LM3404 TI1 | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 30
Technical content
LM3404,LM3404HV LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs Literature Number: SNVS465E
February 8, 2010 1.0A Constant Current Buck Regulator for Driving High Power LEDs General Description The LM3404/04HV are monolithic switching regulators de- signed to deliver constant currents to high power LEDs. Ideal for automotive, industrial, and general lighting applications, they contain a high-side N-channel MOSFET switch with a current limit of 1.5A (typical) for step-down (Buck) regulators. Hysteretic controlled on-time and an external resistor allow the converter output voltage to adjust as needed to deliver a constant current to series and series-parallel connected LED arrays of varying number and type. LED dimming via pulse width modulation (PWM), broken/open LED protection, low- power shutdown and thermal shutdown complete the feature set.
Features
■ Integrated 1.0A MOSFET ■ VIN Range 6V to 42V (LM3404) ■ VIN Range 6V to 75V (LM3404HV) ■ 1.2A Output Current Over Temperature ■ Cycle-by-Cycle Current Limit ■ No Control Loop Compensation Required ■ Separate PWM Dimming and Low Power Shutdown ■ Supports all-ceramic output capacitors and capacitor-less outputs ■ Thermal shutdown protection ■ SO-8 Package, PSOP-8 Package
Applications
■ LED Driver ■ Constant Current Source ■ Automotive Lighting ■ General Illumination ■ Industrial Lighting Typical Application 20205401 © 2010 National Semiconductor Corporation 202054 www.national.com LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs
8-Lead Plastic SO-8 Package 20205456 8-Lead Plastic PSOP-8 Package
Ordering Information
Order Number Package Type NSC Package Drawing Supplied As LM3404MA SO-8 M08A 95 units in anti-static rails LM3404MAX 2500 units on tape and reel LM3404HVMA 95 units in anti-static rails LM3404HVMAX 2500 units on tape and reel LM3404MR PSOP-8 MRA08B 95 units in anti-static rails LM3404MRX 2500 units on tape and reel LM3404HVMR 95 units in anti-static rails LM3404HVMRX 2500 units on tape and reel Pin Descriptions Pin(s) Name Description Application Information 1 SW Switch pin Connect this pin to the output inductor and Schottky diode. 2 BOOT MOSFET drive bootstrap pin Connect a 10 nF ceramic capacitor from this pin to SW.
3 DIM Input for PWM dimming Connect a logic-level PWM signal to this pin to enable/disable the
power MOSFET and reduce the average light output of the LED array. 4 GND Ground pin Connect this pin to system ground.
5 CS Current sense feedback pin Set the current through the LED array by connecting a resistor from
this pin to ground.
6 RON On-time control pin A resistor connected from this pin to VIN sets the regulator controlled
on-time.
7 VCC Output of the internal 7V linear
Bypass this pin to ground with a minimum 0.1 µF ceramic capacitor with X5R or X7R dielectric.
8 VIN Input voltage pin Nominal operating input range for this pin is 6V to 42V (LM3404) or 6V
to 75V (LM3404HV). DAP GND Thermal Pad Connect to ground. Place 4-6 vias from DAP to bottom layer ground plane. www.national.com 2 LM3404/LM3404HV
(LM3404) (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND -0.3V to 45V BOOT to GND -0.3V to 59V SW to GND -1.5V to 45V BOOT to VCC -0.3V to 45V BOOT to SW -0.3V to 14V VCC to GND -0.3V to 14V DIM to GND -0.3V to 7V CS to GND -0.3V to 7V RON to GND -0.3V to 7V Junction Temperature 150°C Storage Temp. Range -65°C to 125°C ESD Rating (Note 2) 2kV Soldering Information Lead Temperature (Soldering, 10sec) 260°C Infrared/Convection Reflow (15sec) 235°C Operating Ratings (LM3404) (Note 1) VIN 6V to 42V Junction Temperature Range −40°C to +125°C Thermal Resistance θJA (SO-8 Package) 155°C/W Thermal Resistance θJA (PSOP-8 Package) (Note 5) 50°C/W 3 www.national.com LM3404/LM3404HV
