LT3645 36V 500mA Step-Down Regulator and 200mA LDO FEATURES DESCRIPTION n The LT®3645 is a dual output regulator combining a 500mA buck regulator and a 200mA low dropout linear regulator (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. n n n n n n n n Wide Input Range: Operation from 3.6V to 36V Overvoltage Lockout Protects Circuit Through 55V Transients on Input 500mA Output Current Switching Regulator High Switching Frequency: 750kHz 200mA Low Dropout Linear Regulator 1.2V to 16V Input; 0.8V to 8V Output 310mV Dropout Voltage VCC2 to OUT2 Precision Programmable Undervoltage Lockout Short-Circuit Robust Internal Soft-Start <2μA Shutdown Current Small Thermally Enhanced 16-Lead (3mm × 3mm) QFN and 12-Lead MSE Packages n The linear regulator operates from the VCC2 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. APPLICATIONS n Cycle-by-cycle current limit and frequency foldback provide protection against shorted outputs. Soft-start and frequency foldback eliminate input current surge during start-up. Automotive CMOS Image Sensors Industrial/Automotive Micro-Controller Supply L, LT, LTC, LTM, 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. TYPICAL APPLICATION Buck Regulator Efficiency 3.3V/5V Step-Down Converter 90 0.1μF 6.2V TO 36V 5V 300mA SW VIN 1μF VOUT = 5V 15μH LT3645 52.3k DA ON OFF EN/UVLO 10μF FB 10k VOUT = 3.3V 70 60 EN2 PGOOD 80 EFFICIENCY (%) BOOST VCC2 NPG 3.3V 200mA OUT2 31.6k 2.2μF FB2 GND 50 VIN = 12V 0 100 400 300 200 LOAD CURRENT (mA) 500 3645 TA01b 10k 3645 TA01a 3645f 1 LT3645 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UVLO (Note 5).......................…….–0.3V to 55V BOOST Voltage……………………………………….55V BOOST Above SW Voltage ............................... …….25V VCC2 Voltage .............................................. –0.3V to 16V VOUT2 Voltage .............................................. –0.3V to 8V FB, FB2 Voltages .......................................... –0.3V to 6V EN2, NPG Voltages .................................... –0.3V to 16V Operating Junction Temperature Range (Note 2) LT3645E ............................................ –40°C to 125°C LT3645I ............................................. –40°C to 125°C LT3645H ............................................ –40°C to 150°C Storage Temperature Range .................. –65°C to 150°C Lead Temperature (Soldering, 10 sec)…… ........... 300°C PIN CONFIGURATION NC VCC2 OUT2 FB2 TOP VIEW 16 15 14 13 NC 1 TOP VIEW 12 NC NC 2 NPG 3 EN/UVLO FB GND DA BOOST SW 11 VIN 17 GND 10 BOOST EN2 4 5 6 7 8 EN/UVLO FB GND DA 9 SW 1 2 3 4 5 6 13 GND 12 11 10 9 8 7 NPG EN2 FB2 OUT2 VCC2 VIN MSE PACKAGE 12-LEAD PLASTIC MSOP θJA = 40°C/W, θJC = 5°C/W TO 10°C/W EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB UD PACKAGE 16-LEAD (3mm × 3mm) PLASTIC QFN θJA = 58.7°C/W, θJC = 7.1°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3645EUD#PBF LT3645EUD#TRPBF LFVS 16-Lead Plastic QFN –40°C to 125°C LT3645IUD#PBF LT3645IUD#TRPBF LFVS 16-Lead Plastic QFN –40°C to 125°C LT3645EMSE#PBF LT3645EMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 125°C LT3645IMSE#PBF LT3645IMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 125°C LT3645HMSE#PBF LT3645HMSE#TRPBF 3645 12-Lead Plastic MSOP –40°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC 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/ 3645f 2 LT3645 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless otherwise noted. (Notes 2, 3) PARAMETER CONDITIONS MIN TYP MAX Undervoltage Lockout on VIN Rising l 3 3.4 3.6 V Overvoltage Lockout on VIN Rising l 36 38.5 40 V l 0.79 0.785 0.8 0.8 0.81 0.813 V l 20 300 nA l 0.015 Overvoltage Lockout Hysteresis 1 Feedback Voltage FB FB Pin Bias Current Feedback Voltage Line Regulation VIN Quiescent Current Not Switching VIN Quiescent Current in Shutdown VEN/UVLO = 0.3V, VCC2 = 0V, VOUT2 = 0V 1.4 Switching Frequency Maximum Duty Cycle 100mA Load Switch Current Limit Rising (Note 4) l DA Pin Current to Stop Osc Switch VCESAT UNITS V %/V 3 mA 0.01 2 μA 675 750 825 kHz 83 87 0.8 1 1.25 1 1.25 0.6 