B340LA

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Technical content

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current The L T®3975 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 42V . Low quiescent current design consumes only 2.7µA of supply current while regulating with no load. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 15mV in a typical application. The L T3975 can supply up to 2.5A of load current and has current limit foldback to limit power dissipation during short circuit. A low dropout voltage of 500mV is maintained when the input voltage drops below the programmed output voltage, such as during automotive cold crank. An internally compensated current mode topology is used for fast transient response and good loop stability. A high efficiency 75mΩ switch is included on the die along with a boost Schottky diode and the necessary oscillator , control, and logic circuitry. An accurate 1.02V threshold enable pin can be driven directly from a microcontroller or used as a programmable undervoltage lockout. A capacitor on the SS pin provides a controlled inrush current (soft-start). A power good flag signals when V OUT reaches 91.6% of the programmed output voltage. The L T3975 is available in a small 16-lead MSOP package with exposed pad for low thermal resistance. No-Load Supply Current 3.3V Step-Down Converter

APPLICATIONS

n Ultralow Quiescent Current: 2.7µA IQ at 12VIN to 3.3VOUT n Low Ripple Burst Mode® Operation Output Ripple < 15mVP-P n Wide Input Range: Operation from 4.3V to 42V n 2.5A Maximum Output Current n Excellent Start-Up and Dropout Performance n Adjustable Switching Frequency: 200kHz to 2MHz n Synchronizable Between 250kHz to 2MHz n Accurate Programmable Undervoltage Lockout n Low Shutdown Current: IQ = 700nA n Power Good Flag n Soft-Start Capability n Thermal Shutdown Protection n Current Limit Foldback with Soft-Start Override n Saturating Switch Design: 75mΩ On Resistance n Small, Thermally Enhanced 16-Lead MSOP Package n Automotive Battery Regulation n Portable Products n Industrial Supplies 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. VIN EN BOOSTOFF ON VIN 4.3V TO 42V PG 0.47µF PDS560 47µF 1210

3975 TA01a

10µF 576k f = 600kHz 78.7k VOUT 3.3V 2.5A 3.3µH L T3975 SS RT SW OUT FB SYNC GND 10pF INPUT VOLTAGE (V) 1.0 INPUT CURRENT (µA) 1.5 2.5 3.0 3.5 4.5 5 25 35

3975 TA01b

2.0 4.0 20 4510 15 30 IN REGULATION

Operating Junction Temperature Range (Note 2) (Note 1) FB SS OUT BOOST SW SW SW NC SYNC PG RT EN V IN VIN VIN NC TOP VIEW GND MSE PACKAGE 16-LEAD PLASTIC MSOP θJA = 40°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB PIN CONFIGURATION The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 2) ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L T3975EMSE#PBF L T3975EMSE#TRPBF 3975 16-Lead Plastic MSOP –40°C to 125°C L T3975IMSE#PBF L T3975IMSE#TRPBF 3975 16-Lead Plastic MSOP –40°C to 125°C L T3975HMSE#PBF L T3975HMSE#TRPBF 3975 16-Lead Plastic MSOP –40°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/ PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input Voltage (Note 3) l 4 4.3 V Dropout Comparator Threshold (VIN – OUT) Falling 430 500 570 mV Dropout Comparator Threshold Hysteresis 25 mV Quiescent Current from VIN VEN Low VEN High, VSYNC Low VEN High, VSYNC Low l 0.7 1.6 1.3 2.7 µA µA µA FB Pin Current V FB = 1.5V l 0.1 12 nA Feedback Voltage l 1.183 1.173 1.197 1.197 1.212 1.222 V V FB Voltage Line Regulation 4.3V < V IN < 40V (Note 3) 0.0003 0.01 %/V Switching Frequency RT = 11.8k RT = 41.2k RT = 294k 1.8 0.8 160 2.25 200 2.7 1.2 240 MHz MHz kHz Minimum Switch On-Time 105 ns Minimum Switch Off-Time (Note 4) 150 200 ns