(LM3404HV) (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND -0.3V to 76V BOOT to GND -0.3V to 90V SW to GND -1.5V to 76V BOOT to VCC -0.3V to 76V BOOT to SW -0.3V to 14V VCC to GND -0.3V to 14V DIM to GND -0.3V to 7V CS to GND -0.3V to 7V RON to GND -0.3V to 7V Junction Temperature 150°C Storage Temp. Range -65°C to 125°C ESD Rating (Note 2) 2kV Soldering Information Lead Temperature (Soldering, 10sec) 260°C Infrared/Convection Reflow (15sec) 235°C Operating Ratings (LM3404HV) (Note 1) VIN 6V to 75V Junction Temperature Range −40°C to +125°C Thermal Resistance θJA (SO-8 Package) 155°C/W Thermal Resistance θJA (PSOP-8 Package) (Note 5) 50°C/W www.national.com 4 LM3404/LM3404HV
Electrical Characteristics VIN = 24V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA = TJ = +25°C. (Note 4) Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/ max specification limits are guaranteed by design, test, or statistical analysis. LM3404 Symbol Parameter Conditions Min Typ Max Units SYSTEM PARAMETERS tON-1 On-time 1 VIN = 10V, RON = 200 kΩ 2.1 2.75 3.4 µs tON-2 On-time 2 VIN = 40V, RON = 200 kΩ 515 675 835 ns LM3404HV Symbol Parameter Conditions Min Typ Max Units SYSTEM PARAMETERS tON-1 On-time 1 VIN = 10V, RON = 200 kΩ 2.1 2.75 3.4 µs tON-2 On-time 2 VIN = 70V, RON = 200 kΩ 325 415 505 ns LM3404/LM3404HV Symbol Parameter Conditions Min Typ Max Units REGULATION AND OVER-VOLTAGE COMPARATORS VREF-REG CS Regulation Threshold CS Decreasing, SW turns on 194 200 206 mV VREF-0V CS Over-voltage Threshold CS Increasing, SW turns off 300 mV ICS CS Bias Current CS = 0V 0.1 µA SHUTDOWN VSD-TH Shutdown Threshold RON / SD Increasing 0.3 0.7 1.05 V VSD-HYS Shutdown Hysteresis RON / SD Decreasing 40 mV OFF TIMER tOFF-MIN Minimum Off-time CS = 0V 270 ns INTERNAL REGULATOR VCC-REG VCC Regulated Output 6.4 7 7.4 V VIN-DO VIN - VCC ICC = 5 mA, 6.0V < VIN < 8.0V 300 mV VCC-BP-TH VCC Bypass Threshold VIN Increasing 8.8 V VCC-BP-HYS VCC Bypass Hysteresis VIN Decreasing 230 mV VCC-Z-6 VCC Output Impedance (0 mA < ICC < 5 mA) VIN = 6V 55 Ω VCC-Z-8 VIN = 8V 50 VCC-Z-24 VIN = 24V 0.4 VCC-LIM VCC Current Limit (Note 3) VIN = 24V, VCC = 0V 16 mA VCC-UV-TH VCC Under-voltage Lock-out Threshold VCC Increasing 5.3 V VCC-UV-HYS VCC Under-voltage Lock-out Hysteresis VCC Decreasing 150 mV VCC-UV-DLY VCC Under-voltage Lock-out Filter Delay 100 mV Overdrive 3 µs IIN-OP IIN Operating Current Non-switching, CS = 0.5V 625 900 µA IIN-SD IIN Shutdown Current RON / SD = 0V 95 180 µA CURRENT LIMIT ILIM Current Limit Threshold 1.2 1.5 1.8 A 5 www.national.com LM3404/LM3404HV
Symbol Parameter Conditions Min Typ Max Units DIM COMPARATOR VIH Logic High DIM Increasing 2.2 V VIL Logic Low DIM Decreasing 0.8 V IDIM-PU DIM Pull-up Current DIM = 1.5V 80 µA MOSFET AND DRIVER RDS-ON Buck Switch On Resistance ISW = 200mA, BST-SW = 6.3V 0.37 0.75 Ω VDR-UVLO BST Under-voltage Lock-out Threshold BST–SW Increasing 1.7 3 4 V VDR-HYS BST Under-voltage Lock-out Hysteresis BST–SW Decreasing 400 mV THERMAL SHUTDOWN TSD Thermal Shutdown Threshold 165 °C TSD-HYS Thermal Shutdown Hysteresis 25 °C THERMAL RESISTANCE θJA Junction to Ambient SOIC-8 Package 155 °C/W Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading. Note 4: Typical specifications represent the most likely parametric norm at 25°C operation. Note 5: θJA of 50°C/W with DAP soldered to a minimum of 2 square inches of 1oz. copper on the top or bottom PCB layer. www.national.com 6 LM3404/LM3404HV
Typical Performance Characteristics VREF vs Temperature (VIN = 24V) 20205450 VREF vs VIN, LM3404 (TA = 25°C) 20205451 VREF vs VIN, LM3404HV (TA = 25°C) 20205452 Current Limit vs Temperature (VIN = 24V) 20205453 Current Limit vs VIN, LM3404 (TA = 25°C) 20205454 Current Limit vs VIN, LM3404HV (TA = 25°C) 20205455 7 www.national.com LM3404/LM3404HV
TON vs VIN, RON = 100 kΩ (TA = 25°C) 20205435 TON vs VIN, (TA = 25°C) 20205436 TON vs VIN, (TA = 25°C) 20205437 TON vs RON, LM3404 (TA = 25°C) 20205444 TON vs RON, LM3404HV (TA = 25°C) 20205438 VCC vs VIN (TA = 25°C) 20205439 www.national.com 8 LM3404/LM3404HV
VO-MAX vs fSW, LM3404 (TA = 25°C) 20205440 VO-MIN vs fSW, LM3404 (TA = 25°C) 20205441 VO-MAX vs fSW, LM3404HV (TA = 25°C) 20205442 VO-MIN vs fSW, LM3404HV (TA = 25°C) 20205443 9 www.national.com LM3404/LM3404HV