ISW = 500mA % 400 Switch Leakage Current A A mV 2 μA Minimum Boost Voltage Above Switch ISW = 500mA 1.6 2.2 V BOOST Pin Current ISW = 500mA 10 18 mA BOOST Schottky Forward Drop IOUT = 50mA 0.7 0.9 V EN/UVLO Threshold High Rising 1.23 1.29 1.17 EN/UVLO Threshold Hysteresis EN/UVLO Input Current 50 VEN/UVLO = 5V VEN/UVLO = 0V 25 Buck Soft-Start Time LDO Minimum Input Voltage VCC2 ILOAD = 200mA, VOUT2 = 0.8V, VIN = 4.0V LDO Feedback Voltage FB2 l LDO FB2 Bias Current l 782 V mV 50 1 μA μA 0.9 1.8 ms 1.1 1.38 V 797 810 mV 20 300 nA LDO Line Regulation 0.020 %/V LDO Load Regulation –1 mV LDO Dropout Voltage (VCC2 to VOUT2) LDO Dropout Voltage (VIN to VOUT2) ILOAD = 10mA ILOAD = 10mA ILOAD = 200mA l ILOAD = 200mA ILOAD = 200mA l LDO Current Limit EN2 Pin Threshold Rising Falling 45 65 145 mV mV mV 1.4 1.7 V V 310 1.1 270 l 210 l l 0.5 LDO Soft-Start Time mA mA 1.3 0.8 1.6 V V 0.6 1.2 ms NPG VCESAT INPG = 1mA, VFB = VFB2 = 850mV 0.4 V NPG Leakage VNPG = 16V, VFB = VFB2 = 750mV 0.5 μA FB2 NPG Threshold, % of Regulation Voltage VFB = 800mV, VFB2 Rising 92 % 88 90 3645f 3 LT3645 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless otherwise noted. (Notes 2, 3) PARAMETER CONDITIONS MIN FB NPG Threshold, % of Regulation Voltage VFB2 = 800mV, VFB Rising 88 NPG Threshold Hysteresis TYP MAX 90 92 UNITS % 25 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 LT3645E is guaranteed to meet performance specifications from 0°C to 125°C. Specifications over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3645I is guaranteed over the full –40°C to 125°C operating temperature range. The LT3645H 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. mV 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 specified 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 VIN and EN/UVLO pins is 55V for nonrepetitive one second transients, and 36V for continuous operation. TYPICAL PERFORMANCE CHARACTERISTICS Efficiency VOUT = 5V Buck Minimum Input Voltage, VOUT = 5V Efficiency VOUT = 3.3V 90 90 80 80 8.0 VIN TO RUN VIN TO START 70 70 VIN = 7V VIN = 12V VIN = 24V 0 100 200 300 400 OUTPUT CURRENT (mA) 50 500 VIN = 7V VIN = 12V VIN = 24V 0 100 200 300 400 LOAD CURRENT (mA) 6.5 6.0 Buck Minimum Input Voltage, VOUT = 3.3V 5.0 500 803 799 4.0 801 FB VOLTAGE (mV) FB VOLTAGE (mV) 4.5 1000 800 802 5.0 100 10 OUTPUT CURRENT (mA) FB2 Voltage 804 VIN TO RUN VIN TO START 5.5 1 3645 G03 FB Voltage 6.5 6.0 5.5 3645 G02 3645 G01 INPUT VOLTAGE (V) 7.0 60 60 50 INPUT VOLTAGE (V) EFFICIENCY (%) EFFICIENCY (%) 7.5 800 799 798 797 796 798 797 796 795 3.5 795 3.0 1 100 10 OUTPUT CURRENT (mA) 1000 3645 G04 794 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 3645 G05 794 –50 0 100 50 TEMPERATURE (°C) 150 3645 G06 3645f 4 LT3645 TYPICAL PERFORMANCE CHARACTERISTICS Buck Power Switch Current Limit 350 950 300 900 250 200 150 100 50 0 3.7 850 800 750 700 0 600 –50 50 100 150 200 250 300 350 400 450 500 SWITCH CURRENT (mA) 0 100 50 TEMPERATURE (°C) 3.2 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 150 3645 G09 LDO Power Transistor Current Limit 800 RISING FALLING 340 790 38.5 38.0 37.5 37.0 320 780 CURRENT LIMIT (mA) SWITCHING FREQUENCY (kHz) 39.0 770 760 750 740 730 100 50 TEMPERATURE (°C) 280 260 240 220 700 –50 150 300 720 710 0 100 50 TEMPERATURE (°C) 200 –50 150 LDO Dropout Voltage to VCC2 LDO Dropout Voltage 300 –0.02 100 50 0 OUT2 = 0.8V OUT2 = 3.3V 0 50 150 100 LOAD CURRENT (mA) 200 3645 G13 OUTPUT VOLTAGE CHANGE (%) 300 DROPOUT VOLTAGE (mV) 0 150 150 LDO Load Regulation 350 200 100 50 TEMPERATURE (°C) 3645 G12 350 250 0 3645 G11 3645 G10 DROPOUT VOLTAGE (mV) 3.4 Switching Frequency 39.5 0 3.5 3645 G08 Overvoltage Lockout 36.5 –50 3.6 3.3 650 3645 G07 THRESHOLD (V) Undervoltage Lockout 3.8 RISING THRESHOLD (V) 1000 CURRENT LIMIT (mA) BUCK POWER SWITCH VCE (mV) Buck Power Switch Voltage Drop 400 250 200 150 100 50 –0.04 –0.06 –0.08 –0.10 –0.12 –0.14 0 –60 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3645 G14 –0.16 0 50 100 150 OUTPUT CURRENT (mA) 200 3645 G15 3645f 5 LT3645 