LTC7138

LTC7138_15 LINER | Alldatasheet

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7138fFor more information www.linear .com/L TC7138 Typical applicaTion FeaTures DescripTion High Efficiency, 140V 400mA Step-Down Regulator The LT C®7138 is a high efficiency step-down DC/DC regulator with internal power switch that draws only 12μA typical DC supply current while maintaining a regulated output voltage at no load. The L TC7138 can supply up to 400mA load current and features a programmable peak current limit that provides a simple method for optimizing efficiency and for reduc- ing output ripple and component size. The L TC7138’s combination of Burst Mode ® operation, integrated power switch, low quiescent current, and programmable peak current limit provides high efficiency over a broad range of load currents. With its wide input range of 4V to 140V and programmable overvoltage lockout, the L TC7138 is a robust regulator suited for regulating from a wide variety of power sources. Additionally, the L TC7138 includes a precise run threshold and soft-start feature to guarantee that the power system start-up is well-controlled in any environment. A feedback comparator output enables multiple L TC7138s to be con- nected in parallel for higher current applications. The LTC7138 is available in a thermally enhanced high voltage-capable 16-lead MSE package with four missing pins. L, L T , L TC, L TM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Efficiency and Power Loss vs Load Current 5V to 140V Input to 5V Output, 400mA Step-Down Regulator

applicaTions

n Wide Operating Input Voltage Range: 4V to 140V n Internal Low Resistance Power MOSFET n No Compensation Required n Adjustable 100mA to 400mA Maximum Output Current n Low Dropout Operation: 100% Duty Cycle n Low Quiescent Current: 12µA n Wide Output Range: 0.8V to VIN n 0.8V ±1% Feedback Voltage Reference n Precise RUN Pin Threshold n Internal or External Soft-Start n Programmable 1.8V , 3.3V , 5V or Adjustable Output n Few External Components Required n Programmable Input Overvoltage Lockout n Thermally Enhanced High Voltage MSOP Package n Industrial Control Supplies n Medical Devices n Distributed Power Systems n Portable Instruments n Battery-Operated Devices n Avionics n Automotive

7138 TA01a

220µH VIN RUN CIN 1µF 250V COUT 22µF VIN 5V TO 140V VOUT 400mAL TC7138 GND VPRG2 OVLO SS VPRG1 VIN = 12V VIN = 48V VIN = 140V LOAD CURRENT (mA) EFFICIENCY (%) POWER LOSS (mW) 100 100 100060 0.1 100 1000

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7138f For more information www.linear .com/L TC7138 absoluTe MaxiMuM raTings SW V IN FBO VPRG2 VPRG1 GND ANODE RUN OVLO ISET SS V FB TOP VIEW GND MSE PACKAGE VARIATION: MSE16 (12) 16-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 40°C/W , θJC = 10°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB pin conFiguraTion orDer inForMaTion LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L TC7138EMSE#PBF L TC7138EMSE#TRPBF 7138 16-Lead Plastic MSOP –40°C to 125°C L TC7138IMSE#PBF L TC7138IMSE#TRPBF 7138 16-Lead Plastic MSOP –40°C to 125°C L TC7138HMSE#PBF L TC7138HMSE#TRPBF 7138 16-Lead Plastic MSOP –40°C to 150°C L TC7138MPMSE#PBF L TC7138MPMSE#TRPBF 7138 16-Lead Plastic MSOP –55°C to 150°C Consult L TC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container . Consult L TC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear .com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear .com/tapeandreel/ Operating Junction Temperature Range (Notes 2, 3, 4) 65°C to 150°C 00°C (Note 1) elecTrical characTerisTics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V , unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply (VIN) VIN Input Voltage Operating Range 4 140 V VOUT Output Voltage Operating Range 0.8 VIN V UVLO V IN Undervoltage Lockout VIN Rising VIN Falling Hysteresis l l 3.5 3.3 3.75 3.5 250 4.0 3.8 V V mV I Q DC Supply Current (Note 5) Active Mode Sleep Mode Shutdown Mode No Load V RUN = 0V 200 1.4 400 µA µA µA V RUN RUN Pin Threshold RUN Rising RUN Falling Hysteresis 1.17 1.06 1.21 1.10 110 1.25 1.14 V V mV I RUN RUN Pin Leakage Current RUN = 1.3V –10 0 10 nA VOVLO OVLO Pin Threshold OVLO Rising OVLO Falling Hysteresis 1.17 1.06 1.21 1.10 110 1.25 1.14 V V mV

