LT3682

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LOAD CURRENT (mA) 0 100 EFFICIENCY (%) 200 300

3645 TA01b

VOUT = 3.3V VIN = 12V VOUT = 5V TYPICAL APPLICATION FEATURES DESCRIPTION 36V 500mA Step-Down Regulator and 200mA LDO The LT®3645 is a dual output regulator combining a 500mA buck regulator and a 200mA low dropout linear regula- tor (LDO). The wide input voltage range of 3.6V to 36V makes the LT3645 suitable for regulating power from a wide variety of sources, including 24V industrial supplies and automotive batteries. Its high operating frequency allows the use of tiny, low cost inductors and capacitors, resulting in a very small solution. Cycle-by-cycle current limit and frequency foldback pro- vide protection against shorted outputs. Soft-start and frequency foldback eliminate input current surge during start-up. The linear regulator operates from the V CC2 pin at voltages down to 1.2V. It supplies 200mA of output current with a typical dropout voltage of 310mV. Other features of the LT3645 include a <2μA shutdown, short circuit protection, soft-start and thermal shutdown. The LT3645 is available in the thermally enhanced 16-lead (3mm × 3mm) QFN package, or a 12-lead MSE package. 3.3V/5V Step-Down Converter

APPLICATIONS

n Automotive CMOS Image Sensors n Industrial/Automotive Micro-Controller Supply L, L T , L TC, L TM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. n Wide Input Range: Operation from 3.6V to 36V Overvoltage Lockout Protects Circuit Through 55V Transients on Input n 500mA Output Current Switching Regulator n High Switching Frequency: 750kHz n 200mA Low Dropout Linear Regulator 1.2V to 16V Input; 0.8V to 8V Output 310mV Dropout Voltage V CC2 to OUT2 n Precision Programmable Undervoltage Lockout n Short-Circuit Robust n Internal Soft-Start n <2μA Shutdown Current n Small Thermally Enhanced 16-Lead (3mm × 3mm) QFN and 12-Lead MSE Packages Buck Regulator Effi ciency 1μF

3645 TA01a

6.2V TO 36V GND SW DA FB FB2 EN2 VCC2 VIN EN/UVLO NPG OUT2 10μF 300mA 3.3V 200mA 15μH 52.3k 31.6k 10k 10k BOOST L T3645 2.2μF 0.1μF ON OFF

V (Note 1) 16 15 14 13 5 6 7 8 TOP VIEW UD PACKAGE 16-LEAD (3mm × 3mm) PLASTIC QFN 1NC NC NPG EN2 NC VIN BOOST SW FB2 OUT2 V CC2 NC EN/UVLO FB GND DA GND θJA = 58.7°C/W , θJC = 7.1°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB EN/UVLO FB GND DA BOOST SW NPG EN2 FB2 OUT2 V CC2 VIN TOP VIEW MSE PACKAGE 12-LEAD PLASTIC MSOP GND θJA = 40°C/W , θJC = 5°C/W TO 10°C/W EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L T3645EUD#PBF L T3645EUD#TRPBF LFVS 16-Lead Plastic QFN –40°C to 125°C L T3645IUD#PBF L T3645IUD#TRPBF LFVS 16-Lead Plastic QFN –40°C to 125°C L T3645EMSE#PBF L T3645EMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 125°C L T3645IMSE#PBF L T3645IMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 125°C L T3645HMSE#PBF L T3645HMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 150°C Consult L TC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container . Consult L TC Marketing for information on non-standard lead based fi nish parts. For more information on lead free part marking, go to: http://www.linear .com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear .com/tapeandreel/ Operating Junction Temperature Range (Note 2)