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Pin Current EN/UVLO PIN Threshold Voltage 1.26 EN/UVLO PIN THRESHOLD VOLTAGE (V) CURRENT INTO EN/UVLO PIN (μA) 120 100 80 60 40 20 0 0 5 10 15 20 25 30 EN/UVLO PIN VOLTAGE 35 1.25 RISING 1.24 1.23 1.22 1.21 1.20 FALLING 1.19 1.18 –50 40 0 50 100 TEMPERATURE (°C) 3645 G16 3645 G17 EN2 Threshold Voltage NPG Threshold Voltage, FB = 0.8V 1.5 750 740 RISING 1.3 730 1.2 FB2 VOLTAGE (mV) EN2 THRESHOLD VOLTAGE (V) 1.4 1.1 1.0 0.9 FALLING 0.8 720 700 680 670 660 100 50 TEMPERATURE (°C) 150 3645 G18 FALLING 690 0.6 0 RISING 710 0.7 0.5 –50 150 650 –50 0 100 50 TEMPERATURE (°C) 150 3645 G19 3645f 6 LT3645 PIN FUNCTIONS (MSOP/QFN) 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 section for more information. The EN/UVLO pin can be tied directly to VIN 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: ⎞ ⎛V R1= R2 • ⎜ OUT – 1⎟ ⎠ ⎝ 0.8 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 switching 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. 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 VCC2 pin must be tied to a voltage source greater than 2.5V to utilize the internal Schottky boost diode. If the VCC2 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: ⎛V ⎞ R3 = R4 t ⎜ OUT2 – 1⎟ ⎝ 0.797 ⎠ 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. VIN (Pin 7/ Pin 11): The VIN pin supplies current to the LT3645’s internal circuitry, to the internal buck power switch, and to the LDO. The VIN pin must be locally bypassed. 3645f 7 8 PGOOD LDO ON ON OFF C1 NPG EN2 GND EN/UVLO START 1.3V 1.23V REFERENCE – + – + – + + – VIN 0.72V START BUCK SOFT-START ERROR AMP SOFT-START OSCILLATOR + + – START LDO 0.8V – + Q + + – LOGIC 0.797V R S ERROR AMP SLOPE COMPENSATION CURRENT COMPARATOR VC LDO DRIVER VIN –40mV BUCK DRIVER Q1 Q2 FB2 OUT2 VCC2 FB GND DA SW BOOST C4 D1 C2 L1 R4 R3 C3 R2 R1 LT3645 BLOCK DIAGRAM 3645f LT3645 OPERATION The LT3645 includes a constant frequency, current mode step-down buck switching regulator together with a lowdropout 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 VIN is less than 3.4V. An overvoltage lockout (OVLO) turns the buck and LDO off when VIN is greater than 38.5V. The part 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 amplifiers. An internal, fixed-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 VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC and the internal slope-compensation. An error amplifier servos the VC node. The output of an external resistor divider between OUT and ground is tied to the VFB 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 VFB pin to 0.8V. The reference voltage of the buck error amplifier is ramped over 900μs during the soft-start period. When VC 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 VC 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 efficient 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 amplifier 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 VFB2 pin and presented to the negative error amp input, forcing the VFB2 pin to 0.797V. The reference voltage of the LDO error amplifier is ramped over 600μs during the soft-start period. The LDO power transistor (Q2) is driven from the VIN 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. 3645f 9 LT3645 APPLICATIONS INFORMATION FB Resistor Networks The output voltages are programmed with resistor dividers between the outputs and the VFB and VFB2 pins. Choose the resistors according to ©V ¹ R1" R2 t ª OUT – 1º « 0.8 » ©V ¹ R3 " R4 t ª OUT2 – 1º « 0.797 » 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 VOUT and VFB 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: 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 VIN(MAX). At input voltages that exceed VIN(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 VOUT = 1.2V near VIN(MAX) = 21.3V. SWITCH VOLTAGE 10V/DIV INDUCTOR CURRENT 0.5A/DIV 3645 F01 VIN = 18V VOUT = 1.2V IOUT = 500mA COUT = 10μF L = 10μH DC = (VOUT + 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: VIN(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 VIN and BOOST pins. For fixed frequency operation, the maximum input voltage is determined by the minimum duty cycle, which is: VIN(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 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 VIN = 22V. SWITCH VOLTAGE 10V/DIV INDUCTOR CURRENT 0.5A/DIV 3645 F02 VIN = 22V VOUT = 1.2V IOUT = 500mA COUT = 10μF L = 10μH Figure 2. 