ELECTRICAL CHARACTERISTICS

ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 2) 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 T3975E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. 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 T3975I is guaranteed over the full –40°C to 125°C operating junction temperature range. The L T3975H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. 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 (in °C/W) is the package thermal impedance. Note 3: Minimum input voltage depends on application circuit. Note 4: The L T3975 contains circuitry that extends the maximum duty cycle if there is sufficient voltage across the boost capacitor . See the Application Information section for more details. Note 5: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 6: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability or permanently damage the device. PARAMETER CONDITIONS MIN TYP MAX UNITS Switch Current Limit VFB = 1V 4 5.4 6.8 A Foldback Switch Current Limit VFB = 0V 3.3 A Switch VCESAT ISW = 1A 80 mV Switch Leakage Current 0.02 1 μA Boost Schottky Forward Voltage ISH = 100mA 730 mV Boost Schottky Reverse Leakage VREVERSE = 12V 0.02 2 μA Minimum Boost Voltage (Note 5) l 1.3 1.8 V BOOST Pin Current ISW = 1A, VBOOST – VSW = 3V 20 32 mA EN Voltage Threshold EN Falling, VIN ≥ 4.3V l 0.92 1.02 1.12 V EN Voltage Hysteresis 60 mV EN Pin Current 0.2 20 nA PG Threshold Offset from VFB VFB Falling 5 8.4 13 % PG Hysteresis as % of Output Voltage 1.7 % PG Leakage VPG = 3V 0.02 1 µA PG Sink Current VPG = 0.4V l 125 480 μA SYNC Low Threshold 0.6 1.0 V SYNC High Threshold 1.18 1.5 V SYNC Pin Current VSYNC = 6V 0.1 nA SS Source Current VSS = 0.5V 0.9 1.8 2.6 μA

TYPICAL PERFORMANCE CHARACTERISTICS Efficiency at 3.3VOUT No-Load Supply Current No-Load Supply Current Reference Voltage Load Regulation Line Regulation Efficiency at 5VOUT Efficiency at 3.3VOUT Efficiency at 5VOUT TA = 25°C, unless otherwise noted. LOAD CURRENT (A) EFFICIENCY (%) 100

3975 G01

0.5 1 1.5 2.5 12V 24V 36V fSW = 800kHz VOUT = 5V L = MSS1260-332NL LOAD CURRENT (A) EFFICIENCY (%)

3975 G02

0.5 1 1.5 2.5 12V 24V 36V FRONT PAGE APPLICATION V OUT = 3.3V L = MSS1260-332NL LOAD CURRENT (mA) 0.01 EFFICIENCY (%) 0.1 1 10 100

3975 G03

fSW = 800kHz VOUT = 5V L = MSS1260-332NL LOAD CURRENT (mA) 0.01 EFFICIENCY (%) 0.1 1 10 100 V OUT = 3.3V L = MSS1260-332NL INPUT VOLTAGE (V) 1.0 INPUT CURRENT (µA) 1.5 2.5 3.0 3.5 4.5 5 25 35

3975 G05

2.0 4.0 20 4510 15 30 IN REGULATION V OUT = 3.3V TEMPERATURE (°C) INPUT CURRENT (µA) 1000 10000 –55 65 125

3975 G06

5 –25 95 15535 100 FRONT PAGE APPLICATION V IN = 12V VOUT = 3.3V DUE TO CATCH DIODE LEAKAGE TEMPERATURE (°C) –55 REFERENCE VOLTAGE (V) 1.190 1.220 1.225 1.230 5 65 95

3975 G07

1.180 1.175 1.210 1.200 1.185 1.215 1.170 1.205 1.195 –25 35 125 155 INPUT VOLTAGE (V) CHANGE IN VOUT (%) –0.05 0.05 1.5 2.5