7138fFor more information www.linear .com/L TC7138 elecTrical characTerisTics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V , unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Output Supply (VFB) VFB(ADJ) Feedback Comparator Threshold (Adjustable Output) VFB Rising, VPRG1 = VPRG2 = 0V L TC7138E, L TC7138I L TC7138H, L TC7138MP l l 0.792 0.788 0.800 0.800 0.808 0.812 V V V FBH Feedback Comparator Hysteresis (Adjustable Output) V FB Falling, VPRG1 = VPRG2 = 0V l 3 5 9 mV IFB Feedback Pin Current VFB = 1V , VPRG1 = VPRG2 = 0V –10 0 10 nA VFB(FIXED) Feedback Comparator Thresholds (Fixed Output) V FB Rising, VPRG1 = SS, VPRG2 = 0V VFB Falling, VPRG1 = SS, VPRG2 = 0V l l 4.94 4.91 5.015 4.985 5.09 5.06 V V V FB Rising, VPRG1 = 0V , VPRG2 = SS VFB Falling, VPRG1 = 0V , VPRG2 = SS l l 3.25 3.23 3.31 3.29 3.37 3.35 V V V FB Rising, VPRG1 = VPRG2 = SS VFB Falling, VPRG1 = VPRG2 = SS l l 1.78 1.77 1.81 1.80 1.84 1.83 V V Operation I PEAK Peak Current Comparator Threshold I SET Floating 100k Resistor from ISET to GND ISET Shorted to GND l l l 540 270 140 610 310 170 680 350 200 mA mA mA I VAL Valley Current Comparator Threshold Relative to I PEAK ISET Floating 100k Resistor from ISET to GND ISET Shorted to GND l l l R ON Power Switch On-Resistance ISW = –100mA 1.8 Ω ILSW Switch Pin Leakage Current VIN = 140V , SW = 0V 0.1 1 μA ISS Soft-Start Pin Pull-Up Current VSS < 2.5V 4 5 6 μA tINT(SS) Internal Soft-Start Time SS Pin Floating 1 ms Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The L TC7138 is tested under pulsed load conditions such that T J ≈ TA. The L TC7138E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The L TC7138I is guaranteed over the –40°C to 125°C operating junction temperature range, the L TC7138H is guaranteed over the –40°C to 150°C operating junction temperature range and the L TC7138MP is tested and guaranteed over the –55°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes; operating lifetime is derated for junction temperatures greater than 125°C. Note that the maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the rated package thermal impedance and other environmental factors. Note 3: The junction temperature (T J, in °C) is calculated from the ambient temperature (TA, in °C) and power dissipation (PD, in Watts) according to the formula: TJ = TA + (PD • θJA) where θJA is 40°C/W for the MSOP package. Note that the maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the rated package thermal impedance and other environmental factors. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. The maximum rated junction temperature will be exceeded when this protection is active. Continuous operation above the specified absolute maximum operating junction temperature may impair device reliability or permanently damage the device. The overtemperature protection level is not production tested. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information.

7138f For more information www.linear .com/L TC7138 Typical perForMance characTerisTics Peak Current and Valley Current T rip Thresholds vs R ISET Efficiency vs Load Current, V OUT = 5V Peak Current and Valley Current T rip Thresholds vs Temperature and I SET Efficiency vs Load Current, V OUT = 3.3V Peak Current and Valley Current T rip Thresholds vs Input Voltage Efficiency vs Load Current, V OUT = 1.8V Efficiency vs Input Voltage, V OUT = 5V Feedback Comparator T rip Threshold vs Temperature RUN and OVLO Thresholds vs Temperature 0.1 100 1000101 LOAD CURRENT (mA) EFFICIENCY (%) 100

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VIN = 12V VIN = 48V VIN = 140V 100 0 25 75 10050 125 150 INPUT VOLTAGE (V) EFFICIENCY (%)

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ILOAD = 1mA ILOAD = 30mA ILOAD = 400mA FIGURE 13 CIRCUIT TEMPERATURE (°C) –55 798 THRESHOLD VOLTAGE (mV)799 800 801 802 –25 5 35 65

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TEMPERATURE (°C) –55 RUN OR OVLO THRESHOLD VOLTAGE (V) 1.20 1.22 1.24 35 95

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1.18 1.16 1.14 1.12 1.10 –25 5 65 125 155 1.08 1.06 RISING FALLING 0.1 100 1000101 FIGURE 13 CIRCUIT VIN = 12V VIN = 48V VIN = 140V LOAD CURRENT (mA) EFFICIENCY (%) 100

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0.1 100 1000101 FIGURE 13 CIRCUIT VIN = 12V VIN = 48V VIN = 140V LOAD CURRENT (mA) EFFICIENCY (%) 100

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R ISET (kΩ) 100 125 150 175 200 225 100 200 300 400 500 600 700 THRESHOLD (mA)

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TEMPERATURE (°C) –55 –25 125 155 100 200 300 400 500 600 700 THRESHOLD (mA)

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V IN VOL TAGE (V) 120 150 100 200 300 400 500 600 700 THRESHOLD (mA)

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7138fFor more information www.linear .com/L TC7138 Typical perForMance characTerisTics Switch On-Resistance vs Input Voltage Switch On-Resistance vs Temperature Load Step T ransient Response Quiescent Supply Current vs Input Voltage Operating Waveforms, V IN = 48V Quiescent Supply Current vs Temperature Operating Waveforms, V IN = 140V Switch Pin Current vs Temperature Short-Circuit and Recovery VIN VOLTAGE (V) VIN SUPPLY CURRENT (µA) 60 90

