disconnects any strings with an open LED from the internal minimum OUT_ voltage detector. This keeps the DC-DC converter output voltage within safe limits and maintains high efficiency. During normal operation, the DC-DC converter output regulation loop uses the mini - mum OUT_ voltage as the feedback input. If any LED string is open, the voltage at the opened OUT_ goes to VLEDGND. The DC-DC converter output voltage then increases to the overvoltage protection threshold set by the voltage-divider network connected between the con- verter output, OVP input, SGND. The overvoltage protec- tion threshold at the DC-DC converter output (V OVP) is determined using the following formula: (see the Typical Operating Circuit)OVP R1V 1.23 1 R2 where 1.23V (typ) is the OVP threshold. Select R1 and R2 such that the voltage at OUT_ does not exceed the absolute maximum rating. As soon as the DC-DC con - verter output reaches the overvoltage protection thresh - old, the PWM controller is switched off setting NDRV low. Any current-sink output with VOUT_ < 300mV (typ) is disconnected from the minimum voltage detector. Connect the OUT_ of all channels without LED connec - tions to LEDGND before power-up to avoid OVP trigger- ing at startup. When an open-LED overvoltage condition occurs, FLT is latched low. Short LED Detection The MAX16814 checks for shorted LEDs at each rising edge of DIM. An LED short is detected at OUT_ if the following condition is met: VOUT_ > VMINSTR + 3 x VRSDT where V OUT_ is the voltage at OUT_, V MINSTR is the minimum current-sink voltage, and V RSDT is the pro - grammable LED short detection threshold set at the RSDT input. Adjust VRSDT using a voltage-divider resis - tive network connected at the V CC output, RSDT input, and SGND. Once a short is detected on any of the strings, the LED strings with the short are disconnected and the FLT out- put flag asserts until the device detects that the shorts are removed on any of the following rising edges of DIM. Connect RSDT directly to V CC to always disable LED short detection. Applications Information DC-DC Converter Three different converter topologies are possible with the DC-DC controller in the MAX16814, which has the ground-referenced outputs necessary to use the con - stant current-sink drivers. If the LED string forward volt - age is always more than the input supply voltage range, use the boost converter topology. If the LED string for - ward voltage falls within the supply voltage range, use the boost-buck converter topology. Boost-buck topology is implemented using either a conventional SEPIC con - figuration or a coupled-inductor boost-buck configura - tion. The latter is basically a flyback converter with 1:1 turns ratio. 1:1 coupled inductors are available with tight coupling sui table for this application. Figure 6 shows the coupled-inductor boost-buck configuration. It is also possible to implement a single inductor boost-buck con- verter using the MAX15054 high-side FET driver. The boost converter topology provides the highest efficiency among the above mentioned topologies. The coupled-inductor boost-buck topology has the advan - tage of not using a coupling capacitor over the SEPIC configuration. Also, the feedback loop compensation for SEPIC becomes complex if the coupling capacitor is not large enough. A coupled-inductor boost-buck is not suit- able for cases where the coupled-inductor windings are not tightly coupled. Considerable leakage inductance requires additional snubber components and degrades the efficiency.
