LT3791

LT3645 36V 500mA Step-Down Regulator and 200mA LDO FEATURES n DESCRIPTION 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. 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. 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 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 R1 R2 R3 R4 VOUT –1 0.8 VOUT2 –1 0.797 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 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: 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 INDUCTOR CURRENT 0.5A/DIV 3645 F01 VIN = 18V VOUT = 1.2V IOUT = 500mA COUT = 10μF L = 10μH 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 Sumida URL www.sumida.com Part Series CDRH4D28 CDRH5D28 CDRH8D28 A916CY D585LC WE-TPC(M) WE-PD2(M) WE-PD(S) Inductance Range (μH) 1.2 to 4.7 2.5 to 10 2.5 to 33 2 to 12 1.1 to 39 1 to 10 2.2 to 22 1 to 27 Size (mm) 4.5 × 4.5 5.5 × 5.5 8.3 × 8.3 6.3 × 6.2 8.1 × 8.0 4.8 × 4.8 5.2 × 5.8 7.3 × 7.3 Toko Würth Elektronik www.toko.com www.we-online.com 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 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. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good value is: 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. Table 2 lists several capacitor vendors. BOOST BOOST Pin Considerations 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. 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. 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 C3 SW VOUT Table 2. Capacitor Vendors AVX Murata Taiyo Yuden Vishay Siliconix TDK www.avxcorp.com www.murata.com www.t-yuden.com www.vishay.com www.tdk.com VIN VIN LT3645 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 8.0 7.5 INPUT VOLTAGE (V) 7.0 6.5 6.0 5.5 5.0 VIN TO RUN VIN TO START INPUT VOLTAGE (V) (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 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 VIN TO RUN VIN TO START 1 100 10 OUTPUT CURRENT (mA) 1000 3645 F04a 1 100 10 OUTPUT CURRENT (mA) 1000 3645 F04b (4a) Typical Minimum Input Voltage, VOUT = 5V Figure 4. (4b) Typical Minimum Input Voltage, VOUT = 3.3V 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 R1 CPL COUT CERAMIC BOOST EN/UVLO LT3645 DA D4 VIN VIN VCC2 FB OUT2 EN2 FB2 BACKUP gm = 100μA/V G = 1A/V RC = 150k CC = 60pF CC SW RC 1M gm ESR 0.8V R2 ELECTROLYTIC 3645 F06 + 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 0 CHANGE IN VALUE (%) X5R –20 –40 –60 Y5V –80 –100 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF With Phase Lead Capacitor 0 2 4 8 6 10 12 DC BIAS VOLTAGE (V) 14 16 3645 F08 Figure 8. Ceramic Capacitor DC Bias Characteristics Figure 7. Frequency Compensation (LDO) 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 CHANGE IN VALUE (%) 40 20 0 –20 –40 –60 –80 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF 50 25 75 0 TEMPERATURE (°C) 100 125 Y5V X5R –100 –50 –25 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. Precision Undervoltage Lockout 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: R7 R8 VIN(MIN) –1 1.23V VCC2 EN2 OUT2 12.4k FB2 10k 2.2μF OUT2 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 SW LT3645 DA FB 10K 31.6K 10μF OUT1 3645 F11 VIN R7 VIN LT3645 EN/UVLO 20V/DIV OUT1 5V/DIV OUT2 2V/DIV NPG 5V/DIV GND 3645 F10 EN/UVLO R8 500μs/DIV 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. 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. 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. 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 SW LT3645 DA FB 10K 31.6K BUCK OUTPUT P1 R9 31.6K OUT1 0.1μF 10μF VCC2 EN2 NPG OUT2 12.4k FB2 10k 2.2μF OUT2 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 NPG EN2 OUT2 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 C3 R2 FB1 R1 FB2 R4 C4 R3 C2 C5 BOOST SW C1 VIN VCC2 D1 DA VIN MAIN PCB BOARD POWER L1 + 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. 