CCM operation, the converter maintains a constant switching frequency that can be selected using the following equation: VF = forward voltage of each LED, n = number of LEDs in series AVERAGE LED CURRENT ACCURACY The COT architecture regulates the valley of ΔVSNS, the AC portion of VSNS. To determine the average LED current (which is also the average inductor current) the valley inductor cur- rent is calculated using the following expression: In this equation tSNS represents the propagation delay of the CS comparator, and is approximately 220 ns. The average inductor/LED current is equal to IL-MIN plus one-half of the in- ductor current ripple, ΔiL: IF = IL = IL-MIN + ΔiL / 2 Detailed information for the calculation of ΔiL is given in the Design Considerations section. MAXIMUM OUTPUT VOLTAGE The 300 ns minimum off-time limits the maximum duty cycle of the converter, DMAX, and in turn the maximum output volt- age, VO(MAX), determined by the following equations: The maximum number of LEDs, n MAX, that can be placed in a single series string is governed by V O(MAX) and the maxi- mum forward voltage of the LEDs used, V F(MAX), using the expression: At low switching frequency the maximum duty cycle and out- put voltage are higher, allowing the LM3404/04HV to regulate output voltages that are nearly equal to input voltage. The following equation relates switching frequency to maximum output voltage, and is also shown graphically in the Typical Performance Characteristics section: MINIMUM OUTPUT VOLTAGE The minimum recommended on-time for the LM3404/04HV is 300 ns. This lower limit for tON determines the minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency. The relationship is deter- mined by the following equation, shown on the same graphs as maximum output voltage in the Typical Performance Char- acteristics section: HIGH VOLTAGE BIAS REGULATOR The LM3404/04HV contains an internal linear regulator with a 7V output, connected between the VIN and the VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as VIN increases. Operation begins when VCC crosses 5.25V. INTERNAL MOSFET AND DRIVER The LM3404/04HV features an internal power MOSFET as well as a floating driver connected from the SW pin to the BOOT pin. Both rise time and fall time are 20 ns each (typical) and the approximate gate charge is 6 nC. The high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an external 10 nF capacitor, CB. VCC charges CB through the internal diode while the power MOSFET is off. When the MOSFET turns on, the internal diode reverse biases. This creates a floating supply equal to the VCC voltage minus the diode drop to drive the MOSFET when its source voltage is equal to VIN. FAST SHUTDOWN FOR PWM DIMMING The DIM pin of the LM3404/04HV is a TTL compatible input for low frequency PWM dimming of the LED. A logic low (be- low 0.8V) at DIM will disable the internal MOSFET and shut off the current flow to the LED array. While the DIM pin is in a logic low state the support circuitry (driver, bandgap, VCC) remains active in order to minimize the time needed to turn the LED array back on when the DIM pin sees a logic high (above 2.2V). A 75 µA (typical) pull-up current ensures that the LM3404/04HV is on when DIM pin is open circuited, elim- inating the need for a pull-up resistor. Dimming frequency, fDIM, and duty cycle, DDIM, are limited by the LED current rise time and fall time and the delay from activation of the DIM pin to the response of the internal power MOSFET. In general, fDIM should be at least one order of magnitude lower than the steady state switching frequency in order to prevent aliasing. PEAK CURRENT LIMIT The current limit comparator of the LM3404/04HV will engage whenever the power MOSFET current (equal to the inductor current while the MOSFET is on) exceeds 1.5A (typical). The power MOSFET is disabled for a cool-down time that is ap- proximately 75x the steady-state on-time. At the conclusion of this cool-down time the system re-starts. If the current limit condition persists the cycle of cool-down time and restarting will continue, creating a low-power hiccup mode, minimizing thermal stress on the LM3404/04HV and the external circuit components. OVER-VOLTAGE/OVER-CURRENT COMPARATOR The CS pin includes an output over-voltage/over-current comparator that will disable the power MOSFET whenever 11 www.national.com LM3404/LM3404HV