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Power-Circuit Design First select a converter topology based on the above factors. Determine the required input supply voltage range, the maximum voltage needed to drive the LED strings including the minimum 1V across the constant LED current sink (V LED), and the total output current needed to drive the LED strings (I LED) as follows: LED STRING STRINGII N = × where ISTRING is the LED current per string in amperes and NSTRING is the number of strings used. Calculate the maximum duty cycle (DMAX) using the fol- lowing equations: For boost configuration: LED D1 IN_MIN MAX LED D1 DS (V V V )D (V V V 0.3V) +−= +−− For SEPIC and coupled-inductor boost-buck-configura - tions: LED D1MAX IN_MIN DS LED D1 (V V )D (V V 0.3V V V ) += −−+ + where VD1 is the forward drop of the rectifier diode in volts (approximately 0.6V), VIN_MIN is the minimum input supply voltage in volts, and V DS is the drain-to-source voltage of the external MOSFET in volts when it is on, and 0.3V is the peak current-sense voltage. Initially, use an approximate value of 0.2V for VDS to calculate DMAX. Calculate a more accurate value of DMAX after the power MOSFET is selected based on the maximum inductor current. Select the switching frequency (fSW) depending on the space, noise, and efficiency constraints. Inductor Selection Boost and Coupled-Inductor Boost-Buck Configurations In all the three converter configurations, the average inductor current varies with the line voltage and the maximum average current occurs at the lowest line voltage. For the boost converter, the average inductor current is equal to the input current. Select the maximum peak-to-peak ripple on the inductor current ( DIL). The recommended peak-to-peak ripple is 60% of the aver - age inductor current. Use the following equations to calculate the maximum average inductor current (ILAVG) and peak inductor cur- rent (ILP) in amperes: LEDAVG MAX IIL 1D= − Allowing the peak-to-peak inductor ripple DIL to be +30% of the average inductor current: AVGIL IL 0.3 2∆= × × and: P AVG ILIL IL 2 ∆= + Calculate the minimum inductance value, L MIN, in hen - ries with the inductor current ripple set to the maximum value: MIN DS MAXMIN SW (VIN V 0.3V) DL f IL where 0.3V is the peak current-sense voltage. Choose an inductor that has a minimum inductance greater than the calculated LMIN and current rating greater than ILP. The recommended saturation current limit of the selected inductor is 10% higher than the inductor peak current for boost configuration. For the coupled-inductor boost-buck, the saturation limit of the inductor with only one winding conducting should be 10% higher than ILP. SEPIC Configuration Power circuit design for the SEPIC configuration is very similar to a conventional boost-buck design with the output voltage referenced to the input supply voltage. For SEPIC, the output is referenced to ground and the inductor is split into two parts (see Figure 5 for the SEPIC configuration). One of the inductors (L2) takes LED cur - rent as the average current and the other (L1) takes input current as the average current. Use the following equations to calculate the average inductor currents (IL1 AVG, IL2 AVG) and peak inductor currents (IL1P, IL2P) in amperes: LED MAXAVG MAX I D 1.1IL1 1D ××= −
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 The factor 1.1 provides a 10% margin to account for the converter losses: AVG LEDIL2 I = Assuming the peak-to-peak inductor ripple DIL is Q30% of the average inductor current: AVGIL1 IL1 0.3 2∆= × × and: P AVG IL1IL1 IL1 2 ∆= + AVGIL2 IL2 0.3 2∆= ×× and: P AVG IL2IL2 IL2 2 ∆= + Calculate the minimum inductance values L1 MIN and L2MIN in henries with the inductor current ripples set to the maximum value as follows: MIN DS MAXMIN SW MIN DS MAXMIN SW (VIN V 0.3V) DL1 f IL1 (VIN V 0.3V) DL2 f IL2 where 0.3V is the peak current-sense voltage. Choose inductors that have a minimum inductance greater than the calculated L1 MIN and L2 MIN and current rating greater than IL1 P and IL2 P, respectively. The recom - mended saturation current limit of the selected inductor is 10% higher than the inductor peak current: For simplifying further calculations, consider L1 and L2 as a single inductor with L1 and L2 connected in parallel. The combined inductance value and current is calculated as follows: MIN MINMIN MIN MIN L1 L2L L1 L2 ×= + and: AVG AVG AVGIL IL1 IL2= + where ILAVG represents the total average current through both the inductors together for SEPIC configuration. Use these values in the calculations for SEPIC configuration in the following sections. Select coupling capacitor C S so that the peak-to-peak ripple on it is less than 2% of the minimum input sup - ply voltage. This ensures that the second-order effects created by the series resonant circuit comprising L1, CS, and L2 does not affect the normal operation of the converter. Use the following equation to calculate the minimum value of CS: LED MAXS IN_MIN SW IDC V 0.02 f ×≥ ×× where CS is the minimum value of the coupling capacitor in farads, I LED is the LED current in amperes, and the factor 0.02 accounts for 2% ripple. Slope Compensation The MAX16814 generates a current ramp for slope com- pensation. This ramp current is in sync with the switch - ing frequency and starts from zero at the beginning of every clock cycle and rises linearly to reach 50FA at the end of the clock cycle. The slope-compensating resistor, RSCOMP, is connected between the CS input and the source of the external MOSFET. This adds a program - mable ramp voltage to the CS input voltage to provide slope compensation.