ELECTRICAL CHARACTERISTICS

PARAMETER CONDITIONS MIN TYP MAX UNITS Undervoltage Lockout on VIN Rising l 3 3.4 3.6 V Overvoltage Lockout on VIN Rising l 36 38.5 40 V Overvoltage Lockout Hysteresis 1V Feedback Voltage FB l 0.79 0.785 0.8 0.8 0.81 0.813 V FB Pin Bias Current l 20 300 nA Feedback Voltage Line Regulation l 0.015 %/V VIN Quiescent Current Not Switching 1.4 3 mA VIN Quiescent Current in Shutdown V EN/UVLO = 0.3V , VCC2 = 0V , VOUT2 = 0V 0.01 2 μA Switching Frequency 675 750 825 kHz Maximum Duty Cycle 100mA Load l 83 87 % Switch Current Limit Rising (Note 4) 0.8 1 1.25 A DA Pin Current to Stop Osc 0.6 1 1.25 A Switch VCESAT ISW = 500mA 400 mV Switch Leakage Current 2μ A Minimum Boost Voltage Above Switch I SW = 500mA 1.6 2.2 V BOOST Pin Current I SW = 500mA 10 18 mA BOOST Schottky Forward Drop I OUT = 50mA 0.7 0.9 V EN/UVLO Threshold High Rising 1.17 1.23 1.29 V EN/UVLO Threshold Hysteresis 50 mV EN/UVLO Input Current V EN/UVLO = 5V VEN/UVLO = 0V 25 50 μA μA Buck Soft-Start Time 0.9 1.8 ms LDO Minimum Input Voltage VCC2 ILOAD = 200mA, VOUT2 = 0.8V , VIN = 4.0V 1.1 1.38 V LDO Feedback Voltage FB2 l 782 797 810 mV LDO FB2 Bias Current l 20 300 nA LDO Line Regulation 0.020 %/V LDO Load Regulation –1 mV LDO Dropout Voltage (VCC2 to VOUT2)I LOAD = 10mA ILOAD = 10mA ILOAD = 200mA l 310 145 mV mV mV LDO Dropout Voltage (V IN to VOUT2)I LOAD = 200mA ILOAD = 200mA l 1.1 1.4 1.7 V V LDO Current Limit l 210 270 mA mA EN2 Pin Threshold Rising Falling l l 0.5 1.3 0.8 1.6 V V LDO Soft-Start Time 0.6 1.2 ms NPG VCESAT INPG = 1mA, VFB = VFB2 = 850mV 0.4 V NPG Leakage V NPG = 16V , VFB = VFB2 = 750mV 0.5 μA FB2 NPG Threshold, % of Regulation Voltage V FB = 800mV , VFB2 Rising 88 90 92 % The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless otherwise noted. (Notes 2, 3)

Buck Minimum Input Voltage, VOUT = 3.3V Effi ciency VOUT = 5V Effi ciency V OUT = 3.3V Buck Minimum Input Voltage, VOUT = 5V PARAMETER CONDITIONS MIN TYP MAX UNITS FB NPG Threshold, % of Regulation Voltage V FB2 = 800mV , VFB Rising 88 90 92 % NPG Threshold Hysteresis 25 mV ELECTRICAL CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless otherwise noted. (Notes 2, 3) Note1: 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 T3645E is guaranteed to meet performance specifi cations from 0°C to 125°C. Specifi cations over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The L T3645I is guaranteed over the full –40°C to 125°C operating temperature range. The L T3645H is guaranteed over the full –40°C to 150°C operating temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. Note 3: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum junction operating temperature when overtemperature protection is active. Continuous operation above the specifi ed maximum operating junction temperature may result in device degradation or failure. Note 4: Current Measurements are performed when the outputs are not switching. Slope compensation reduces current limit at high duty cycles. Note 5: Absolute Maximum Voltage at V IN and EN/UVLO pins is 55V for nonrepetitive one second transients, and 36V for continuous operation. FB2 Voltage TYPICAL PERFORMANCE CHARACTERISTICS OUTPUT CURRENT (mA) EFFICIENCY (%) 100 200 300

3645 G01

VIN = 7V VIN = 12V VIN = 24V LOAD CURRENT (mA) EFFICIENCY (%) 100 200 300

3645 G02

VIN = 7V VIN = 12V VIN = 24V OUTPUT CURRENT (mA) 5.0 INPUT VOLTAGE (V) 5.5 6.5 6.0 7.5 7.0 8.0 10 100

3645 G03

TEMPERATURE (°C) –50 –30 –10

794 FB VOLTAGE (mV)

3645 G05

TEMPERATURE (°C) –50 0 794 FB VOLTAGE (mV) 795 797 796 798 799 800

3645 G06

OUTPUT CURRENT (mA) 3.0 INPUT VOLTAGE (V)4.0 3.5 5.0 4.5 6.0 5.5 6.5 10 100

3645 G04

TYPICAL PERFORMANCE CHARACTERISTICS Buck Power Switch Voltage Drop Buck Power Switch Current Limit Undervoltage Lockout Overvoltage Lockout Switching Frequency TEMPERATURE (°C) –50 0