3975 G08

–0.10 –0.15 –0.20 0.5 1 2 0.10 0.15 0.20 VIN = 12V VOUT = 5V INPUT VOLTAGE (V) –0.05 CHANGE IN VOUT (%) –0.04 –0.02 –0.01 0.05 0.02 15 25 30 45

2975 G09

–0.03 0.03 0.04 0.01 10 20 35 40 VOUT = 5V LOAD = 1A

TYPICAL PERFORMANCE CHARACTERISTICS Current Limit Foldback Soft-Start Switch VCESAT BOOST Pin Current Minimum On-Time Minimum Off-Time Thermal Derating Switch Current Limit Switch Current Limit TA = 25°C, unless otherwise noted. SWITCH CURRENT (A) VCESAT (mV) 100 150 200 250 0.5 1 1.5 2.0

3975 G15

2.5 3.0 SWITCH CURRENT (A) BOOST PIN CURRENT (mA)10 0.5 1 1.5 2

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2.5 TEMPERATURE (°C) –50 LOAD CURRENT (A) 0.5 1.0 1.5 2.0 0 50 100 150

3975 G10

2.5 3.0 –25 25 75 125 I-GRADE H-GRADE VIN = 12V VOUT = 5V LIMITED BY MAXIMUM JUNCTION TEMPERATURE θJA = 40°C/W DUTY CYCLE 0.8

3975 G11

0.2 0.4 0.6 1 CURRENT LIMIT (A) TEMPERATURE (°C) –55 CURRENT LIMIT (A) 6.5

3975 G12

5.0 4.0 –25 5 65 3.5 3.0 7.0 6.0 5.5 4.5 95 125 155 30% DUTY CYCLE FB PIN VOLTAGE (V) CURRENT LIMIT (A) 0.2 0.4 0.6 0.8

3975 G13

1.0 1.2 30% DUTY CYCLE VSS = 3V SS PIN VOLTAGE (V) CURRENT LIMIT (A) 0.5 1 1.5 2

3975 G14

2.5 VFB = 1V VFB = 0V 30% DUTY CYCLE TEMPERATURE (°C) –55 MINIMUM ON-TIME (ns) 100 160 170 180 5 65 95

3975 G17

–25 35 125 155 VSYNC = 0V fSW = 2MHz LOAD = 1A LOAD = 2.5A TEMPERATURE (°C) –55

100 MINIMUM OFF-TIME (ns)

3975 G18

–25 35 155 VSYNC = 0V fSW = 2MHz LOAD = 1A LOAD = 2.5A

TYPICAL PERFORMANCE CHARACTERISTICS Internal Undervoltage Lockout (UVLO) EN Thresholds PG Thresholds Minimum Input Voltage, V OUT = 5V Minimum Input Voltage, V OUT = 3.3V Burst Frequency Switching Frequency RT Programmed Switching Frequency Frequency Foldback TA = 25°C, unless otherwise noted. TEMPERATURE (°C) –55 SWITCHING FREQUENCY (kHz) 660 720 780 35 95

3975 G19

–25 5 65 125 155 480 420 SWITCHING FREQUENCY (MHz) 0.2 RT RESISTOR (k/uni03A9) 150 200 250 350 0.4 1.2 1.6

3975 G20

FB PIN VOLTAGE (V) 700 600 500 400 300 200 100 0.6 1

3975 G21

0.2 0.4 0.8 1.2 SWITCHING FREQUENCY (kHz) TEMPERATURE (°C) –55 INPUT VOLTAGE (V) 35 95

3975 G22

–25 5 65 125 155 TEMPERATURE (°C) –55 EN THRESHOLD (V) 1.08

3975 G23

1.05 1.03 –25 5 65 1.02 1.01 1.09 1.07 1.06 1.04 95 125 155 EN RISING EN FALLING TEMPERATURE (°C) –55 PG THRESHOLD (V) 1.11