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VIN VOLTAGE (V)

1.0 SWITCH ON-RESISTANCE (/uni03A9)

2.0 1.5 2.5 3.0

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TEMPERATURE (°C) –55 SWITCH ON-RESISTANCE (/uni03A9)

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–25 5 65 95 125 155 ISW = 250mA TEMPERATURE (°C) –55 –25 VIN SUPPLY CURRENT (µA)10 5 65 95

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VIN = 140V SLEEP SHUTDOWN TEMPERATURE (°C) –55 SWITCH PIN CURRENT (µA)

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–25 5 65 –15 –10 95 125 155 VIN = 140V SLEEP MODE SW = 0.8V CURRENT INTO SW SW = 0V CURRENT OUT OF SW OUTPUT VOL TAGE 100mV/DIV LOAD CURRENT 200mA/DIV 200µs/DIVVIN = 48V VOUT = 3.3V 10mA TO 400mA LOAD STEP FIGURE 14 CIRCUIT

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10µs/DIVVIN = 48V VOUT = 3.3V IOUT = 300mA FIGURE 14 CIRCUIT

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10µs/DIVVIN = 140V VOUT = 3.3V IOUT = 300mA FIGURE 14 CIRCUIT

7138 G17

500µs/DIV FIGURE 14 CIRCUIT

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7138f For more information www.linear .com/L TC7138 pin FuncTions SW (Pin 1): Switch Node Connection to Inductor and Catch Diode Cathode. This pin connects to the drain of the internal power MOSFET switch. V IN (Pin 3): Main Supply Pin. A ceramic bypass capacitor should be tied between this pin and GND. FBO (Pin 5): Feedback Comparator Output. Connect to the V FB pins of additional L TC7138s to combine the output current. The typical pull-up current is 20µA. The typical pull- down impedance is 70Ω. See Applications Information. V PRG2, VPRG1 (Pins 6, 7): Output Voltage Selection. Short both pins to ground for a resistive divider programmable output voltage. Short V PRG1 to SS and short V PRG2 to ground for a 5V output voltage. Short V PRG1 to ground and short V PRG2 to SS for a 3.3V output voltage. Short both pins to SS for a 1.8V output voltage. GND (Pin 8, Exposed Pad Pin 17): Ground. The exposed pad must be soldered to the PCB ground plane for rated electrical and thermal performance. V FB (Pin 9): Output Voltage Feedback. When configured for an adjustable output voltage, connect to an external resistive divider to divide the output voltage down for comparison to the 0.8V reference. For the fixed output configuration, directly connect this pin to the output. SS (Pin 10): Soft-Start Control Input. A capacitor to ground at this pin sets the output voltage ramp time. A 50µA current initially charges the soft-start capacitor until switching begins, at which time the current is reduced to its nominal value of 5µA. The output voltage ramp time from zero to its regulated value is 1ms for every 6.25nF of capacitance from SS to GND. If left floating, the ramp time defaults to an internal 1ms soft-start. I SET (Pin 11): Peak Current Set Input. A resistor from this pin to ground sets the peak current comparator threshold. Leave floating for the maximum peak current (610mA typi- cal) or short to ground for minimum peak current (170mA typical). The valley current is typically 60% of the peak current set by this pin. The maximum output current is 75% of the peak current. The 5µA current that is sourced out of this pin when switching is reduced to 1µA in sleep. Optionally, a capacitor can be placed from this pin to GND to trade off efficiency for light load output voltage ripple. See Applications Information. OVLO (Pin 12): Overvoltage Lockout Input. Connect to the input supply through a resistor divider to set the over- voltage lockout level. A voltage on this pin above 1.21V disables the internal MOSFET switch. Normal operation resumes when the voltage on this pin decreases below 1.10V. Exceeding the OVLO lockout threshold triggers a soft-start reset, resulting in a graceful recovery from an input supply transient. This pin must be grounded if the OVLO is not used. RUN (Pin 14): Run Control Input. A voltage on this pin above 1.21V enables normal operation. Forcing this pin below 0.7V shuts down the L TC7138, reducing quiescent current to approximately 1.4µA. Optionally, connect to the input supply through a resistor divider to set the undervoltage lockout. ANODE (Pin 16): Catch Diode Anode Sense. This pin is the anode connection for the catch diode. An internal sense resistor is connected between this pin and the exposed pad ground.