3645f 10 LT3645 APPLICATIONS INFORMATION Table 1. Inductor Vendors Vendor URL Part Series Inductance Range (μH) Size (mm) Sumida www.sumida.com CDRH4D28 CDRH5D28 CDRH8D28 1.2 to 4.7 2.5 to 10 2.5 to 33 4.5 × 4.5 5.5 × 5.5 8.3 × 8.3 Toko www.toko.com A916CY D585LC 2 to 12 1.1 to 39 6.3 × 6.2 8.1 × 8.0 Würth Elektronik www.we-online.com WE-TPC(M) WE-PD2(M) WE-PD(S) 1 to 10 2.2 to 22 1 to 27 4.8 × 4.8 5.2 × 5.8 7.3 × 7.3 Now the required on time has decreased below the minimum on time of 100ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the part skips a few pulses so that the average inductor current meets and does not exceed the load current requirement. The LT3645 is robust enough to survive prolonged operation under these conditions as long as the peak inductor current does not exceed 1.2A. Inductor saturation due to high current may further limit performance in this operating region. Inductor Selection and Maximum Output Current Choose the inductor value according to: L = 2.2 •(VOUT + VD)/ƒ where VD is the forward voltage drop of the catch diode (~0.4V), f is the switching frequency in MHz and L is in μH. With this value, there will be no subharmonic oscillation for applications with 50% or greater duty cycle. For robust operation in fault conditions, the saturation current should be above 1.5A. To keep efficiency high, the series resistance (DCR) should be less than 0.1Ω. Table 1 lists several inductor vendors. If the buck load current is less than 500mA, then a lower valued inductor can be used. Catch Diode Depending on load current, a 500mA to 1A Schottky diode is recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the overvoltage lockout voltage (38.5V). The ON Semiconduc- tor MBRA140T3 and Central Semiconductor CMMSH1-40 are good choices, as they are rated for 1A continuous forward current and a maximum reverse voltage of 40V. Input Filter Network Bypass VIN with a 1μF or higher ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage and should not be used. A 1μF ceramic capacitor is adequate to bypass the LT3645 and will easily handle the ripple current. However, if the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance might be necessary. This can be provided with a low performance (high ESR) electrolytic capacitor in parallel with the ceramic device. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3645 input and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 1μF capacitor is capable of this task, but only if it is placed close to the LT3645 and catch diode (see the PCB layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3645. A ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT3645 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3645’s voltage rating. This situation can easily be avoided. For more details, see Linear Technology Application Note 88. 3645f 11 LT3645 APPLICATIONS INFORMATION Output Capacitor BOOST Pin Considerations The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3645 to produce the DC output. In this role it determines the output ripple so low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3645’s control loop. The external capacitor C2 and an internal Schottky diode connected between the VCC2 and BOOST pins form a charge pump circuit which is used to generate a boost voltage that is higher than the input voltage (VIN). In most application circuits where the duty cycle is less than 50%, use C2 = 0.1μF. If the duty cycle is higher than 50% then use C2 = 0.22μF. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good value is: The BOOST pin must be at least 2.2V above the SW pin to fully saturate the NPN power switch (Q1). The forward drop of the internal Schottky diode is 0.8V. This means that VCC2 must be tied to a supply greater than 2.6V. COUT = 26.4/(VOUT • ƒ) where f is the switching frequency in MHz and COUT is in μF. This choice will provide low output ripple and good transient response. COUT = 10μF is a good choice for output voltages above 2.5V. For lower output voltages use 22μF or higher. Transient performance can be improved with a high value capacitor, but a phase lead capacitor across the feedback resistor R1 may be required to get the full benefit (see the Compensation section). Using a small output capacitor results in an increased loop crossover frequency. Use X5R or X7R types and keep in mind that a ceramic capacitor biased with VOUT will have less than its nominal capacitance. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier and should be 0.1Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. VCC2 may be tied to a supply between 2.2V and 2.6V if an external Schottky diode (such as a BAS70) is connected from VCC2 (anode) to BOOST (cathode). If no voltage supply greater than 2.6V is available, then an external boost Schottky diode can be tied from the VIN pin (anode) to the BOOST pin (cathode) as shown in Figure 3. In this configuration, the BOOST capacitor will be charged to approximately the VIN voltage, and will change if VIN changes. In this configuration the maximum operating VIN is 25V, because when VIN = 25V, then when the power switch Q1 turns on, VSW ~ 25V, and since the boost capacitor is charged to 25V, the BOOST pin will be at 50V. This connection is not as efficient as the others because the BOOST pin current comes from a higher voltage. The minimum operating voltage of an LT3645 application is limited by the undervoltage lockout (~3.4V) and by the maximum duty cycle as outlined above. For proper startup, the minimum input voltage is also limited by the D2 Table 2 lists several capacitor vendors. C3 BOOST Table 2. Capacitor Vendors AVX LT3645 www.avxcorp.com Murata www.murata.com Taiyo Yuden www.t-yuden.com Vishay Siliconix www.vishay.com TDK www.tdk.com VIN VIN SW VOUT GND 3645 F03 VBOOST – VSW % VIN MAX VBOOST % 2VIN Figure 3. 3645f 12 LT3645 APPLICATIONS INFORMATION 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 VIN 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 regulator. Both soft-starts are reset if the EN/UVLO pin is low, if VIN drops below 3.4V (undervoltage), if VIN exceeds 36V (overvoltage), or when the die temperature exceeds 160°C (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 battery 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 VIN pin is allowed to float and the EN/UVLO pin is held high (either by a logic signal or because it is tied to VIN), then the LT3645’s internal circuitry will draw its quiescent current through its SW pin. This is fine 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 VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3645 can 8.0 7.0 6.5 6.0 5.5 5.0 VIN TO RUN VIN TO START 6.0 INPUT VOLTAGE (V) 7.5 INPUT VOLTAGE (V) 6.5 VIN TO RUN VIN TO START 5.5 5.0 4.5 4.0 3.5 1 100 10 OUTPUT CURRENT (mA) 3.0 1000 3645 F04a 1 100 10 OUTPUT CURRENT (mA) 1000 3645 F04b (4b) Typical Minimum Input Voltage, VOUT = 3.3V (4a) Typical Minimum Input Voltage, VOUT = 5V Figure 4. 3645f 13 LT3645 APPLICATIONS INFORMATION pull large currents from the output through the SW pin and the VIN pin. Figure 5 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. G OUT COUT CERAMIC R1 CPL BOOST EN/UVLO gm SW RC LT3645 1M ESR 0.8V + R2 CC DA D4 VIN ELECTROLYTIC VCC2 VIN FB BACKUP 3645 F06 gm = 100μA/V G = 1A/V RC = 150k CC = 60pF OUT2 EN2 FB2 Figure 6. Model for Loop Response NPG GND 3645 F05 Figure 5. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input. The LT3645 Runs Only When the Input Is Present Frequency Compensation (Buck) The LT3645 uses current mode control to regulate the loop. This simplifies loop compensation. In particular, the LT3645 does not require the ESR of the output capacitor for stability, allowing the use of ceramic capacitors to achieve low output ripple and small circuit size. A low ESR output capacitor will typically provide for a greater margin of circuit stability than an otherwise equivalent capacitor with higher ESR, although the higher ESR will tend to provide a faster loop response. Figure 6 shows an equivalent circuit for the LT3645 control loop. The error amplifier (gm) is a transconductance type with finite output impedance. The power section, consisting of the modulator, power switch, and inductor, is modeled as a transconductance amplifier (G) generating an output current proportional to the voltage at the VC node. Note that the output capacitor integrates this current, and that the capacitor on the VC node (CC) integrates the error amplifier output current, resulting in two poles in the loop. RC provides a zero. With the recommended output capacitor, the loop crossover occurs above the RCCC zero. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. With a larger ceramic capacitor that will have lower ESR, crossover may be lower and a phase lead capacitor connected across R1 in the feedback divider may improve the transient response. Large electrolytic capacitors may have an ESR 3645f 14 LT3645 APPLICATIONS INFORMATION large enough to create an additional zero, and the phase lead might not be necessary. If the output capacitor is different than the recommended capacitor, stability should be checked across all operating conditions, including input voltage and temperature. Figure 7 shows the transient response of the LT3645 with a few output capacitor choices. The output is 3.3V. The load current is stepped from 0.25A to 0.5A and back to 0.25A, and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The second photo shows the improved response (faster recovery) resulting from a phase lead capacitor. No Phase Lead Capacitor pin and the FB2 pin. Capacitors up to 1nF can be used. This bypass capacitor reduces system noise as well. Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior across temperature and applied voltage. The most common dielectrics used are specified with EIA temperature characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but they tend to have strong voltage and temperature coefficients as shown in Figures 8 and 9. When used with a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an effective value as low as 1μF to 2μF for the DC bias voltage applied and over the operating 20 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF With Phase Lead Capacitor CHANGE IN VALUE (%) 0 X5R –20 –40 –60 Y5V –80 –100 0 2 4 14 8 6 10 12 DC BIAS VOLTAGE (V) 16 3645 F08 Figure 8. Ceramic Capacitor DC Bias Characteristics Figure 7. 40 Frequency Compensation (LDO) CHANGE IN VALUE (%) 20 The LT3645 LDO requires an output capacitor for stability. It is designed to be stable with most low ESR capacitors (typically ceramic, tantalum or low ESR electrolytic). A minimum output capacitor of 2.2μF with an ESR of 0.5Ω or less is recommended to prevent oscillations. Larger values of output capacitance decrease peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT3645, increase the effective output capacitor value. For improvement in transient performance, place a capacitor across the OUT2 X5R 0 –20 –40 Y5V –60 –80 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF –100 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3645 F09 Figure 9. Ceramic Capacitor Temperature Characteristics 3645f 15 LT3645 APPLICATIONS INFORMATION temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values. Care still must be exercised when using X5R and X7R capacitors; the X5R and X7R codes only specify operating temperature range and maximum capacitance change over temperature. Capacitance change due to DC bias with X5R and X7R capacitors is better than Y5V and Z5U capacitors, but can still be significant enough to drop capacitor values below appropriate levels. Capacitor DC bias characteristics tend to improve as component case size increases, but expected capacitance at operating voltage should be verified. Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric microphone works. For a ceramic capacitor the stress can be induced by vibrations in the system or thermal transients. With the resistor divider connected, the part will only operate at input voltages greater than VIN(MIN). Note that the resistor divider will always draw current from VIN. To reduce this current, the user might use large value resistors for R7 and R8. This is acceptable as long as R7 and R8 are selected such that they can supply 10μA to the EN/UVLO pin. A good value for R8 is 100k. Output Voltage Sequencing There are a few applications available for sequencing the buck and LDO output voltages. In Figures 11 and 12, the buck output (OUT1) is programmed to 3.3V, while the LDO output (OUT2) is programmed to 1.8V. Figure 11 shows a standard configuration where OUT1 and OUT2 come up as soon as possible. In this configuration, 4.7μH OUT1 SW LT3645 31.6K DA 10μF FB Precision Undervoltage Lockout 10K The EN/UVLO pin has an accurate 1.23V threshold that can be used to shutdown the part when the input voltage drops below a specified level. To perform this function, a resistor divider between the EN/UVLO pin and the VIN pin can be tied as shown in Figure 10. The resistor values can be determined from the following equation: VCC2 EN2 12.4k 10k 3645 F11 EN/UVLO 20V/DIV VIN R7 LT3645 OUT1 5V/DIV OUT2 2V/DIV EN/UVLO R8 2.2μF FB2 ©V ¹ R7 " R8 t ª IN(MIN) – 1º « 1.23V » VIN OUT2 OUT2 NPG 5V/DIV GND 500μs/DIV 3645 F10 Figure 11. OUT1 and OUT2 Come Up as Soon as Possible Figure 10. Precision UVLO Circuit 3645f 16 LT3645 APPLICATIONS INFORMATION there is a small delay before OUT2 begins ramping up as OUT2 has to wait until VCC2 is above 2V before power can be supplied to OUT2. When both OUT2 and the buck output are in regulation, the NPG pin will pull low, turning on PFET P1 and supplying power to OUT1. Figure 12 utilizes the NPG pin to sequence the outputs such that OUT1 comes into regulation after OUT2 is already in regulation. When the part is off, the buck output, OUT1 and OUT2 will be 0V. The NPG pin will be high impedance, PFET P1 will be off and OUT1 will be disconnected from the buck output. When the part is turned on, first the buck output will come up to 3.3V. Once the Buck output is in regulation, the LDO output, OUT2 will come up to 1.8V. The NPG pin is capable of sinking 1mA and will pull the gate of P1 down to 300mV. Therefore R9 should be chosen such that: R9 < (VOUT1 – 300mV)/1mA Where R7 is in Ω. For a 3.3V buck output application, PFET P1 must be able to source 300mA to OUT1 from the buck output with ~3V of gate drive. Note that PFET 4.7μH BUCK OUTPUT P1 SW LT3645 R9 31.6K 31.6K DA OUT1 0.1μF 10μF FB 10K VCC2 EN2 NPG OUT2 OUT2 12.4k 2.2μF FB2 10k 3645 F12 EN/UVLO, 20V/DIV BUCK OUTPUT, 5V/DIV OUT1, 5V/DIV OUT2 2V/DIV NPG 5V/DIV 500μs/DIV Figure 12. OUT2 Comes Up Before OUT1 3645f 17 LT3645 APPLICATIONS INFORMATION P1 has a finite on-resistance which will result in power dissipation and some loss in efficiency. 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 currents flow in the LT3645’s VIN 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 OUT1 EN/UVLO C3 OUT2 NPG EN2 R2 C4 R4 FB1 FB2 R1 R3 C2 C5 BOOST SW C1 VIN VCC2 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 temperature 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 efficiency 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 D1 Application Notes 19, 35, and 44 contain more detailed descriptions and design information for step-down regulators 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. DA VIN L1 MAIN PCB BOARD POWER + 3645 F13 VIA TO LOCAL GROUND PLANE OUTLINE OF LOCAL GROUND PLANE Figure 13. 