rest of the circuit increase. the converter on-time, when C IN supplies the load current. due to low forward drop and near-zero reverse recovery time. used to estimate the operating die temperature of the device. FIGURE 5. LED Current From SW Pin
will not affect regular PWM dimming operation. the SW pin to ground according to the chart below. FIGURE 6. CS Pin, Transient Path
FIGURE 10. VIN Pin with Typical Input Protection
A moderate switching frequency is needed in this application to balance the requirements of magnetics size and efficiency. RON is selected from the equation for switching frequency as follows: RON = 7.1 / (1.34 x 10-10 x 4 x 105) = 132.5 kΩ The closest 1% tolerance resistor is 133 k Ω. The switching frequency and on-time of the circuit can then be found using the equations relating RON and tON to fSW: fSW = 7.1 / (1.33 x 105 x 1.34 x 10-10) = 398 kHz tON = (1.34 x 10-10 x 1.33 x 105) / 24 = 743 ns OUTPUT INDUCTOR Since an output capacitor will be used to filter some of the AC ripple current, the inductor ripple current can be set higher than the LED ripple current. A value of 40% P-P is typical in many buck converters: ΔiL = 0.4 x 0.7 = 0.28A With the target ripple current determined the inductance can be chosen: The closest standard inductor value is 47 µH. The average current rating should be greater than 700 mA to prevent over- heating in the inductor. Separation between the LM3404 drivers and the LED arrays means that heat from the inductor will not threaten the lifetime of the LEDs, but an overheated inductor could still cause the LM3404 to enter thermal shut- down. The inductance of the standard part chosen is ±20%. With this tolerance the typical, minimum, and maximum inductor cur- rent ripples can be calculated: = 266 mAP-P = 223 mAP-P = 330 mAP-P The peak LED/inductor current is then estimated: IL(PEAK) = IL + 0.5 x ΔiL(MAX) IL(PEAK) = 0.7 + 0.5 x 0.330 = 866 mA In the case of a short circuit across the LED array, the LM3404 will continue to deliver rated current through the short but will reduce the output voltage to equal the CS pin voltage of 200 mV. The inductor ripple current and peak current in this con- dition would be equal to: ΔiL(LED-SHORT) = [(24 – 0.2) x 7.43 x 10-7] / 38 x 10-6 = 465 mAP-P IL(PEAK) = 0.7 + 0.5 x 0.465 = 933 mA In the case of a short at the switch node, the output, or from the CS pin to ground the short circuit current limit will engage at a typical peak current of 1.5A. In order to prevent inductor saturation during these fault conditions the inductor’s peak current rating must be above 1.5A. A 47 µH off-the shelf in- ductor rated to 1.4A (peak) and 1.5A (average) with a DCR of 0.1Ω will be used. USING AN OUTPUT CAPACITOR This application does not require high frequency PWM dim- ming, allowing the use of an output capacitor to reduce the size and cost of the output inductor. To select the proper out- put capacitor the equation from Buck Regulators with Output Capacitors is re-arranged to yield the following: The target tolerance for LED ripple current is 100 mAP-P, and a typical value for rD of 1.8Ω at 700 mA can be read from the LED datasheet. The required capacitor impedance to reduce the worst-case inductor ripple current of 333 mA P-P is there- fore: A ceramic capacitor will be used and the required capacitance is selected based on the impedance at 400 kHz: CO = 1/(2 x π x 0.77 x 4 x 105) = 0.51 µF This calculation assumes that impedance due to the equiva- lent series resistance (ESR) and equivalent series inductance (ESL) of CO is negligible. The closest 10% tolerance capacitor value is 1.0 µF. The capacitor used should be rated to 25V or more and have an X7R dielectric. Several manufacturers pro- duce ceramic capacitors with these specifications in the 0805 case size. A typical value for ESR of 3 mΩ can be read from the curve of impedance vs. frequency in the product datasheet. www.national.com 20 LM3404/LM3404HV