7138fFor more information www.linear .com/L TC7138 block DiagraM COUT CIN VIN VOUT PEAK CURRENT COMPARATOR VALLEY CURRENT COMPARATOR FEEDBACK COMPARATOR VOL TAGE REFERENCE VPRG2 GND GND SS SS V PRG1 GND SS GND SS 1.0M 4.2M 2.5M 1.0M 800k 800k 800k V OUT ADJUSTABLE 5V FIXED 3.3V FIXED 1.8V FIXED START-UP: 50µA NORMAL: 5µA IMPLEMENT DIVIDER EXTERNALL Y FOR ADJUSTABLE VERSION V IN 1SW L1 ANODE ISET LOGIC SS 20µA FBO 70/uni03A9 10 GND8 GND 17 VFB VPRG1 VPRG2 7138 BD 0.800V OVLO 1.21V 1.21V RUN14 ISET ACTIVE: 5µA SLEEP: 1µA 1.3V

trips and enables burst cycles. switch is turned on again and another cycle commences. Figure 1. Burst Mode Operation noise on the sensitive input to the feedback comparator.

7138fFor more information www.linear .com/L TC7138 Start-Up and Shutdown If the voltage on the RUN pin is less than 0.7V , the L TC7138 enters a shutdown mode in which all internal circuitry is disabled, reducing the DC supply current to 1.4µA. When the voltage on the RUN pin exceeds 1.21V , normal operation of the main control loop is enabled. The RUN pin comparator has 110mV of internal hysteresis, and therefore must fall below 1.1V to disable the main control loop. An internal 1ms soft-start function limits the ramp rate of the output voltage on start-up to prevent excessive input supply droop. If a longer ramp time and consequently less supply droop is desired, a capacitor can be placed from the SS pin to ground. The 5µA current that is sourced out of this pin will create a smooth voltage ramp on the capacitor . If this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the ramp rate on the SS pin. The internal and external soft-start functions are reset on start-up, after an undervoltage or overvoltage event on the input supply, and after an overtemperature shutdown. Peak Inductor Current Programming The peak current comparator nominally limits the peak inductor current to 610mA. This peak inductor current can be adjusted by placing a resistor from the I SET pin to ground. The 5µA current sourced out of this pin through the resistor generates a voltage that adjusts the peak cur- rent comparator threshold. The valley current threshold tracks the peak current threshold setting, and is typically 60% of the peak current. During sleep mode, the current sourced out of the I SET pin is reduced to 1µA. The ISET current is increased back to 5µA on the first switching cycle after exiting sleep mode. The ISET current reduction in sleep mode, along with adding a filtering network, RISET and CISET, from the ISET pin to ground, provides a method of reducing light load output voltage ripple at the expense of lower efficiency and slightly degraded load step transient response. For applications requiring higher output current, the L TC7138 provides a feedback comparator output pin (FBO) for combining the output current of multiple L TC7138s. operaTion By connecting the FBO pin of a master L TC7138 to the VFB pin of one or more slave L TC7138s, the output currents can be combined to source 400mA times the number of L TC7138s. Dropout Operation When the input supply decreases toward the output sup- ply, the duty cycle increases to maintain regulation. The P-channel MOSFET switch in the L TC7138 allows the duty cycle to increase all the way to 100%. At 100% duty cycle, the P-channel MOSFET stays on continuously, providing output current equal to the peak current, which is greater than the maximum load current when not in dropout. Input Voltage and Overtemperature Protection When using the L TC7138, care must be taken not to exceed any of the ratings specified in the Absolute Maximum Rat- ings section. As an added safeguard, however , the L TC7138 incorporates an overtemperature shutdown feature. If the junction temperature reaches approximately 180°C, the L TC7138 will enter thermal shutdown mode. The power switch will be turned off and the SW node will become high impedance. After the part has cooled below 160°C, it will r estart. The overtemperature level is not production tested. The L TC7138 additionally implements protection features which inhibit switching when the input voltage is not within a programmable operating range. By use of a resistive divider from the input supply to ground, the RUN and OVLO pins serve as a precise input supply voltage moni- tor . Switching is disabled when either the RUN pin falls below 1.1V or the OVLO pin rises above 1.21V , which can be configured to limit switching to a specific range of input supply voltage. Furthermore, if the input voltage falls below 3.5V typical (3.8V maximum), an internal undervoltage detector disables switching. When switching is disabled, the L TC7138 can safely sustain input voltages up to the absolute maximum rating of 140V . Input supply undervoltage or overvoltage events trigger a soft-start reset, which results in a graceful recovery from an input supply transient. (Refer to Block Diagram)

followed by capacitors CIN and COUT. plications section for more information. maximum value of 540mA (405mA average output current). described in the Optimizing Output Voltage Ripple section. Figure 2. RISET Selection

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highest efficiency in most L TC7138 applications. damage to the part may occur . value but is very dependent of the inductance selected. wire and therefore copper losses will increase. Figure 3. Recommended Inductor Values for Maximum Efficiency