3975 G24

1.08 1.06 –25 5 65 1.05 1.04 1.12 1.10 1.09 1.07 95 125 155 FB RISING FB FALLING LOAD CURRENT (A) INPUT VOLTAGE (V) 5.5 6.0 6.5

3975 G25

5.0 4.5 4.0 0.5 1 1.5 2.5 VOUT = 5V fSW = 800kHz TO RUN/TO START LOAD CURRENT (A) INPUT VOLTAGE (V) 4.0 4.5 5.0

3975 G26

3.5 3.0 2.5 0.5 1 1.5 2.5 VOUT = 3.3V FRONT PAGE APPLICATION TO RUN/TO START LOAD CURRENT (mA) SWITCHING FREQUENCY (kHz) 500 600 700 160

3975 G27

VOUT = 5V fSW = 800kHz VOUT = 3.3V fSW = 600kHz

TYPICAL PERFORMANCE CHARACTERISTICS Start-Up/Dropout Performance Start-Up/Dropout Performance Boost Capacitor Charger Boost Diode Forward Voltage Dropout Comparator Thresholds Burst Mode Switching Waveforms Dropout Switching Waveforms Full Frequency Switching Waveforms TA = 25°C, unless otherwise noted. TEMPERATURE (°C) –55 SS PIN CURRENT (µA) 2.4

3975 G28

1.8 1.4 –25 5 65 1.2 1.0 2.6 2.2 2.0 1.6 95 125 155 VSS = 0.5V OUT PIN VOLTAGE (V) OUT PIN CURRENT (mA) 120

3975 G29

VBST = VIN BOOST DIODE CURRENT (A) BOOST DIODE VOLTAGE (V) 0.8 1.0 1.2

3975 G30

0.6 0.4 0.5 1 1.5 0.2 1.6 1.4 TEMPERATURE (°C) –55

400 DROPOUT THRESHOLD (mV)

3975 G31

–25 35 155 VOUT RISING VOUT FALLING VIN 1V/DIV VOUT 1V/DIV VOUT VIN 100ms/DIV2.5/uni03A9 LOAD (2A IN REGULATION)

3975 G32

(5mA IN REGULATION)

3975 G33

0.5A/DIV 5µs/DIVVIN = 12V VOUT = 3.3V ILOAD = 20mA COUT = 47µF

3975 G34

1µs/DIVVIN = 12V VOUT = 3.3V ILOAD = 1A COUT = 47µF

3975 G35

5µs/DIVVIN = 5V VOUT SET FOR 5V ILOAD = 0.5A COUT = 47µF

3975 G36

FB (Pin 1): The L T3975 regulates the FB pin to 1.197V . Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and the output. Typically, this capacitor is 10pF . SS (Pin 2): A capacitor is tied between SS and ground to slowly ramp up the peak current limit of the L T3975 on start-up. There is an internal 1.8μA pull-up on this pin. The soft-start capacitor is actively discharged when the EN pin goes low, during undervoltage lockout or thermal shutdown. Float this pin to disable soft-start. OUT (Pin 3): This pin is an input to the dropout comparator which maintains a minimum dropout of 500mV between V IN and OUT . The OUT pin connects to the anode of the internal boost diode. This pin also supplies the current to the L T3975’s internal regulator when OUT is above 3.2V . Connect this pin to the output when the programmed output voltage is less than 16V . BOOST (Pin 4): This pin is used to provide a drive volt - age, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pins 5, 6, 7): The SW pin is the output of an internal power switch. Connect these pins to the inductor , catch diode, and boost capacitor . NC (Pins 8, 9): No Connects. These pins are not connected to internal circuitry. V IN (Pins 10, 11, 12): The VIN pin supplies current to the L T3975’s internal circuitry and to the internal power switch. These pins must be locally bypassed. EN (Pin 13): The part is in shutdown when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.08V going up and 1.02V going down. The EN threshold is only accurate when V IN is above 4.3V . If VIN is lower than 3.9V , internal UVLO will place the part in shutdown. Tie to VIN if shutdown feature is not used. RT (Pin 14): A resistor is tied between RT and ground to set the switching frequency. PG (Pin 15): The PG pin is the open-drain output of an internal comparator . PGOOD remains low until the FB pin is within 8.4% of the final regulation voltage. PGOOD is valid when V IN is above 2V . SYNC (Pin 16): This is the external clock synchronization input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchroni - zation, which will include pulse skipping at low output loads. When in pulse-skipping mode, quiescent current increases to 11µA in a typical application at no load. Do not float this pin. GND (Exposed Pad Pin 17): Ground. The exposed pad must be soldered to the PCB. TYPICAL PERFORMANCE CHARACTERISTICSTA = 25°C, unless otherwise noted. Load T ransient: 0.5A to 2.5A Load T ransient: 20mA to 2A VOUT 200mV/DIV IL 1A/DIV 20µs/DIV12VIN 3.3VOUT COUT = 47µF