3645f 18 LT3645 TYPICAL APPLICATIONS 5V Step-Down Converter with 3.3V Logic Rail 0.1μF BOOST VIN 12V 1μF 15μH LT3645 EN/UVLO 52.3k MBRM140 DA ON OFF 5V 300mA SW 10μF FB 10k EN2 NPG PGOOD VCC2 3.3V 200mA OUT2 31.6k 2.2μF FB2 GND 10k 3645 TA02 3.3V Step-Down Converter with 1.8V Logic Rail 0.1μF BOOST VIN 12V 1μF 10μH LT3645 DA ON OFF 3.3V 300mA SW EN/UVLO MBRM140 31.6k 10μF FB 10k EN2 PGOOD NPG VCC2 1.8V 200mA OUT2 12.4k 2.2μF FB2 GND 10k 3645 TA03 3645f 19 LT3645 TYPICAL APPLICATIONS 3.3V Step-Down Converter with 1.8V Core Rail 0.1μF L1 10μH BOOST VIN 12V 1μF OUT1 3.3V 300mA SW LT3645 31.6k DA 0.1μF 10μF FB 10k EN/UVLO ON OFF 31.6K EN2 VCC2 NPG OUT2 1.8V 200mA OUT2 12.4k 2.2μF FB2 GND 10k 3645 TA04 2.5V Step-Down Converter with 1.2V Logic Rail BAT85 0.1μF BOOST VIN 12V 1μF 4.7μH LT3645 DA ON OFF 2.5V 300mA SW EN/UVLO MBRM140 21.5k 10μF FB 10k EN2 PGOOD NPG VCC2 1.2V 200mA OUT2 4.99k 2.2μF FB2 GND 10k 3645 TA05 3645f 20 LT3645 TYPICAL APPLICATIONS 3.3V Step-Down Converter with 5V Logic Rail 0.1μF BOOST VIN 12V 1μF 6.8μH LT3645 DA ON OFF 3.3V 450mA SW EN/UVLO 31.6k MBRM140 10μF FB 10k PGOOD EN2 NPG VCC2 5.5V 5V 50mA OUT2 0.1μF 52.3k 2.2μF FB2 GND 10k 3645 TA06 3645f 21 LT3645 PACKAGE DESCRIPTION MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev D) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 t 0.102 (.112 t .004) 5.23 (.206) MIN 2.845 t 0.102 (.112 t .004) 0.889 t 0.127 (.035 t .005) 6 1 1.651 t 0.102 3.20 – 3.45 (.065 t .004) (.126 – .136) 0.12 REF 12 0.65 0.42 t 0.038 (.0256) (.0165 t .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 4.039 t 0.102 (.159 t .004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY 7 NO MEASUREMENT PURPOSE 0.406 t 0.076 (.016 t .003) REF 12 11 10 9 8 7 DETAIL “A” 0s – 6s TYP 3.00 t 0.102 (.118 t .004) (NOTE 4) 4.90 t 0.152 (.193 t .006) GAUGE PLANE 0.53 t 0.152 (.021 t .006) 1 2 3 4 5 6 DETAIL “A” 1.10 (.043) MAX 0.18 (.007) SEATING PLANE 0.22 – 0.38 (.009 – .015) TYP 0.650 (.0256) BSC 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 0.86 (.034) REF 0.1016 t 0.0508 (.004 t .002) MSOP (MSE12) 0910 REV D 3645f 22 LT3645 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 p0.05 3.50 p 0.05 1.45 p 0.05 2.10 p 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 p0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 s 45o CHAMFER R = 0.115 TYP 0.75 p 0.05 15 16 PIN 1 TOP MARK (NOTE 6) 0.40 p 0.10 1 1.45 p 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 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 0.25 p 0.05 0.50 BSC 3645f 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23 LT3645 TYPICAL APPLICATION 1.8V Step-Down Converter with 0.8V Logic Rail 0.1μF BOOST 4.7μH VIN 12V 1μF LT3645 DA ON OFF 1.8V 500mA SW MBRM140 12.4k 10μF FB EN/UVLO 10k EN2 PGOOD NPG VCC2 + 0.8V 200mA OUT2 FB2 2.2μF GND 0.1μF V 3V – ALTERNATE POWER SOURCE 3645 TA07 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3694 36V, 70V Transient Protection, 2.6A, 2.5MHz High Efficiency Step-Down DC/DC Converter with Dual LDO Controllers VIN: 3.6V to 36V, Transient to 70V, VOUT(MIN) = 0.75V, IQ = 1mA, ISD < 1μA, 4mm × 5mm QFN-28, TSSOP-20E LT3509 36V, 60V Transient Protection, Dual 700mA, 2.2MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1μA, 3mm × 4mm DFN-14, MSOP-16E LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD < 1μA, High Efficiency MicroPower Step-Down DC/DC 3mm × 3mm QFN-16 Converter with POR Reset and Watchdog Timer LT3682 36V, 60VMax, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter LT3970 40V, 350mA (IOUT), 2.2MHz, High Efficiency VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10, Step-Down DC/DC Converter with Only 2.5μA of MSOP-10 Quiescent Current LT3990 62V, 350mA (IOUT), 2.2MHz, High Efficiency VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10, Step-Down DC/DC Converter with Only 2.5μA of MSOP-10 Quiescent Current LT3791 38V, 1.2A, 2.2MHz High Efficiency 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 LT3991 55V, 1.2A, 2.2MHz High Efficiency 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 LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency 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 LT3685 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD < 1μA, 3mm × 3mm QFN-12 3645f 24 Linear Technology Corporation LT 0511 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011