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7138f For more information www.linear .com/L TC7138 applicaTions inForMaTion teristics. The choice of which style inductor to use mainly depends on the price versus size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, TDK, Toko, and Sumida. Catch Diode Selection The catch diode (D1 from Block Diagram) conducts current only during the switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT VIN – VOUT VIN where IOUT is the output load current. The maximum av- erage diode current occurs with a shorted output at the high line. For this worst-case condition, the diode current will approach 75% of the programmed peak current. The diode reverse voltage rating should be greater than the maximum operating input voltage. When the OVLO pin is used to limit the maximum operating input voltage, the diode reverse voltage should be greater than the OVLO pin setting, but may be lower than the maximum input voltage during overvoltage lockout. For high efficiency at full load, it is important to select a catch diode with a low reverse recovery time and low for- ward voltage drop. As a result, Schottky diodes are often used as catch diodes. However , Schottky diodes generally exhibit much higher leakage than silicon diodes. In sleep, the catch diode leakage current will appear as load current, and may significantly reduce light load efficiency. Diodes with low leakage often have larger forward voltage drops at a given current, so a trade-off can exist between light load and full load efficiency. The selection of Schottky diodes with high reverse voltage ratings is limited relative to that of silicon diodes. There- fore, for low reverse leakage and part availability, some applications may prefer a silicon diode. If a silicon diode is necessary , be sure to select a diode with a specified low reverse recovery time to maximize efficiency. CIN and COUT Selection The input capacitor , CIN, is needed to filter the trapezoidal current at the source of the high side MOSFET . CIN should be sized to provide the energy required to magnetize the inductor without causing a large decrease in input voltage (∆V IN). The relationship between CIN and ∆VIN is given by: CIN > L •IPEAK 2 2• VIN • ∆VIN It is recommended to use a larger value for C IN than calculated by the previous equation since capacitance decreases with applied voltage. In general, a 1µF X7R ce- ramic capacitor is a good choice for C IN in most L TC7138 applications. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. RMS current is given by: IRMS = IOUT(MAX) • VOUT VIN

• VIN VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based only on 2000 hours of life which makes it advisable to further derate the capacitor , or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. The output capacitor , C OUT, filters the inductor’s ripple current and stores energy to satisfy the load current when the L TC7138 is in sleep. The output ripple has a lower limit of V OUT/160 due to the 5mV typical hysteresis of the feed- back comparator . The time delay of the comparator adds an additional ripple voltage that is a function of the load current. During this delay time, the L TC7138 continues to switch and supply current to the output. The output ripple

change in output voltage during a single switching cycle.

• 100% Typically, a capacitor that satisfies the voltage ripple re - quirement is adequate to filter the inductor ripple. To avoid overheating, the output capacitor must also be sized to handle the ripple current generated by the inductor . The worst-case ripple current in the output capacitor is given by IRMS = IPEAK/2. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer , aluminum electrolytic, and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important only to use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long- term reliability. Ceramic capacitors have excellent low ESR characteristics but can have high voltage coefficient and audible piezoelectric effects. The high quality factor (Q) of ceramic capacitors in series with trace inductance can also lead to significant input voltage ringing. Input Voltage Steps If the input voltage falls below the regulated output voltage, the body diode of the internal MOSFET will conduct current from the output supply to the input supply. If the input voltage falls rapidly, the voltage across the inductor will be significant and may saturate the inductor . A large current will then flow through the MOSFET body diode, resulting in excessive power dissipation that may damage the part. If rapid voltage steps are expected on the input supply, put a small silicon or Schottky diode in series with the V IN pin to prevent reverse current and inductor saturation, shown below as D1 in Figure 4. The diode should be sized for a reverse voltage of greater than the regulated output volt- age, and to withstand repetitive currents higher than the maximum peak current of the LTC7138.

Figure 4. Preventing Current Flow to the Input

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Figure 6. Setting the Output Voltage with External Resistors IN large enough to damage the part. IN to dampen the ringing of the input supply. ditional information on suppressing input supply transients. or electrolytic capacitor at the output. voltage options, directly connect the VFB pin to VOUT. noise pick-up on the sensitive VFB trace.

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Figure 5. Series RC to Reduce VIN Ringing current is much greater than the leakage.

shown in Figure 9 to meet specific VIN voltage requirements.

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Figure 7. Setting the Output Voltage with

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Figure 8. RUN Pin Interface to Logic Figure 9. Adjustable UV and OV Lockout

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on the allowable DC current that can be drawn from VIN. UVLO using the previous equations with R5 = 0Ω. figurations for driving the RUN pin from logic.