600 CURRENT LIMIT (mA)

3645 G08

TEMPERATURE (°C) –50 –30 –10 3.2 RISING THRESHOLD (V) 3.3 3.5 3.4 3.6 3.7 3.8 10 30 50 70 90 130

3645 G09

TEMPERATURE (°C) –50 0

36.5 THRESHOLD (V)

37.0 38.0 37.5 38.5 39.0 39.5

3645 G10

TEMPERATURE (°C) –50 0 700 SWITCHING FREQUENCY (kHz) 750 740 730 720 710 770 760 780 790 800

3645 G11

TEMPERATURE (°C) –50 0

200 CURRENT LIMIT (mA)

3645 G12

TEMPERATURE (°C) –60 –40 –20 20 40 600 DROPOUT VOLTAGE (mV) 150 100 250 200 300 350

3645 G14

OUTPUT CURRENT (mA) 0 50 100 150 OUTPUT VOLTAGE CHANGE (%) –0.12 –0.16 –0.14 –0.06 –0.08 –0.10 –0.04 –0.02

3645 G15

LDO Power Transistor Current Limit LDO Dropout Voltage to V CC2 LDO Dropout Voltage LDO Load Regulation LOAD CURRENT (mA) 05 0 DROPOUT VOLTAGE (mV) 150 100 250 200 300 350 100

3645 G13

OUT2 = 0.8V OUT2 = 3.3V SWITCH CURRENT (mA) 0 50 100 150 200 250 300 350 400 450 BUCK POWER SWITCH VCE (mV) 200 150 100 250 300 350 400

3645 G07

TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Pin Current EN2 Threshold Voltage NPG Threshold Voltage, FB = 0.8V EN/UVLO PIN Threshold Voltage TEMPERATURE (°C) –50 0 50 100 EN/UVLO PIN THRESHOLD VOLTAGE (V) 1.21 1.18 1.20 1.19 1.22 1.23 1.24 1.25 1.26