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Use the following equation to calculate the value of slope compensation resistance, RSCOMP. For boost configuration: ( )LED IN_MIN CS SCOMP MIN SW V 2V R 3 R L 50 A f 4 − ×× = × ××F For SEPIC and coupled-inductor boost-buck: ( )LED IN_MIN CS SCOMP MIN SW V V R3 R L 50 A f 4 − ×× = × ××F where VLED and VIN_MIN are in volts, RSCOMP and RCS are in ohms, LMIN is in henries and fSW is in hertz. The value of the switch current-sense resistor, R CS, can be calculated as follows: For boost: ( )( )CSMAX LED IN_MIN CSLP MN SW D V 2V R 3 0.396 0.9 I R 4L f × − ×× ×=× + ×× For SEPIC and boost-buck: ( )( )CSMAX LED IN_MIN CSLP MN SW D V V R3 0.396 0.9 I R 4L f × − ×× ×=× + ×× where 0.396 is the minimum value of the peak cur - rent-sense threshold. The current-sense threshold also includes the slope compensation component. The mini - mum current-sense threshold of 0.396 is multiplied by 0.9 to take tolerances into account. Output Capacitor Selection For all the three converter topologies, the output capaci- tor supplies the load current when the main switch is on. The function of the output capacitor is to reduce the converter output ripple to accep table levels. The entire output-voltage ripple appears across constant current- sink outputs because the LED string voltages are s table due to the constant current. For the MAX16814, limit the peak-to-peak output voltage ripple to 200mV to get stable output current. The ESR, ESL, and the bulk capacitance of the output capacitor contribute to the output ripple. In most of the applications, using low-ESR ceramic capacitors can dramatically reduce the output ESR and ESL effects. To reduce the ESL and ESR effects, connect multiple ceramic capacitors in parallel to achieve the required bulk capacitance. To minimize audible noise during PWM dimming, the amount of ceramic capacitors on the output are usually minimized. In this case, an additional electrolytic or tantalum capacitor provides most of the bulk capacitance. External MOSFET Selection The external MOSFET should have a voltage rating suf - ficient to withstand the maximum output voltage together with the rectifier diode drop and any possible overshoot due to ringing caused by parasitic inductances and capacitances. The recommended MOSFET VDS voltage rating is 30% higher than the sum of the maximum output voltage and the rectifier diode drop. The recommended continuous drain current rating of the MOSFET (ID), when the case temperature is at +70NC, is greater than that calculated below: The MOSFET dissipates power due to both switching losses and conduction losses. Use the following equa - tion to calculate the conduction losses in the MOSFET: COND AVG MAX DS(ON)P IL D R= ×× where RDS(ON) is the on-state drain-to-source resistance of the MOSFET. Use the following equation to calculate the switching losses in the MOSFET: AVG LED GD SWSW GON GOFF IL V C f 11P 2 II where I GON and I GOFF are the gate currents of the MOSFET in amperes, with V GS at the threshold voltage in volts, when it is turned on and turned off, respectively. CGD is the gate-to-drain MOSFET capacitance in farads. Rectifier Diode Selection Using a Schottky rectifier diode produces less forward drop and puts the least burden on the MOSFET during reverse recovery. A diode with considerable reverse- recovery time increases the MOSFET switching loss. Select a Schottky diode with a voltage rating 20% higher than the maximum boost-converter output voltage and current rating greater than that calculated in the follow - ing equation: AVGD MAX