7138f For more information www.linear .com/L TC7138 applicaTions inForMaTion Similarly, for applications that do not require a precise UVLO, the RUN pin can be tied to VIN. In this configuration, the UVLO threshold is limited to the internal V IN UVLO thresholds as shown in the Electrical Characteristics table. The resistor values for the OVLO can be computed using the previous equations with R3 = 0Ω. Be aware that the OVLO pin cannot be allowed to exceed its absolute maximum rating of 6V . To keep the voltage on the OVLO pin from exceeding 6V , the following relation should be satisfied: VIN(MAX) • R5 R3+ R4+ R5    < 6V If this equation cannot be satisfied in the application, connect a 4.7V Zener diode between the OVLO pin and ground to clamp the OVLO pin voltage. Soft-Start Soft-start is implemented by ramping the effective refer- ence voltage from 0V to 0.8V . To increase the duration of the soft-start, place a capacitor from the SS pin to ground. An internal 5µA pull-up current will charge this capacitor . The value of the soft-start capacitor can be calculated by the following equation: CSS = Soft-Start Time • 5µA 0.8V The minimum soft-start time is limited to the internal soft-start timer of 1ms. When the L TC7138 detects a fault condition (input supply undervoltage/overvoltage or overtemperature) or when the RUN pin falls below 1.1V , the SS pin is quickly pulled to ground and the internal soft-start timer is reset. This ensures an orderly restart when using an external soft-start capacitor . Note that the soft-start capacitor may not be the limiting factor in the output voltage ramp. The maximum output current, which is equal to half of the peak current, must charge the output capacitor from 0V to its regulated value. For small peak currents or large output capacitors, this ramp time can be significant. Therefore, the output voltage ramp time from 0V to the regulated V OUT value is limited to a minimum of Ramp Time ≥ 1.33•COUT IPEAK VOUT Optimizing Output Voltage Ripple After the peak current resistor and inductor have been selected to meet the load current and frequency require- ments, an optional capacitor , CISET can be added in parallel with RISET to reduce the output voltage ripple dependency on load current. At light loads the output voltage ripple will be a maximum. The peak inductor current is controlled by the voltage on the I SET pin. The current out of the I SET pin is 5µA while the L TC7138 is active and is reduced to 1µA during sleep mode. The I SET current will return to 5µA on the first switching cycle after sleep mode. Placing a parallel RC network to ground on the I SET pin filters the ISET voltage as the L TC7138 enters and exits sleep mode, which in turn will affect the output voltage ripple, efficiency, and load step transient performance. Higher Current Applications For applications that require more than 400mA, the L TC7138 provides a feedback comparator output pin (FBO) for driving additional L TC7138s. When the FBO pin of a master L TC7138 is connected to the V FB pin of one or more slave L TC7138s, the master controls the burst cycle of the slaves. Figure 10 shows an example of a 5V , 800mA regulator using two L TC7138s. The master is configured for a 5V fixed output with external soft-start and V IN UVLO/OVLO levels set by the RUN and OVLO pins. Since the slave is directly controlled by the master , its SS pin should be float- ing, RUN should be tied to V IN, and OVLO should be tied to ground. Furthermore, the slave should be configured for a 1.8V fixed output (V PRG1 = VPRG2 = SS) to set the

7138 F10

Figure 10. 5V , 800mA Regulator higher resistance MOSFET , which dissipates more power . output current that is equal to the peak current of the part. circuitry to trigger at 180°C and shut down the L TC7138. meet high voltage clearance and creepage requirements. the criteria described in the Inductor Selection section.

7138f For more information www.linear .com/L TC7138 applicaTions inForMaTion Design Example As a design example, consider using the L TC7138 in an application with the following specifications: VIN = 36V to 72V (48V nominal), VOUT = 12V , IOUT = 400mA, and that switching is enabled when VIN is between 30V and 90V . First, calculate the inductor value: L = 220µH• 90V 150V = 132µH Choose a 150µH inductor as a standard value. Next, verify that this meets the L MIN requirement at the maximum input voltage: LMIN = 90V •150ns Therefore, the minimum inductor requirement is satisfied and the 150μH inductor value may be used. Next, C IN and COUT are selected. For this design, CIN should be sized for a current rating of at least: IRMS = 400mA• 12V 36V • 36V 12V –1 ≅ 189mARMS The value of CIN is selected to keep the input from droop- ing less than 1V at low line: CIN > 150µH•0.61A2 2•36V •1V ≅ 0.76µF Since the capacitance of capacitors decreases with DC bias, a 1µF capacitor should be chosen. The catch diode should have a reverse voltage rating of greater than the overvoltage lockout setting of 90V . It should also be rated for an average forward current of at least: ID(AVG) = 400mA 90V –12V 90V = 347mA For margin, select a catch diode with a reverse breakdown of at least 100V and an average current of 400mA or higher . C OUT will be selected based on a value large enough to satisfy the output voltage ripple requirement. For a 1% output ripple (120mV), the value of the output capacitor can be calculated from: COUT ≥ 0.61A•2•10–6 120mV –12V 160 ≅ 27µF COUT also needs an ESR that will satisfy the output voltage ripple requirement. The required ESR can be calculated from: ESR < 120mV 0.61A ≅ 197mΩ A 33µF ceramic capacitor has significantly less ESR than 197mΩ. The output voltage can now be programmed by choosing the values of R1 and R2. Since the output volt- age is higher than 10V , the L TC7138 should be set for a 5V fixed output with an external divider to divide the 12V output down to 5V . R2 is chosen to be less than 200k to keep the output voltage variation to less than 1% due to the internal 5M resistor tolerance. Set R2 = 196k and calculate R1 as: R1= 12V – 5V 5V • 196kΩ 5MΩ( ) = 264kΩ Choose a standard value of 267k for R1.

choose R3 = 2.2M and scale R4 and R5 by 2.2M/2.4M. than the rising thresholds, or 27V and 81V respectively. complete schematic for this design example. plane is recommended to minimize ground impedance. the AC current into the internal power MOSFET .