3645 G17

TEMPERATURE (°C) –50 0

650 FB2 VOLTAGE (mV)

3645 G19

CURRENT INTO EN/UVLO PIN (μA) 100 120

3645 G16

TEMPERATURE (°C) –50 0 0.5 EN2 THRESHOLD VOLTAGE (V) 0.9 0.8 0.7 0.6 1.1 1.0 1.2 1.3 1.4 1.5

3645 G18

EN/UVLO (Pin 1/Pin 5): The EN/UVLO pin is used to enable the buck switching regulator and the low dropout linear regulator (LDO). An accurate threshold of 1.23V allows the user to set the undervoltage lockout point with a simple resistor divider, see Precision Undervoltage Lockout sec- tion for more information. The EN/UVLO pin can be tied directly to V IN if the UVLO or shutdown is not used. FB (Pin 2/Pin 6): The FB pin programs the buck output voltage. The LT3645 regulates the FB pin to 0.8V. The feedback resistor divider tap should be connected to this pin. The output voltage is programmed according to the following equation: R1= R2 • VOUT 0.8 –1⎛ ⎝⎜ ⎞ where R1 connects between OUT and FB and R2 connects between FB and GND. A good value for R2 is 10k. GND (Pin 3, Exposed Pad Pin 13/Pin 7, Exposed Pad Pin 17): The GND pin should be tied to a local ground plane below the LT3645 and the circuit components. Return the feedback dividers from FB and FB2 to this pin. The exposed pad must be soldered to the PCB and electrically connected to ground. Use a large ground plane and thermal vias to optimize thermal performance. DA (Pin 4/Pin 8): The DA pin senses the external catch diode current and prevents the buck regulator from switch- ing if the sensed current is too high. Connect the anode of the external Schottky catch diode to this pin. BOOST (Pin 5/Pin 10): The BOOST pin provides a drive voltage to the internal bipolar NPN power switch. Tie a 0.1μF capacitor between the BOOST and SW pins. SW (Pin 6/Pin 9): The SW pin is the output of the internal buck power switch. Connect the inductor and the cathode of the external catch Schottky diode to this pin. V IN (Pin 7 / Pin 11): The VIN pin supplies current to the L T 3 6 4 5 ’ s i n t e r n a l c i r c u i t ry , t o t h e i n t e r n a l b u c k p o w e r switch, and to the LDO. The V IN pin must be locally bypassed. VCC2 (Pin 8/Pin 14): The VCC2 pin supplies current to the linear regulator’s output device. The VCC2 pin is also the anode of an internal Schottky diode used to generate the BOOST voltage. The V CC2 pin must be tied to a voltage source greater than 2.5V to utilize the internal Schottky boost diode. If the V CC2 pin is tied to a voltage lower than 2.5V, then an external Schottky diode must be connected between a power supply greater than 2.5V (anode) and the BOOST pin (cathode). Bypass this pin to ground with a 0.1μF capacitor close to the part. OUT2 (Pin 9/Pin 15): The OUT2 pin is the output of the LDO. Connect a capacitor of at least 0.47μF from this pin to ground. See Frequency Compensation (LDO) section for more details. FB2 (Pin 10/Pin 16): The FB2 pin programs the LDO output voltage. The LT3645 regulates the FB2 pin to 0.797V. The feedback resistor divider tap should be connected to this pin. The output voltage is programmed according to the following equation: R3 = R4 t VOUT2 0.797 –1⎛ ⎝⎜ ⎞ where R3 connects between OUT2 and FB2 and R4 connects between FB2 and GND. A good value for R4 is 10k. EN2 (Pin 11/Pin 4): The EN2 pin is used to enable the linear regulator. Pull this pin above 1.6V to enable the LDO. Pull EN2 below 0.5V to disable the LDO. NPG (Pin 12/Pin 3): The NPG pin is an open-collector output used to indicate that both buck and LDO output voltages are in regulation. The NPG pin pulls low when FB and FB2 both exceed 720mV. NC (Pins 1, 2, 12, 13, QFN Only): No Connect Pins. Tie these to ground. (MSOP/QFN)

1.23V 1.3V STARTBUCK STARTLDO ERRORAMP CURRENT COMPARATORVC BUCKDRIVER LOGIC –40mV BOOST 0.8V REFERENCE PGOOD EN2 NPG GND OSCILLATOR SOFT-START SQ R SW DA GND C4 R3 FB VCC2 VIN OUT2 LDO DRIVER ERRORAMP 0.797V 0.72V FB2 SOFT-START LDO ON ON OFF SLOPE COMPENSATION

The L T3645 includes a constant frequency, current mode step-down buck switching regulator together with a low- dropout regulator (LDO). If EN/UVLO is less than ~0.7V, both the buck and LDO are off, the output is disconnected and the input current is less than 2μA. The buck turns on when EN/UVLO is greater than 1.23V. An undervoltage lockout (UVLO) turns the buck and LDO off when V IN is less than 3.4V . An overvoltage lockout (OVLO) turns the buck and LDO off when V IN is greater than 38.5V. The par t will withstand nonrepetitive one second input voltage transients up to 55V. An internal thermal shutdown circuit monitors the die temperature and shuts both the buck and LDO off if the die temperature exceeds ~160°C. The thermal shutdown has 10 degrees of hysteresis. An internal regulator provides power to the control circuitry and produces the 0.8V feedback voltage for the buck and LDO error amplifi ers. An internal, fi xed-frequency oscillator in the step-down regulator enables an RS flip-flop, turning on the internal power switch Q1. A comparator monitors 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 and the internal slope-compensation. An error amplifier servos the V C node. The output of an external resistor divider between OUT and ground is tied to the V FB pin and presented to the negative error amp input. The positive input to the error amp is a 0.8V reference, so the voltage loop forces the V FB pin to 0.8V. The reference voltage of the buck error amplifi er is ramped over 900μs during the soft-start period. When V C rises, it results in an increase in output current, and when VC falls, it results in less output current. Current limit is provided by an active clamp on the V C node. The buck power switch (Q1) is driven from the BOOST pin. An external capacitor and internal diode are used to generate a voltage at the BOOST pin that is higher than the input supply, which allows the driver to fully saturate the internal bipolar NPN power switch for effi cient operation. An external diode can be used to make the BOOST drive more effective at low output voltages. The oscillator reduces the LT3645’s operating frequency during the soft-start period. This frequency foldback helps to control the output current during startup. The current in the external catch diode (D1) is sensed through the DA pin. If the catch diode current exceeds 0.9A, the oscillator frequency is decreased. This prevents current runaway during startup or overload. The LDO only operates if EN/UVLO is greater than 1.23V and EN2 is greater than 1.3V. If EN/UVLO is low and EN2 is high, the LDO will not start. When EN2 > 1.3V and EN/ UVLO > 1.23V, the LDO power transistor will turn on and regulate the output at the OUT2 pin. An error amplifi er driving Q2 has its positive input at the 0.797V reference. The output of an external resistor divider between OUT2 and ground is tied to the V FB2 pin and presented to the negative error amp input, forcing the VFB2 pin to 0.797V. The reference voltage of the LDO error amplifi er is ramped over 600μs during the soft-start period. The LDO power transistor (Q2) is driven from the V IN pin. Q2 is a bipolar NPN which draws its collector current from the VCC2 pin. The NPG pin is an open-collector output that indicates when both buck and LDO outputs are in at least 90% in regulation. When FB and FB2 rise above 720mV, the NPG pin is pulled low.