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 12V 1μF VIN LT3645 DA ON OFF EN/UVLO FB 10k EN2 PGOOD NPG VCC2 OUT2 31.6k FB2 GND 10k 3645 TA02 15μH SW MBRM140 52.3k 10μF 5V 300mA 3.3V 200mA 2.2μF 3.3V Step-Down Converter with 1.8V Logic Rail 0.1μF BOOST 12V 1μF VIN LT3645 DA ON OFF EN/UVLO FB 10k EN2 PGOOD NPG VCC2 OUT2 12.4k FB2 GND 10k 3645 TA03 10μH SW MBRM140 31.6k 10μF 3.3V 300mA 1.8V 200mA 2.2μF 3645f 19 LT3645 TYPICAL APPLICATIONS 3.3V Step-Down Converter with 1.8V Core Rail 0.1μF BOOST 12V 1μF VIN LT3645 DA FB ON OFF EN/UVLO EN2 VCC2 NPG OUT2 12.4k FB2 GND 10k 3645 TA04 L1 10μH SW 31.6k 10μF 10k 31.6K OUT1 3.3V 300mA 0.1μF OUT2 1.8V 200mA 2.2μF 2.5V Step-Down Converter with 1.2V Logic Rail BAT85 0.1μF BOOST 12V 1μF VIN LT3645 DA ON OFF EN/UVLO FB 10k EN2 PGOOD NPG VCC2 OUT2 4.99k FB2 GND 10k 3645 TA05 4.7μH SW MBRM140 21.5k 10μF 2.5V 300mA 1.2V 200mA 2.2μF 3645f 20 LT3645 TYPICAL APPLICATIONS 3.3V Step-Down Converter with 5V Logic Rail 0.1μF BOOST 12V 1μF VIN LT3645 DA ON OFF EN/UVLO FB 10k MBRM140 SW 31.6k 10μF 6.8μH 3.3V 450mA PGOOD NPG EN2 VCC2 OUT2 52.3k FB2 GND 10k 3645 TA06 5.5V 5V 50mA 0.1μF 2.2μF 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 (.112 0.102 .004) 2.845 (.112 1 0.102 .004) 6 0.35 REF 0.889 (.035 0.127 .005) 5.23 (.206) MIN 1.651 (.065 0.102 3.20 – 3.45 .004) (.126 – .136) 0.12 REF DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY 7 NO MEASUREMENT PURPOSE 12 0.65 0.42 0.038 (.0256) (.0165 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 4.039 0.102 (.159 .004) (NOTE 3) 12 11 10 9 8 7 0.406 0.076 (.016 .003) REF 0.254 (.010) GAUGE PLANE DETAIL “A” 0 – 6 TYP 4.90 0.152 (.193 .006) 3.00 0.102 (.118 .004) (NOTE 4) 0.53 0.152 (.021 .006) DETAIL “A” 0.18 (.007) 123456 1.10 (.043) MAX 0.86 (.034) REF SEATING PLANE 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 0.22 – 0.38 (.009 – .015) TYP 0.650 (.0256) BSC 0.1016 (.004 0.0508 .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 0.05 3.50 0.05 2.10 1.45 0.05 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 15 16 0.40 1 1.45 0.10 (4-SIDES) 2 0.10 PIN 1 NOTCH R = 0.20 TYP OR 0.25 45 CHAMFER 3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) (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 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 12V 1μF VIN LT3645 DA ON OFF EN/UVLO FB 10k EN2 PGOOD NPG VCC2 OUT2 0.8V 200mA 0.1μF MBRM140 SW 12.4k 10μF 4.7μH 1.8V 500mA + V 3V – ALTERNATE POWER SOURCE FB2 GND 2.2μF 3645 TA07 RELATED PARTS PART NUMBER DESCRIPTION LT3694 36V, 70V Transient Protection, 2.6A, 2.5MHz High Efficiency Step-Down DC/DC Converter with Dual LDO Controllers 36V, 60V Transient Protection, Dual 700mA, 2.2MHz High Efficiency Step-Down DC/DC Converter COMMENTS VIN: 3.6V to 36V, Transient to 70V, VOUT(MIN) = 0.75V, IQ = 1mA, ISD < 1μA, 4mm × 5mm QFN-28, TSSOP-20E VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1μA, 3mm × 4mm DFN-14, MSOP-16E LT3509 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 36V, 60VMax, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD < 1μA, 3mm × 3mm QFN-12 LT3682 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 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 38V, 1.2A, 2.2MHz High Efficiency MicroPower Step-Down DC/DC Converter with IQ = 2.8μA 55V, 1.2A, 2.2MHz High Efficiency MicroPower Step-Down DC/DC Converter with IQ = 2.8μA 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode® Operation 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter VIN: 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E VIN: 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E 3645f LT3990 LT3791 LT3991 LT3480 LT3685 24 Linear Technology Corporation (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0511 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 www.linear.com © LINEAR TECHNOLOGY CORPORA TION 2011