A preliminary value for R SNS was determined in selecting ΔiL. This value should be re-evaluated based on the calcula- tions for ΔiF: tSNS = 220 ns, RSNS = 0.33Ω Sub-1Ω resistors are available in both 1% and 5% tolerance. A 1%, 0.33Ω device is the closest value, and a 0.33W, 1206 size device will handle the power dissipation of 162 mW. With the resistance selected, the average value of LED current is re-calculated to ensure that current is within the ±5% toler- ance requirement. From the expression for average LED current: = 706 mA, 1% above 700 mA INPUT CAPACITOR Following the calculations from the Input Capacitor section, ΔvIN(MAX) will be 24V x 2% P-P = 480 mV. The minimum re- quired capacitance is: To provide additional safety margin the a higher value of 3.3 µF ceramic capacitor rated to 50V with X7R dielectric in an 1210 case size will be used. From the Design Considerations section, input rms current is: IIN-RMS = 0.7 x Sqrt(0.28 x 0.72) = 314 mA Ripple current ratings for 1210 size ceramic capacitors are typically higher than 2A, more than enough for this design. RECIRCULATING DIODE The input voltage of 24V ±5% requires Schottky diodes with a reverse voltage rating greater than 30V. The next highest standard voltage rating is 40V. Selecting a 40V rated diode provides a large safety margin for the ringing of the switch node and also makes cross-referencing of diodes from differ- ent vendors easier. The next parameters to be determined are the forward current rating and case size. In this example the low duty cycle (D = 7.1 / 24 = 28%) places a greater thermal stress on D1 than on the internal power MOSFET of the LM3404. The estimated average diode current is: ID = 0.706 x 0.72 = 509 mA A Schottky with a forward current rating of 1A would be ade- quate, however reducing the power dissipation is critical in this example. Higher current diodes have lower forward volt- ages, hence a 2A-rated diode will be used. To determine the proper case size, the dissipation and temperature rise in D1 can be calculated as shown in the Design Considerations section. VD for a case size such as SMB in a 40V, 2A Schottky diode at 700 mA is approximately 0.3V and the θJA is 75°C/ W. Power dissipation and temperature rise can be calculated as: PD = 0.509 x 0.3 = 153 mW TRISE = 0.153 x 75 = 11.5°C CB AND CF The bootstrap capacitor CB should always be a 10 nF ceramic capacitor with X7R dielectric. A 25V rating is appropriate for all application circuits. The linear regulator filter capacitor CF should always be a 100 nF ceramic capacitor, also with X7R dielectric and a 25V rating. EFFICIENCY To estimate the electrical efficiency of this example the power dissipation in each current carrying element can be calculated and summed. Electrical efficiency, η, should not be confused with the optical efficacy of the circuit, which depends upon the LEDs themselves. Total output power, PO, is calculated as: PO = IF x VO = 0.706 x 7.1 = 5W Conduction loss, PC, in the internal MOSFET: PC = (IF2 x RDSON) x D = (0.7062 x 0.8) x 0.28 = 112 mW Gate charging and VCC loss, PG, in the gate drive and linear regulator: PG = (IIN-OP + fSW x QG) x VIN PG = (600 x 10-6 + 4 x 105 x 6 x 10-9) x 24 = 72 mW Switching loss, PS, in the internal MOSFET: PS = 0.5 x VIN x IF x (tR + tF) x fSW PS = 0.5 x 24 x 0.706 x 40 x 10-9 x 4 x 105 = 136 mW AC rms current loss, PCIN, in the input capacitor: PCIN = IIN(rms)2 x ESR = 0.3172 0.003 = 0.3 mW (negligible) DCR loss, PL, in the inductor PL = IF2 x DCR = 0.7062 x 0.1 = 50 mW Recirculating diode loss, PD = 153 mW Current Sense Resistor Loss, PSNS = 164 mW Electrical efficiency, η = PO / (PO + Sum of all loss terms) = Temperature Rise in the LM3404 IC is calculated as: TLM3404 = (PC + PG + PS) x θJA = (0.112 + 0.072 + 0.136) x 155 = 49.2°C 21 www.national.com LM3404/LM3404HV
ponent selection for an outdoor general lighting application. in Table 2 at the end of this datasheet. FIGURE 13. Schematic for Design Example 2 inductor used should have a de-rating of about 50%, or 1A.