T h e o u t pu t v o l t a g e s a r e pr o gr a mm e d w i t h r e s i s t or di v i d er s between the outputs and the V FB and VFB2 pins. Choose the resistors according to R1"R2 t VOUT 0.8 –1© «ª ¹ R3 "R4 t VOUT2 0.797 –1© «ª ¹ R2 and R4 should be 20k or less to avoid bias current errors. In the step-down converter, an optional phase lead capacitor of 22pf between V OUT and V FB reduces light-load ripple. Input Voltage Range The maximum operating input voltage for the LT3645 is 36V. The minimum input voltage is determined by either the LT3645’s minimum operating voltage of 3.6V or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC = (V OUT + VD)/(VIN – VSW + VD) where VD is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). This leads to a minimum input voltage of: V IN(MIN) = ((VOUT + VD)/DCMAX) – VD + VSW with DCMAX = 0.83 for the LT3645. The maximum input voltage is determined by the absolute maximum ratings of the V IN and BOOST pins. For fi xed frequency operation, the maximum input voltage is de- termined by the minimum duty cycle, which is: V IN(MAX) = ((VOUT + VD)/DCMIN) – VD + VSW with DCMIN = 0.075 for the LT3645. Note that this is a restriction on the operating input voltage for continuous mode operation. The circuit will continue to regulate the output up until the overvoltage lockout input voltage (38.5V). The part will tolerate transient input APPLICATIONS INFORMATION voltages up to 55V, but once the input voltage exceeds 36V, the power switch will shut off and stop regulating the output voltage until the input voltage falls below 36V. Minimum On Time The LT3645 will operate at the correct frequency while the input voltage is below V IN(MAX). At input voltages that exceed V IN(MAX), the LT3645 will still regulate the output properly (up to 38.5V); however, the LT3645 will skip pulses to regulate the output voltage resulting in increased output voltage ripple. Figure 1 illustrates switching waveforms for a LT3645 application with V OUT = 1.2V near VIN(MAX) = 21.3V. Figure 1. As the input voltage is increased, the part is required to switch for shorter periods of time. Delays associated with turning off the power switch dictate the minimum on time of the part. The minimum on time for the LT3645 is 100ns. Figure 2 illustrates the switching waveforms when the input voltage is increased to V IN = 22V.

3645 F01

0.5A/DIV VIN = 18V VOUT = 1.2V IOUT = 500mA COUT = 10μF L = 10μH

3645 F02

0.5A/DIV VIN = 22V VOUT = 1.2V IOUT = 500mA COUT = 10μF L = 10μH Figure 2.

than 500mA, then a lower valued inductor can be used. forward current and a maximum reverse voltage of 40V. concerns the maximum input voltage rating of the LT3645. Table 1. Inductor Vendors

stabilize the LT3645’s control loop. sistance (ESR) and provide the best ripple performance. results in an increased loop crossover frequency. choose one that is intended for use in switching regulators. capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Table 2. Capacitor Vendors CC2 must be tied to a supply greater than 2.6V. from VCC2 (anode) to BOOST (cathode). Figure 3. In this confi guration, the BOOST capacitor will be capacitor is charged to 25V, the BOOST pin will be at 50V. the BOOST pin current comes from a higher voltage.