1.2 ILI
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Feedback Compensation During normal operation, the feedback control loop reg - ulates the minimum OUT_ voltage to 1V when LED string currents are enabled during PWM dimming. When LED currents are off during PWM dimming, the control loop turns off the converter and stores the steady-state condi- tion in the form of capacitor voltages, mainly the output filter capacitor voltage and compensation capacitor voltage. For the MAX16814A_ _ and the MAX16814U_ _, when the PWM dimming pulses are less than five switching clock cycles, the feedback loop regulates the converter output voltage to 95% of the OVP threshold. The worst-case condition for the feedback loop is when the LED driver is in normal mode regulating the minimum OUT_ voltage to 1V. The switching converter small-signal transfer function has a right-half plane (RHP) zero for boost configuration if the inductor current is in continuous conduction mode. The RHP zero adds a 20dB/decade gain together with a 90 N-phase lag, which is difficult to compensate. The worst-case RHP zero frequency (f ZRHP) is calcu - lated as follows: For boost configuration: LED MAXZRHP LED V (1 D )f 2 LI −= π× × For SEPIC and coupled-inductor boost-buck configura - tions: LED MAXZRHP LED MAX V (1 D )f 2 LI D −= π× × × where fZRHP is in hertz, V LED is in volts, L is the induc - tance value of L1 in henries, and I LED is in amperes. A simple way to avoid this zero is to roll off the loop gain to 0dB at a frequency less than one fifth of the RHP zero frequency with a -20dB/decade slope. The switching converter small-signal transfer function also has an output pole. The effective output impedance together with the output filter capacitance determines the output pole frequency fP1 that is calculated as follows: For boost configuration: LEDP1 LED OUT If 2V C= ×π× × For SEPIC and coupled-inductor boost-buck configurations: LED MAXP1 LED OUT IDf 2V C ×= ×π× × where fP1 is in hertz, VLED is in volts, ILED is in amperes, and COUT is in farads. Compensation components, R COMP and C COMP, per - form two functions. C COMP introduces a low-frequency pole that presents a -20dB/decade slope to the loop gain. RCOMP flattens the gain of the error amplifier for frequencies above the zero formed by R COMP and CCOMP. For compensation, this zero is placed at the output pole frequency f P1 so that it provides a -20dB/ decade slope for frequencies above fP1 to the combined modulator and compensator response. The value of R COMP needed to fix the total loop gain at fP1 so that the total loop gain crosses 0dB with -20dB/ decade slope at 1/5 the RHP zero frequency is calcu - lated as follows: For boost configuration: ZRHP CS LEDCOMP P1 COMP LED MAX f RIR 5 f GM V (1 D ) For SEPIC and coupled-inductor boost-buck configura - tions: ZRHP CS LED MAXCOMP P1 COMP LED MAX f RI DR 5 f GM V (1 D ) where R COMP is the compensation resistor in ohms, fZRHP and f P2 are in hertz, R CS is the switch current- sense resistor in ohms, and GM COMP is the transcon - ductance of the error amplifier (600FS). The value of CCOMP is calculated as follows: COMP COMP Z1 1C 2R f= π× × where f Z1 is the compensation zero placed at 1/5 of the crossover frequency that is, in turn, set at 1/5 of the fZRHP. If the output capacitors do not have low ESR, the ESR zero frequency may fall within the 0dB crossover fre - quency. An additional pole may be required to cancel out this pole placed at the same frequency. This is usu - ally implemented by connecting a capacitor in parallel
remains between 20mA to 150mA.