voltage of 200 mV. The inductor ripple current and peak cur- rent in this condition would be equal to: ΔiL(LED-SHORT) = [(48 – 0.2) x 3.3 x 10-6] / 264 x 10-6 = 0.598AP-P In the case of a short at the switch node, the output, or from the CS pin to ground the short circuit current limit will engage at a typical peak current of 1.5A. In order to prevent inductor saturation during these fault conditions the inductor’s peak current rating must be above 1.5A. A 330 µH off-the shelf in- ductor rated to 1.9A (peak) and 1.0A (average) with a DCR of 0.56Ω will be used. USING AN OUTPUT CAPACITOR This application uses sub-1 kHz frequency PWM dimming, allowing the use of a small output capacitor to reduce the size and cost of the output inductor. To select the proper output capacitor the equation from Buck Regulators with Output Ca- pacitors is re-arranged to yield the following: The target tolerance for LED ripple current is 50 mA P-P, and the typical value for r D is 10 Ω with ten LEDs in series. The required capacitor impedance to reduce the worst-case steady-state inductor ripple current of 160 mAP-P is therefore: A ceramic capacitor will be used and the required capacitance is selected based on the impedance at 223 kHz: CO = 1/(2 x π x 4.5 x 2.23 x 105) = 0.16 µF This calculation assumes that impedance due to the equiva- lent series resistance (ESR) and equivalent series inductance (ESL) of CO is negligible. The closest 10% tolerance capacitor value is 0.15 µF. The capacitor used should be rated to 50V or more and have an X7R dielectric. Several manufacturers produce ceramic capacitors with these specifications in the 0805 case size. ESR values are not typically provided for such low value capacitors, however is can be assumed to be under 100 mΩ, leaving plenty of margin to meet to LED ripple current requirement. The low capacitance required allows the use of a 100V rated, 1206-size capacitor. The rating of 100V en- sures that the capacitance will not decrease significantly when the DC output voltage is applied across the capacitor. RSNS A preliminary value for R SNS was determined in selecting ΔiL. This value should be re-evaluated based on the calcula- tions for ΔiF: tSNS = 220 ns, RSNS = 0.43Ω Sub-1Ω resistors are available in both 1% and 5% tolerance. A 1%, 0.43Ω device is the closest value, and a 0.25W, 0805 size device will handle the power dissipation of 110 mW. With the resistance selected, the average value of LED current is re-calculated to ensure that current is within the ±10% toler- ance requirement. From the expression for average LED current: = 505 mA INPUT CAPACITOR Following the calculations from the Input Capacitor section, ΔvIN(MAX) will be 48V x 2% P-P = 960 mV. The minimum re- quired capacitance is: To provide additional safety margin a 2.2 µF ceramic capac- itor rated to 100V with X7R dielectric in an 1812 case size will be used. From the Design Considerations section, input rms current is: IIN-RMS = 0.5 x Sqrt(0.73 x 0.27) = 222 mA Ripple current ratings for 1812 size ceramic capacitors are typically higher than 2A, more than enough for this design, and the ESR is approximately 3 mΩ. RECIRCULATING DIODE The input voltage of 48V requires Schottky diodes with a re- verse voltage rating greater than 50V. The next highest stan- dard voltage rating is 60V. Selecting a 60V rated diode provides a large safety margin for the ringing of the switch node and also makes cross-referencing of diodes from differ- ent vendors easier. The next parameters to be determined are the forward current rating and case size. In this example the high duty cycle (D = 35.2 / 48 = 73%) places a greater thermal stress on the in- ternal power MOSFET than on D1. The estimated average diode current is: ID = 0.5 x 0.27 = 135 mA A Schottky with a forward current rating of 0.5A would be ad- equate, however reducing the power dissipation is critical in this example. Higher current diodes have lower forward volt- ages, hence a 1A-rated diode will be used. To determine the proper case size, the dissipation and temperature rise in D1 can be calculated as shown in the Design Considerations section. VD for a case size such as SMA in a 60V, 1A Schottky diode at 0.5A is approximately 0.35V and the θJA is 75°C/W. Power dissipation and temperature rise can be calculated as: PD = 0.135 x 0.35 = 47 mW TRISE = 0.047 x 75 = 3.5°C 23 www.national.com LM3404/LM3404HV