3975 G37

20µs/DIV 3975 G38 12VIN 3.3VOUT COUT = 47µF

R VINVIN EN BOOST 0.5V SW SHDN SWITCH LATCH SS 1.8µA VOUT OPT OUT RT GND ERROR AMP FB RT PG 1.097V 1.02V S Q 3975 BD INTERNAL 1.197V REF SYNC – SHDN +

The L T3975 is a constant frequency, current mode step- down regulator . An oscillator , with frequency set by RT , sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the V IN and SW pins, turning the switch off when this current reaches a level determined by the voltage at V C (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the V C node. If the error ampli- fier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the V C pin provides current limit. The VC pin is also clamped by the voltage on the SS pin; soft-start is implemented by generating a voltage ramp at the SS pin using an external capacitor . An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the V IN pin, but if the OUT pin is connected to an external volt - age higher than 3.2V , bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. If the EN pin is low, the L T3975 is shut down and draws 700nA from the input. When the EN pin falls below 1.02V , the switching regulator will shut down, and when the EN pin rises above 1.08V , the switching regulator will become active. This accurate threshold allows programmable undervoltage lockout. The switch driver operates from either V IN or from the BOOST pin. An external capacitor is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the L T3975 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.7μA. In a typical application, 2.7μA will be consumed from the supply when regulating with no load. The oscillator reduces the L T3975’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during start- up and overload. The L T3975 can provide up to 2.5A of output current. A current limit foldback feature throttles back the cur - rent limit during overload conditions to limit the power dissipation. When SS is below 2V , the L T3975 overrides the current limit foldback circuit to avoid interfering with start-up. Thermal shutdown further protects the part from excessive power dissipation, especially in elevated ambient temperature environments. If the input voltage decreases towards the programmed output voltage, the L T3975 will start to skip switch-off times and decrease the switching frequency to maintain output regulation. As the input voltage decreases below the programmed output voltage, the output voltage will be regulated 500mV below the input voltage. This enforced minimum dropout voltage limits the duty cycle and keeps the boost capacitor charged during dropout conditions. Since sufficient boost voltage is maintained, the internal switch can fully saturate yielding low dropout performance. The L T3975 contains a power good comparator which trips when the FB pin is at 91.6% of its regulated value. The PG output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when V IN is above 2V . When the L T3975 is shut down the PG pin is actively pulled low.

be greater than 1.7μA when regulating. Figure 1. Switching Frequency in Burst Mode Operation a typical application. See Figure 2. Figure 2. Burst Mode Operation effects of output capacitor ESR and ESL.

3975 F01

3975 F02

minimum load to full frequency, so is not recommended. low supply current at light loads. where RT is in kΩ and fSW is in MHz. Table 1. Switching Frequency vs RT Value minimum on-time is a strong function of temperature.