3645 F03

boost circuit. If the input voltage is ramped slowly, or if the LT3645 is turned on with the EN/UVLO pin when the output is already in regulation, then the boost capacitor might not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load generally goes to zero once the circuit has started. The worst case situation is when V IN is ramping very slowly. Figure 4a shows the minimum input voltage needed to start a 5V application versus output current. Figure 4b shows the minimum input voltage needed to start a 3.3V application versus output current. Soft-Start The LT3645 includes a 500μs internal soft-start for the buck converter and a 500μs soft-start for the LDO regula- tor. Both soft-starts are reset if the EN/UVLO pin is low, if V IN drops below 3.4V (undervoltage), if VIN exceeds 36V (overvoltage), or when the die temperature exceeds 160°C Figure 4. (4a) Typical Minimum Input Voltage, VOUT = 5V (4b) Typical Minimum Input Voltage, VOUT = 3.3V (thermal shutdown). The soft-start for the LDO can also be reset by pulling the EN2 pin low. The soft-start functions act to reduce the maximum input current during startup. Soft-start can not be disabled in the LT3645. Reversed Input Protection In some systems, the output will be held high when the input to the LT3645 is absent. This may occur in bat- tery charging applications or in battery backup systems where a battery or some other supply is diode OR’d with the LT3645’s output. If the V IN pin is allowed to fl oat and the EN/UVLO pin is held high (either by a logic signal or because it is tied to V IN), then the LT3645’s internal circuitry will draw its quiescent current through its SW pin. This is fi ne if the system can tolerate a few mA in this state. You can reduce this current by grounding the EN/ UVLO pin, then the SW pin current will drop to essentially zero. However, if the V IN pin is grounded while the output is held high, then parasitic diodes inside the LT3645 can OUTPUT CURRENT (mA) 5.0 INPUT VOLTAGE (V) 5.5 6.5 6.0 7.5 7.0 8.0 10 100

3645 F04a

OUTPUT CURRENT (mA) 3.0 INPUT VOLTAGE (V)4.0 3.5 5.0 4.5 6.0 5.5 6.5 10 100

3645 F04b

a shorted or reversed input. equivalent circuit for the LT3645 control loop. Figure 5. Diode D4 Prevents a Shorted Input from Discharging a Figure 6. Model for Loop Response capacitor, the loop crossover occurs above the RCCC zero.

3645 F05

3645 F06

resulting from a phase lead capacitor. The LT3645 LDO requires an output capacitor for stability. bypass capacitor reduces system noise as well. and temperature coeffi cients as shown in Figures 8 and 9. Figure 8. Ceramic Capacitor DC Bias Characteristics Figure 9. Ceramic Capacitor Temperature Characteristics

3645 F08

1210 CASE SIZE, 10μF

3645 F09

EN/UVLO pin. A good value for R8 is 100k. output (OUT2) is programmed to 1.8V. Figure 10. Precision UVLO Circuit Figure 11. OUT1 and OUT2 Come Up as Soon as Possible

3645 F10

3645 F11

regulation, the LDO output, OUT2 will come up to 1.8V. Figure 12. OUT2 Comes Up Before OUT1