dielectric and a 25V rating. much upon the layout of the PCB as the component selection. FIGURE 14. Buck Converter Current Loops resents the high current path during the off-time.
routed with a short, thick shape, preferably on the component side of the PCB. Multiple vias in parallel should be used right at the pad of the input capacitor to connect the component side shapes to the ground plane. A second pulsating current loop that is often ignored is the gate drive loop formed by the SW and BOOT pins and capacitor CB. To minimize this loop at the EMI it generates, keep CB close to the SW and BOOT pins. CURRENT SENSING The CS pin is a high-impedance input, and the loop created by RSNS, RZ (if used), the CS pin and ground should be made as small as possible to maximize noise rejection. RSNS should therefore be placed as close as possible to the CS and GND pins of the IC. REMOTE LED ARRAYS In some applications the LED or LED array can be far away (several inches or more) from the LM3404/04HV, or on a sep- arate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large or separated from the rest of the converter, the output capacitor should be placed close to the LEDs to reduce the effects of parasitic inductance on the AC impedance of the capacitor. The current sense resistor should remain on the same PCB, close to the LM3404/04HV. 25 www.national.com LM3404/LM3404HV
TABLE 1. BOM for Design Example 1
1 TDK
TABLE 2. BOM for Design Example 2
1 Coilcraft
Physical Dimensions inches (millimeters) unless otherwise noted 27 www.national.com LM3404/LM3404HV
LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy PowerWise® Solutions www.national.com/powerwise Applications & Markets www.national.com/solutions Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless PowerWise® Design University www.national.com/training THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. Copyright© 2010 National Semiconductor Corporation For the most current product information visit us at www.national.com National Semiconductor Americas Technical Support Center Email: support@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Technical Support Center Email: europe.support@nsc.com National Semiconductor Asia Pacific Technical Support Center Email: ap.support@nsc.com National Semiconductor Japan Technical Support Center Email: jpn.feedback@nsc.com www.national.com
Texas InstrumentsIncorporatedand itssubsidiaries(TI)reservetherighttomake corrections,modifications,enhancements,improvements, and otherchanges toitsproductsand servicesatany timeand todiscontinueany productorservicewithoutnotice.Customersshould obtainthelatestrelevantinformationbeforeplacingordersand shouldverifythatsuch informationiscurrentand complete.Allproductsare soldsubjecttoTI’s termsand conditionsofsalesuppliedatthetimeoforderacknowledgment. TIwarrantsperformanceofitshardwareproductstothespecificationsapplicableatthetimeofsaleinaccordancewithTI’s standard warranty.Testingand otherqualitycontroltechniquesareused totheextentTIdeems necessarytosupportthiswarranty.Exceptwhere mandated by governmentrequirements,testingofallparametersofeach productisnotnecessarilyperformed. TIassumes no liabilityforapplicationsassistanceorcustomerproductdesign.Customersareresponsiblefortheirproductsand applicationsusingTIcomponents.To minimizetherisksassociatedwithcustomerproductsand applications,customersshouldprovide adequatedesignand operatingsafeguards. TIdoes