A good choice of switching frequency should allow ad - equate input voltage range (see next two sections) and keep the inductor and capacitor values small. Maximum Input Voltage Range The L T3975 can operate from input voltages of up to 42V . Often the highest allowed V IN during normal operation (VIN(OP-MAX)) is limited by the minimum duty cycle rather than the absolute maximum ratings of the VIN pin. It can be calculated using the following equation: VIN(OP-MAX) = VOUT + VD fSW •tON(MIN) – VD + VSW where tON(MIN) is the minimum switch on-time. A lower switching frequency can be used to extend normal opera- tion to higher input voltages. The circuit will tolerate inputs above the maximum op - erating input voltage and up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency. However , during such transients where V IN is higher than VIN(OP-MAX), the L T3975 will enter pulse-skipping operation where some switching pulses are skipped to maintain output regulation. The output voltage ripple and inductor current ripple will be higher than in typical operation. Do not overload when V IN is greater than VIN(OP-MAX). Minimum Input Voltage Range The minimum input voltage is determined by either the L T3975’s minimum operating voltage of 4.3V , its maximum duty cycle, or the enforced minimum dropout voltage. See the Typical Performance Characteristics section for the minimum input voltage across load for outputs of 3.3V and 5V . The duty cycle is the fraction of time that the internal switch is on during a clock cycle. Unlike many fixed fre - quency regulators, the L T3975 can extend its duty cycle by remaining on for multiple clock cycles. The L T3975 will not switch off at the end of each clock cycle if there is sufficient voltage across the boost capacitor (C3 in the Block Diagram). Eventually, the voltage on the boost capacitor falls and requires refreshing. When this occurs, the switch will turn off, allowing the inductor current to recharge the boost capacitor . This places a limitation on the maximum duty cycle as follows: DCMAX = β SW β SW + 1 where βSW is equal to the beta of the internal power switch. The beta of the power switch is typically about 50, which leads to a DC MAX of about 98%. This leads to a minimum input voltage of approximately: VIN(MIN1) = VOUT + VD DCMAX – VD + VSW where VOUT is the output voltage, V D is the catch diode drop, VSW is the internal switch drop and DC MAX is the maximum duty cycle. The final factor affecting the minimum input voltage is the minimum dropout voltage. When the OUT pin is tied to the output, the L T3975 regulates the output such that it stays 500mV below V IN. This enforced minimum drop- out voltage is due to reasons that are covered in the next section. This places a limitation on the minimum input voltage as follows: VIN(MIN2) = VOUT + VDROPOUT(MIN) where V OUT is the programmed output voltage and VDROPOUT(MIN) is the minimum dropout voltage of 500mV . Combining these factors leads to the overall minimum input voltage: VIN(MIN) = Max (VIN(MIN1), VIN(MIN2), 4.3V) Minimum Dropout Voltage To achieve a low dropout voltage, the internal power switch must always be able to fully saturate. This means that the boost capacitor , which provides a base drive higher than V IN, must always be able to charge up when the part starts up and then must also stay charged during all operating conditions.

off, and the part may re-enter Burst Mode operation. measured dropout voltage, can be significantly reduced. and switching frequency will determine the ripple current. is the inductor value is μH. (>30V), the saturation current should be above 8.5A. Table 2. Inductor Vendors of the switch current limit (ILIM) and the ripple current. Figure 3. VIN to VOUT Performance