1.23 V1500I 1220RR
Figure 5. SEPIC Configuration
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 PCB Layout Considerations LED driver circuits based on the MAX16814 device use a high-frequency switching converter to generate the voltage for LED strings. Take proper care while laying out the circuit to ensure proper operation. The switching- converter part of the circuit has nodes with very fast volt- age changes that could lead to undesirable effects on the sensitive parts of the circuit. Follow the guidelines below to reduce noise as much as possible: 1) Connect the bypass capacitor on V CC and DRV as close to the device as possible and connect the capacitor ground to the analog ground plane using vias close to the capacitor terminal. Connect SGND of the device to the analog ground plane using a via close to SGND. Lay the analog ground plane on the inner layer, preferably next to the top layer. Use the analog ground plane to cover the entire area under critical signal components for the power converter. 2) Have a power ground plane for the switching-con - verter power circuit under the power components (input filter capacitor, output filter capacitor, inductor, MOSFET, rectifier diode, and current-sense resistor). Connect PGND to the power ground plane as close to PGND as possible. Connect all other ground con - nections to the power ground plane using vias close to the terminals. 3) There are two loops in the power circuit that carry high-frequency switching currents. One loop is when the MOSFET is on (from the input filter capacitor positive terminal, through the inductor, the internal MOSFET, and the current-sense resistor, to the input capacitor negative terminal). The other loop is when the MOSFET is off (from the input capacitor positive terminal, through the inductor, the rectifier diode, output filter capacitor, to the input capacitor nega - tive terminal). Analyze these two loops and make the loop areas as small as possible. Wherever possible, have a return path on the power ground plane for the switching currents on the top layer copper traces, or through power components. This reduces the loop area considerably and provides a low-inductance path for the switching currents. Reducing the loop area also reduces radiation during switching. 4) Connect the power ground plane for the constant- current LED driver part of the circuit to LEDGND as close to the device as possible. Connect SGND to PGND at the same point.
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Pin Configurations Typical Operating Circuit OUT1 OVPCSNDRVIN EN DRV VCC UP TO 40V RCSRSCOMP MAX16814 OUT2 OUT3 OUT4 SETI FLT VCC RSETI RT RSDT RT PGND LEDGNDSGND L VIN 4.75V TO 40V N R1 DIM COMP RCOMP CCOMP PGND CS OUT4 OUT3IN V CC DRV NDRV TOP VIEW MAX16814 LEDGND OUT2 OUT1RT COMP
138 DIMFLT
129 SGNDOVP
1110 RSDTSETI
EP* *EXPOSED PAD. TSSOP EN FLT IN OUT3 OUT2 OUT1 OUT4 1 2 NDRV 4 5 15 14 12 11 DRV V CC SGND RSDT SETI OVP MAX16814 COMP LEDGND PGND EP*
10 DIMCS
Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814 Chip Information PROCESS: BiCMOS DMOS
Package Information
For the latest package outline information and land patterns, go to www.maximintegrated.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.
20 TSSOP-EP U20E+1 21-0108 90-0114
20 TQFN-EP T2044+3 21-0139 90-0037
20 QFND-EP
(Side Wettable) G2044Y+1 21-0576 90-0360
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000 25 © 2013 Maxim Integrated Products, Inc. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller MAX16814
Revision History
DATE DESCRIPTION PAGES CHANGED 0 7/09 Initial release — 1 9/09 Correction to slope compensation description and block diagram 10, 18 2 11/09 Correction to synchronization description frequency and minor edits 1–4, 8, 12–20, 22, 25 3 2/10 Correction to CSYNC formula 13 4 6/10 Added MAX16814BE _ _ parts; corrected specification 1–4, 8, 13, 25 5 3/11 Correction to output current accuracy specification and Absolute Maximum Ratings 1, 2, 4 6 10/11 Correction to the last formula and description 19 7 1/13 Added side-wettable package option and updated EN leakage in Electrical Characteristics 1, 2, 4, 8, 9, 23, 24