notwarrantorrepresentthatany license,eitherexpressorimplied,isgrantedunderany TIpatentright,copyright,mask work right, orotherTIintellectualpropertyrightrelatingtoany combination,machine,orprocessinwhichTIproductsorservicesareused.Information publishedby TIregardingthird-partyproductsorservicesdoes notconstitutea licensefromTItouse such productsorservicesora warrantyorendorsementthereof.Use ofsuch informationmay requirea licensefroma thirdpartyunderthepatentsorotherintellectual propertyofthethirdparty,ora licensefromTIunderthepatentsorotherintellectualpropertyofTI. ReproductionofTIinformationinTIdatabooks ordatasheetsispermissibleonlyifreproductioniswithoutalterationand isaccompanied by allassociatedwarranties,conditions,limitations,and notices.Reproductionofthisinformationwithalterationisan unfairand deceptive businesspractice.TIisnotresponsibleorliableforsuch altereddocumentation.Informationofthirdpartiesmay be subjecttoadditional restrictions. ResaleofTIproductsorserviceswithstatementsdifferentfromorbeyond theparametersstatedby TIforthatproductorservicevoidsall expressand any impliedwarrantiesfortheassociatedTIproductorserviceand isan unfairand deceptivebusinesspractice.TIisnot responsibleorliableforany such statements. TIproductsarenotauthorizedforuse insafety-criticalapplications(suchas lifesupport)where a failureoftheTIproductwouldreasonably be expectedtocause severepersonalinjuryordeath,unlessofficersofthepartieshave executedan agreementspecificallygoverning such use.Buyersrepresentthattheyhave allnecessaryexpertiseinthesafetyand regulatoryramificationsoftheirapplications,and acknowledgeand agreethattheyaresolelyresponsibleforalllegal,regulatoryand safety-relatedrequirementsconcerningtheirproducts and any use ofTIproductsinsuch safety-criticalapplications,notwithstandingany applications-relatedinformationorsupportthatmay be providedby TI.Further,Buyersmust fullyindemnifyTIand itsrepresentativesagainstany damages arisingoutoftheuse ofTIproductsin such safety-criticalapplications. TIproductsareneitherdesignednorintendedforuse inmilitary/aerospaceapplicationsorenvironmentsunlesstheTIproductsare specificallydesignatedby TIas military-gradeor"enhanced plastic." Onlyproductsdesignatedby TIas military-grademeet military specifications.Buyersacknowledgeand agreethatany such use ofTIproductswhichTIhas notdesignatedas military-gradeissolelyat theBuyer's risk,and thattheyaresolelyresponsibleforcompliancewithalllegaland regulatoryrequirementsinconnectionwithsuch use. TIproductsareneitherdesignednorintendedforuse inautomotiveapplicationsorenvironmentsunlessthespecificTIproductsare designatedby TIas compliantwithISO/TS 16949 requirements.Buyersacknowledgeand agreethat,iftheyuse any non-designated productsinautomotiveapplications,TIwillnotbe responsibleforany failuretomeet such requirements. FollowingareURLs where you can obtaininformationon otherTexas Instrumentsproductsand applicationsolutions: Products Applications Audio www.ti.com/audio Communicationsand Telecom www.ti.com/communications Amplifiers amplifier.ti.com Computers and Peripherals www.ti.com/computers Data Converters dataconverter.ti.com Consumer Electronics www.ti.com/consumer-apps DLP ® Products www.dlp.com Energyand Lighting www.ti.com/energy DSP dsp.ti.com Industrial www.ti.com/industrial Clocksand Timers www.ti.com/clocks Medical www.ti.com/medical Interface interface.ti.com Security www.ti.com/security Logic logic.ti.com Space,Avionicsand Defense www.ti.com/space-avionics-defense Power Mgmt power.ti.com Transportationand Automotive www.ti.com/automotive Microcontrollers microcontroller.ti.com Videoand Imaging www.ti.com/video RFID www.ti-rfid.com OMAP MobileProcessors www.ti.com/omap WirelessConnectivity www.ti.com/wirelessconnectivity TIE2E Community Home Page e2e.ti.com MailingAddress:Texas Instruments,PostOfficeBox 655303,Dallas,Texas 75265 Copyright© 2011,Texas InstrumentsIncorporated