3645 F12

P1 has a fi nite on-resistance which will result in power dissipation and some loss in effi ciency. For higher buck output voltage applications, a smaller PFET may be used since the gate drive will be higher. PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 13 shows the recommended component placement with trace, ground plane, and via locations. Note that large, switched current s fl ow in the LT3645’s V IN and SW pins, the catch diode (D1), and the input capacitor (C1). The loop formed by these components should be as small as possible and tied to system ground in only one place. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground system ground in only one place. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location (ideally at the ground terminal of the output capacitor C1). The SW and BOOST nodes should be kept as small as possible. Finally, keep the FB nodes small so that the ground pin and ground traces will shield them from the SW and BOOST nodes. Include vias near the exposed GND pad of the LT3645 to help remove heat from the LT3645 to the ground plane. High Temperature Considerations The die temperature of the LT3645 must be lower than the maximum rating of 125°C (150°C for H-grade). This is generally not a concern unless the ambient tempera- ture is above 85°C. For higher temperatures, extra care should be taken in the layout of the circuit to ensure good heat sinking at the LT3645. The maximum load current should be derated as the ambient temperature approaches 125°C. The die temperature is calculated by multiplying the LT3645 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3645 can be estimated by calculating the total power loss from an effi ciency measurement and subtracting the catch diode loss. The resulting temperature rise at full load is nearly independent of input voltage. Thermal resistance depends upon the layout of the circuit board, but 68°C/W is typical for the QFN (UD) package, and 40°C/W is typical for the MSE package. Thermal shutdown will turn off the Buck and LDO when the die temperature exceeds 160°C, but it is not a warrant to allow operation at die temperatures exceeding 125°C (150°C for H-grade). Other Linear Technology Publications Application Notes 19, 35, and 44 contain more detailed descriptions and design information for step-down regu- lators and other switching regulators. The LT1376 data sheet has an extensive discussion of output ripple, loop compensation, and stability testing. Design Note 318 shows how to generate a bipolar output supply using a step-down regulator. Figure 13.

3645 F13

OUTLINE OF LOCAL GROUND PLANE

1μF

3645 TA02

10μF 300mA 3.3V 200mA 15μH MBRM140 52.3k 31.6k 10k 10k BOOST L T3645 2.2μF 0.1μF ON OFF 1μF

3645 TA03

10μF 3.3V 300mA 1.8V 200mA 10μH MBRM140 31.6k 12.4k 10k 10k BOOST L T3645 2.2μF 0.1μF ON OFF TYPICAL APPLICATIONS 5V Step-Down Converter with 3.3V Logic Rail 3.3V Step-Down Converter with 1.8V Logic Rail

1μF

3645 TA05

10μF 2.5V 300mA 1.2V 200mA 4.7μH BAT85 MBRM140 21.5k 4.99k 10k 10k BOOST L T3645 2.2μF 0.1μF ON OFF TYPICAL APPLICATIONS 2.5V Step-Down Converter with 1.2V Logic Rail 3.3V Step-Down Converter with 1.8V Core Rail 1μF

3645 TA04

10μF OUT2 1.8V 200mA 10μH 31.6k 12.4k 10k 10k BOOST L T3645 2.2μF 0.1μF OUT1 3.3V 300mA 0.1μF 31.6K ON OFF

1μF

3645 TA06

10μF 3.3V 450mA 5.5V 50mA 6.8μH MBRM140 31.6k 52.3k 10k 10k BOOST L T3645 2.2μF 0.1μF 0.1μF ON OFF 3.3V Step-Down Converter with 5V Logic Rail

MSOP (MSE12) 0910 REV D 0.53 t 0.152 (.021 t .006) SEATING PLANE 0.18 (.007) 1.10 (.043) MAX 0.22 – 0.38 (.009 – .015) TYP 0.86 (.034) REF 0.650 (.0256) BSC 12 11 10 9 8 7 DETAIL “B” 1 6 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 0.254 (.010) 0s – 6s TYP DETAIL “A” DETAIL “A” GAUGE PLANE RECOMMENDED SOLDER PAD LAYOUT BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 t 0.102 (.112 t .004)2.845 t 0.102 (.112 t .004) 4.039 t 0.102 (.159 t .004) (NOTE 3) 1.651 t 0.102 (.065 t .004) 0.1016 t 0.0508 (.004 t .002) 12 345 6 3.00 t 0.102 (.118 t .004) (NOTE 4) 0.406 t 0.076 (.016 t .003) REF 4.90 t 0.152 (.193 t .006) DETAIL “B” CORNER TAIL IS PART OF THE LEADFRAME FEATURE. FOR REFERENCE ONL Y NO MEASUREMENT PURPOSE

0.12 REF

0.35 REF 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 0.889 t 0.127 (.035 t .005) 0.42 t 0.038 (.0165 t .0015) TYP 0.65 (.0256) BSC 12-Lead Plastic MSOP, Exposed Die Pad (Reference L TC DWG # 05-08-1666 Rev D)