3975 F03

The L T3975 limits its peak switch current in order to protect itself and the system from overload and short-circuit faults. The L T3975’s switch current limit (I LIM) is typically 5.4A at low duty cycles and decreases linearly to 4.4A at DC = 0.8. When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor: ∆ IL = 1–DC( ) • VOUT + VD( ) L •fSW where fSW is the switching frequency of the L T3975, DC is the duty cycle and L is the value of the inductor . Therefore, the maximum output current that the L T3975 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. The inductor value may have to be increased if the inductor ripple current does not allow sufficient maximum output current (I OUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application. The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a higher maximum load current and reduces the output voltage ripple. If your load is lower than the maximum load current, than you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor , or one with a lower DCR resulting in higher efficiency. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on the input voltage. In addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. For details of maximum output current and discontinuous operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (V OUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations, see Application Note 19. One approach to choosing the inductor is to start with the simple rule given above, look at the available induc- tors, and choose one to meet cost or space goals. Then use the equations above to check that the L T3975 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinuous operation occurs when I OUT is less than ΔIL/2. Current Limit Foldback and Thermal Protection The L T3975 has a large peak current limit to ensure a 2.5A max output current across duty cycle and current limit distribution, as well as allowing a reasonable inductor ripple current. During a short-circuit fault, having a large current limit can lead to excessive power dissipation and temperature rise in the L T3975, as well as the inductor and catch diode. To limit this power dissipation, the L T3975 starts to fold back the current limit when the FB pin falls below 0.8V . The L T3975 typically lowers the peak current limit about 40% from 5.4A to 3.3A. During start-up, when the output voltage and FB pin are low, current limit foldback could hinder the L T3975’s ability to start up into a large load. To avoid this potential problem, the L T3975’s current limit foldback will be disabled until the SS pin has charged above 2V . Therefore, the use of a soft-start capacitor will keep the current limit foldback feature out of the way while the L T3975 is starting up. The L T3975 has thermal shutdown to further protect the part during periods of high power dissipation, particularly in high ambient temperature environments. The thermal shutdown feature detects when the L T3975 is too hot and shuts the part down, preventing switching. When the thermal event passes and the L T3975 cools, the part will restart and resume switching. A thermal shutdown event actively discharges the soft-start capacitor . Input Capacitor Bypass the input of the L T3975 circuit with a ceramic capaci- tor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7μF to 10μF ceramic capacitor is adequate to bypass the L T3975 and will easily handle the ripple cur- rent. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor .

switching current into a tight local loop, minimizing EMI. placed close to the L T3975 (see the PCB Layout section). concerns the maximum input voltage rating of the L T3975. Technology Application Note 88 for a complete discussion. provide low output ripple and good transient response. cost but transient performance will suffer . operating conditions (applied voltage and temperature). Table 3. Recommended Ceramic Capacitor Vendors Technology Application Note 88 for a complete discussion.

not the worst-case condition due to current limit foldback. Peak reverse voltage is equal to the regulator input voltage. For inputs up to 40V , a 40V diode is adequate. An additional consideration is reverse leakage current. Schottky diodes and their manufacturers. drop than the internal boost diode. Table 4. Schottky Diodes. The Reverse Current Values Listed For output voltages less than 2.5V , there are two options. the BOOST pin must not be exceeded.

Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current

3875 F04

Figure 4. Five Circuits for Generating the Boost Voltage

3975 F05a

3975 F05b

shows the minimum input voltage for 3.3V and 5V outputs.

T should be selected for 200kHz. by the inductor size, input voltage and output voltage. sufficient for all synchronization frequencies. to the user when the output voltage is within regulation. the PG pin will pull low to indicate the power is not good. the PG pin as the input voltage is increased. Figure 8. PG Pin Voltage Versus Input Voltage when PG

3975 F08

against a shorted or reversed input.

these layers will spread the heat dissipated by the L T3975. temperature approaches the maximum junction rating. corresponding increase in the input quiescent current. current at high temperatures.

3975 F09

Figure 9. Diode D4 Prevents a Shorted Input from Discharging Figure 10. Layout Showing a Good PCB Design

3975 F10

5V Step-Down Converter 4V Step-Down Converter with a High Impedance Input Source 12V Step-Down Converter 2.5V Step-Down Converter 5V , 2MHz Step-Down Converter with Power Good 1.8V Step-Down Converter VIN EN BOOSTOFF ON VIN 5.7V TO 42V PG 0.47µF PDS360 47µF 1210