1. Keep the switching node, SW , away from all sensitive

FB, and create increased output ripple.

7138 F11

Figure 11. 36V to 72V Input to 12V Output, 400mA Regulator

7138 F12

Figure 12. Example PCB Layout

7138 F13b

7138 TA04b

7138 F13

7138 F14

7138 TA04a

• 3•IPEAK MAXIMUM INPUT VOL TAGE = 140 –|VOUT| L1: TDK SLF12555-221MR72 D1: ST MICRO STTH102A Soft-Start Waveform Maximum Load Current vs Input Voltage

Figure 13. High Efficiency 400mA Regulator Figure 14. 3.3V/400mA Regulator with 75ms Soft-Start

7138fFor more information www.linear .com/L TC7138 L1 CURRENT 500mA/DIV VIN/VOUT 5V/DIV L2 CURRENT 500mA/DIV 1s/DIV 7138 TA05b VIN VOUT L1 CURRENT 500mA/DIV VIN 50V/DIV VOUT 10V/DIV L2 CURRENT 500mA/DIV 200ms/DIV 7138 TA05c TRANSIENT TO 140V 72V VIN INPUT VOLTAGE (V) EFFICIENCY (%) 100 150120

7138 TA03b

7138 TA03a

100µH VIN CIN 1µF 250V X7R COUT 4.7µF 50V X7R V OUT 25V LED 400mA VIN 32V TO 140V FBO VPRG2 ISETVDIM VPRG1 SS RUN L TC7138 GND 42.2k 27.4k 3.3V PWM L1: TDK SLF10145T-101M D1: TOSHIBA CRH01 M1: VISHAY SILICONIX Si2356DS V DIM = 0.1V TO 1V FOR 10:1 ANALOG DIMMING PWM = SQUARE WAVE FOR DIGITAL DIMMING 30V OVERVOL TAGE PROTECTION ON V OUT ANODE Low Dropout Startup and Shutdown Overvoltage Lockout Operation Efficiency vs Input Voltage Typical applicaTions

7138 TA05a

100µH VIN RUN CIN1 1µF 250V X7R CIN2 1µF 250V X7R COUT 47µF 16V X5R V OUT* 12V 800mA VIN 4V TO 90V UP TO 140V TRANSIENT SS FBOVPRG1 VPRG2 OVLO L TC7138 (MASTER) GND VFB ISET SW 100µH VIN RUN FBO SS VPRG2 VPRG1 OVLO ANODE L TC7138 (SLAVE) GND 13.7k 267k 196k L1/L2: WÜRTH 744 770 910 1 D1/D2: CENTRAL SEMI CMSH1-100M-L TN OUT = VIN FOR VIN < 12V ANODE 4V to 90V Input to 12V/800mA Output Regulator with Overvoltage Lockout

7138f For more information www.linear .com/L TC7138 Typical applicaTions 36V to 140V to 36V/400mA with 120mA Input Current Limit 5V to 140V Input to 5V/400mA Output with 20kHz Minimum Switching Frequency L1: TDK SLF12555T-101M1R1 D1: ROHM RF101L2S

7138 TA06a

100µH VIN RUN CIN 1µF 250V X7R COUT 4.7µF 50V X7R V OUT 36V 400mA* VIN 36V TO 140V ISET OVLO L TC7138 GND FBO 220k 35.7k 4.02k 470k V PRG1 VPRG2 INPUT CURRENT LIMIT= VOUT 2.5 • R2 R1+R2 • 1+5µA•R1 VIN  ≈ VOUT 2.5 • R2 R1+R2 *MAXIMUM LOAD CURRENT= VIN 36V •120mA≤400mA ANODE Maximum Load and Input Current vs Input Voltage Input Current vs Load Current Switching Frequency vs Load Current

7138 TA08a

150µH VIN RUN CIN 1µF 250V COUT 22µF VOUT 400mA VIN 5V TO 140V VPRG1VPRG2 OVLO L TC7138 GND ISET FBO SS 953k 6.8/uni03A9 2N7000 100k 175k OUT SET IN DIV L TC6994-1 GND L1: COIL TRONICS DR74-101-R D1: DIODES INC MURS120-13-F VIN INPUT VOLTAGE (V) MAXIMUM CURRENT (mA) 400 500 150140 130 120 110