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- t i o n t h a t t h e i n t e r c o n n e c t i o n o f i t s c i r c u i t s a s d e s c r i b e d h e r e i n w i l l n o t i n f r i n g e o n e x i s t i n g p a t e n t r i g h t s . 3.00 p 0.10 (4 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 1.45 p 0.05 (4 SIDES) NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE PIN 1 TOP MARK (NOTE 6) 0.40 p 0.10 BOTTOM VIEW—EXPOSED PAD 1.45 p 0.10 (4-SIDES) 0.75 p 0.05 R = 0.115 TYP 0.25 p 0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.25 s 45o CHAMFER 15 16

0.50 BSC

0.200 REF

2.10 p 0.05 3.50 p 0.05 0.70 p0.05 0.00 – 0.05 (UD16) QFN 0904 0.25 p0.05 16-Lead Plastic QFN (3mm × 3mm) (Reference L TC DWG # 05-08-1691) PACKAGE DESCRIPTION

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com © LINEAR TECHNOLOGY CORPORA TION 2011 LT 0511 • PRINTED IN USA RELATED PARTS TYPICAL APPLICATION PART NUMBER DESCRIPTION COMMENTS L T3694 36V , 70V T ransient Protection, 2.6A, 2.5MHz High Effi ciency Step-Down DC/DC Converter with Dual LDO Controllers V IN: 3.6V to 36V , T ransient to 70V , VOUT(MIN) = 0.75V , IQ = 1mA, ISD < 1μA, 4mm × 5mm QFN-28, TSSOP-20E L T3509 36V , 60V T ransient Protection, Dual 700mA, 2.2MHz High Effi ciency Step-Down DC/DC Converter V IN: 3.6V to 36V , T ransient to 60V , VOUT(MIN) = 0.8V , IQ = 1.9mA, ISD < 1μA, 3mm × 4mm DFN-14, MSOP-16E L T3689 36V , 60V T ransient Protection, 800mA, 2.2MHz High Effi ciency MicroPower Step-Down DC/DC Converter with POR Reset and Watchdog Timer V IN: 3.6V to 36V , T ransient to 60V , VOUT(MIN) = 0.8V , IQ = 75μA, ISD < 1μA, 3mm × 3mm QFN-16 L T3682 36V , 60VMax, 1A, 2.2MHz High Effi ciency Micropower Step-Down DC/DC Converter VIN: 3.6V to 36V , T ransient to 60V , VOUT(MIN) = 0.8V , IQ = 75μA, ISD < 1μA, 3mm × 3mm QFN-12 L T3970 40V , 350mA (I OUT), 2.2MHz, High Effi ciency Step-Down DC/DC Converter with Only 2.5μA of Quiescent Current V IN: 4.2V to 40V , VOUT(MIN) = 1.21V , IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10 L T3990 62V , 350mA (I OUT), 2.2MHz, High Effi ciency Step-Down DC/DC Converter with Only 2.5μA of Quiescent Current V IN: 4.2V to 40V , VOUT(MIN) = 1.21V , IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10 L T3791 38V , 1.2A, 2.2MHz High Effi ciency MicroPower Step-Down DC/DC Converter with IQ = 2.8μA VIN: 4.3V to 38V , VOUT(MIN) = 1.2V , IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E L T3991 55V , 1.2A, 2.2MHz High Effi ciency MicroPower Step-Down DC/DC Converter with IQ = 2.8μA VIN: 4.3V to 55V , VOUT(MIN) = 1.2V , IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E L T3480 36V with T ransient Protection to 60V , 2A (I OUT), 2.4MHz, High Effi ciency Step-Down DC/DC Converter with Burst Mode ® Operation VIN: 3.6V to 38V , VOUT(MIN) = 0.78V , IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E L T3685 36V with T ransient Protection to 60V , 2A (I OUT), 2.4MHz, High Effi ciency Step-Down DC/DC Converter V IN: 3.6V to 38V , VOUT(MIN) = 0.78V , IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E 1.8V Step-Down Converter with 0.8V Logic Rail 1μF

3645 TA07

10μF 1.8V 500mA 0.8V 200mA AL TERNATE POWER SOURCE 4.7μH MBRM140 12.4k 10k BOOST L T3645 2.2μF 0.1μF V ON OFF 0.1μF