3975 TA02

10µF 316k f = 800kHz L = IHLP-2020CZ-01 54.9k VOUT 2.5A 4.7µH L T3975 SS RT SW OUT FB SYNC GND 10pF VIN EN BOOSTOFF ON VIN 12.9V TO 42V PG 0.47µF PDS360 22µF 1210

3975 TA03

10µF 110k f = 800kHz L = IHLP-3232CZ-01 54.9k VOUT 12V 2.5A 10µH L T3975 SS RT SW OUT FB SYNC GND 10pF VIN EN BOOST OFF ON VIN 5.9V TO 18V (42V TRANSIENTS) 0.47µF PDS360 22µF 1210

3975 TA04

4.7µF 316k 150k f = 2MHz L = IHLP-2525CZ-01 14.7k VOUT 2.5A PGOOD 2.2µH L T3975 SS RT SW OUT PG FB SYNC GND 10pF VIN PG BOOST EN 0.47µF PDS360 47µF 1210

3975 TA05

100µF 24V 10µF 432k f = 800kHz L = IHLP-2525EZ-01 54.9k 499k 5.49M VOUT 2.5A 4.7µH L T3975 SS RT SW OUT FB SYNC GND 10pF V VIN EN BOOST OFF ON VIN 4.3V TO 42V PG 0.47µF PDS360 100µF 1210

3975 TA06

10µF 909k f = 400kHz L = IHLP-2525EZ-01 130k VOUT 2.5V 2.5A 6.8µH DFLS160 L T3975 SS RT SW OUT FB SYNC GND 10pF VIN EN BOOSTOFF ON VIN 4.3V TO 27V PG 0.47µF PDS360 100µF 1210

3975 TA07

10µF f = 500kHz L = IHLP-2020CZ-01 97.6k VOUT 1.8V 2.5A 3.3µH DFLS160 L T3975 SS RT SW OUT FB SYNC GND 499k 10pF

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. PACKAGE DESCRIPTION Please refer to http://www.linear .com/designtools/packaging/ for the most recent package drawings. MSOP (MSE16) 0911 REV E 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 16151413121110 1 2 3 4 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 5.23 (.206) 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 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E)

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com  LINEAR TECHNOLOGY CORPORA TION 2012 LT 0812 • PRINTED IN USA RELATED PARTS TYPICAL APPLICATION 1.2V Step-Down Converter VIN EN BOOSTOFF ON VIN 4.3V TO 27V (42V TRANSIENT) PG 0.47µF PDS360 3.3V 100µF 1210

3975 TA08

10µF f = 400kHz L = IHLP-2525EZ-01 130k VOUT 1.2V 2.5A 4.7µH DFLS160 L T3975 SS RT SW OUT FB SYNC GND PART NUMBER DESCRIPTION COMMENTS L T3480 36V with T ransient Protection to 60V , 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN = 3.6V to 38V , T ransients to 60V , VOUT(MIN) = 0.78V , IQ = 70µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOP-10E L T3980 58V with T ransient Protection to 80V , 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN = 3.6V to 58V , T ransients to 80V , VOUT(MIN) = 0.79V , IQ = 75µA, ISD < 1µA, 3mm × 4mm DFN-16, MSOP-16E L T3971 38V , 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.8µA of Quiescent Current VIN = 4.2V to 38V , VOUT(MIN) = 1.2V , IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOP-10E L T3991 55V , 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.8µA of Quiescent Current VIN = 4.2V to 55V , VOUT(MIN) = 1.2V , IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOP-10E L T3970 40V , 350mA (IOUT), 2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.5µA of Quiescent Current VIN = 4.2V to 40V , VOUT(MIN) = 1.2V , IQ = 2.5µA, ISD < 0.7µA, 2mm × 3mm DFN-10, MSOP-10E L T3990 62V , 350mA (IOUT), 2.2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.5µA of Quiescent Current VIN = 4.2V to 62V , VOUT(MIN) = 1.2V , IQ = 2.5µA, ISD < 0.7µA, 3mm × 3mm DFN-10, MSOP-16E