7138 TA06b

0.1 100 1000101 LOAD CURRENT (mA) SWITCHING FREQUENCY (kHz) 100

7138 TA08b

0.01 0.1 WITHOUT FREQUENCY LIMIT WITH FREQUENCY LIMIT VIN = 48V 0.1 100 1000101 LOAD CURRENT (mA) INPUT CURRENT (mA) 100

7138 TA08c

0.01

0.1 WITHOUT FREQUENCY LIMIT

VIN = 48V

7138fFor more information www.linear .com/L TC7138 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However , no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. MSOP (MSE16(12)) 0213 REV D 0.53 ±0.152 (.021 ±.006) SEATING PLANE 0.18 (.007) 1.10 (.043) MAX 0.17 –/uni00A00.27 (.007 – .011) TYP 0.86 (.034) REF 0.50 (.0197) BSC 1.0 (.039) BSC 1.0 (.039) BSC 16 14 121110 1 3 5 6 7 8 1 8 NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 0.254 (.010) 0° – 6° TYP DETAIL “A” DETAIL “A” GAUGE PLANE 5.10 (.201) MIN 3.20 – 3.45 (.126 – .136) 0.889 ±0.127 (.035 ±.005) RECOMMENDED SOLDER PAD LAYOUT 0.305 ±0.038 (.0120 ±.0015) TYP 0.50 (.0197) BSC BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 2.845 ±0.102 (.112 ±.004) 4.039 ±0.102 (.159 ±.004) (NOTE 3) 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 (.065 ±.004) 0.1016 ±0.0508 (.004 ±.002) 3.00 ±0.102 (.118 ±.004) (NOTE 4) 0.280 ±0.076 (.011 ±.003) REF 4.90 ±0.152 (.193 ±.006) DETAIL “B” DETAIL “B” CORNER TAIL IS PART OF THE LEADFRAME FEATURE. FOR REFERENCE ONL Y NO MEASUREMENT PURPOSE

0.12 REF

0.35 REF Variation: MSE16 (12) 16-Lead Plastic MSOP with 4 Pins Removed Exposed Die Pad (Reference LTC DWG # 05-08-1871 Rev D) package DescripTion Please refer to http://www.linear .com/designtools/packaging/ for the most recent package drawings.

7138f For more information www.linear .com/L TC7138  LINEAR TECHNOLOGY CORPORATION 2015 LT 0115 • PRINTED IN USA Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com/L TC7138 relaTeD parTs Typical applicaTion 12V/400mA Automotive Supply *VOUT ≅ VIN FOR VIN < 12V VOUT 12V* 400mA

7138 TA07

220µH VIN RUN CIN 1µF 250V X7R COUT 22µF 16V X7R V IN 4V TO 140V SS OVLOVPRG1 VPRG2 L TC7138 GND FBO 267k 196k L1: COILCRAFT MSS1246T-224KL D1: DIODES INC SBR1U200P1-7 PART NUMBER DESCRIPTION COMMENTS L TC3638 140V , 250mA Micropower Step-Down DC/DC Regulator V IN: 4V to 140V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD = 1.4µA, LTC3639 150V , 100mA Synchronous Micropower Step-Down DC/DC Regulator VIN: 4V to 150V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD = 1.4µA, LTC3637 76V , 1A High Efficiency Step-Down DC/DC Regulator V IN: 4V to 76V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD = 3µA, 3mm × 5mm DFN16, MSOP16E Packages LTC3630A 76V , 500mA Synchronous Step-Down DC/DC Regulator V IN: 4V to 76V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD = 5µA, 3mm × 5mm DFN16, MSOP16E Packages LTC3810 100V Synchronous Step-Down DC/DC Controller VIN: 6.4V to 100V , VOUT(MIN) = 0.8V , IQ = 2mA, ISD < 240µA, LTC3631/L TC3631-3.3 L TC3631-5 45V (T ransient to 60V), 100mA Synchronous Step-Down DC/DC Regulator VIN: 4.5V to 45V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD < 3µA, 3mm × 3mm DFN8, MSOP8 Packages LTC3642 45V (T ransient to 60V), 50mA Synchronous Step-Down DC/DC Regulator VIN: 4.5V to 45V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD < 3µA, 3mm × 3mm DFN8, MSOP8 Packages LTC3632 50V (T ransient to 60V), 20mA Synchronous Step-Down DC/DC Regulator VIN: 4.5V to 45V , VOUT(MIN) = 0.8V , IQ = 12µA, ISD < 3µA, 3mm × 3mm DFN8, MSOP8 Packages LTC3891 60V Synchronous Step-Down DC/DC Controller with Burst Mode Operation V IN: 4V to 60V , VOUT(MIN) = 0.8V , IQ = 50µA, ISD < 14µA, 3mm × 4mm QFN20, TSSOP20E Packages LTC4366-1/L TC4366-2 High Voltage Surge Stopper VIN: 9V to >500V , Adjustable Output Clamp Voltage, ISD < 14µA, 2mm × 3mm DFN8, TSOT-8 Packages Efficiency and Power Loss vs Load Current VIN = 24V VIN = 48V VIN = 120V LOAD CURRENT (mA) EFFICIENCY (%) POWER LOSS (mW) 100 100

7138 TA07b

0.1 100 1000101