LT3972IMSE-PBF

LT3972 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current FEATURES ■ DESCRIPTION The LT®3972 is an adjustable frequency (200kHz to 2.4MHz) monolithic buck switching regulator that accepts input voltages up to 33V (62V Maximum). A high efficiency 95mΩ switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. Current mode topology is used for fast transient response and good loop stability. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 15mV in a typical application. In addition, the LT3972 can further enhance low output current efficiency by drawing bias current from the output when VOUT is above 3V. Shutdown reduces input supply current to less than 1μA while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A power good flag signals when VOUT reaches 91% of the programmed output voltage. The LT3972 is available in 10-Pin MSOP and 3mm × 3mm DFN packages with exposed pads for low thermal resistance. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Range: Operation from 3.6V to 33V Over-Voltage Lockout Protects Circuits Through 62V Transients 3.5A Maximum Output Current Low Ripple (30V), the saturation current should be above 5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types. where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on time (~100ns). Note that a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage transients of up to 33V are acceptable regardless of the switching frequency. In this mode, the LT3972 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT3972’s minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD –V +V 1– fSW tOFF(MIN) D SW where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A reasonable starting point for selecting 3972f 10 LT3972 APPLICATIONS INFORMATION Table 1. Inductor Vendors VENDOR Murata TDK Toko Sumida URL www.murata.com www.componenttdk.com www.toko.com www.sumida.com PART SERIES LQH55D SLF10145 D75C D75F CDRH74 CR75 CDRH8D43 MPLC073 MPBI0755 TYPE Open Shielded Shielded Open Shielded Open Shielded Shielded Shielded NEC www.nec.com Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 3.5A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See AN19. Input Capacitor Bypass the input of the LT3972 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 10μF to 22μF ceramic capacitor is adequate to bypass the LT3972 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a lower performance electrolytic capacitor. 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 LT3972 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 10μF capacitor is capable of this task, but only if it is placed close to the LT3972 and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3972. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3972 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3972’s voltage rating. This situation is easily avoided (see the Hot Plugging Safety section). For space sensitive applications, a 4.7μF ceramic capacitor can be used for local bypassing of the LT3972 input. However, the lower input capacitance will result in increased input current ripple and input voltage ripple, and may couple noise into other circuitry. Also, the increased voltage ripple will raise the minimum operating voltage of the LT3972 to ~3.7V. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3972 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3972’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 VOUT fSW where fSW is in MHz, and COUT is the recommended output capacitance in μF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lower 3972f 11 LT3972 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR Panasonic PHONE (714) 373-7366 URL www.panasonic.com PART SERIES Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic TPS Series COMMANDS EEF Series Kemet Sanyo (864) 963-6300 (408) 749-9714 www.kemet.com www.sanyovideo.com T494, T495 POSCAP Murata AVX Taiyo Yuden (408) 436-1300 www.murata.com www.avxcorp.com (864) 963-6300 www.taiyo-yuden.com value of output capacitor can be used to save space and cost but transient performance will suffer. See the Frequency Compensation section to choose an appropriate compensation network. When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or 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 0.05Ω 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. Catch Diode The catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT (VIN – VOUT)/VIN where IOUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a Schottky diode with a reverse voltage rating greater than the input voltage. The overvoltage protection feature in the LT3972 will keep the switch off when VIN > 35V which allows the use of 40V rated Schottky even when VIN ranges up to 62V. Table 3 lists several Schottky diodes and their manufacturers. Table 3. Diode Vendors PART NUMBER On Semiconductor MBRA340 Diodes Inc. PDS340 B340A B340LA VR (V) 40 40 40 40 IAVE (A) 3 3 3 3 VF AT 3A (mV) 500 500 500 450 Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3972 due to their piezoelectric nature. When in Burst Mode operation, the LT3972’s switching frequency depends on the load current, and at very light loads the LT3972 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3972 operates at a lower current limit during Burst Mode operation, the noise is nearly silent to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. 3972f 12 LT3972 APPLICATIONS INFORMATION Frequency Compensation The LT3972 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3972 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 2. Generally a capacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. Figure 2 shows an equivalent circuit for the LT3972 control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. 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. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. Figure 3 shows the transient response when the load current is stepped from 1A to 3A and back to 1A. LT3972 CURRENT MODE POWER STAGE gm = 5.3mho SW ERROR AMPLIFIER FB ESR gm = 500μmho 3M C1 POLYMER OR TANTALUM R2 CERAMIC R1 CPL OUTPUT VC CF RC CC Figure 2. Model for Loop Response VOUT 100mV/DIV IL 1A/DIV VIN = 12V VOUT = 3.3V Figure 3. Transient Load Response of the LT3972 Front Page Application as the Load Current is Stepped from 1A to 3A. VOUT = 5V – + 0.8V + C1 GND 3680 F02 10μs/DIV 3680 F03 3972f 13 LT3972 APPLICATIONS INFORMATION Low-Ripple Burst Mode and Pulse-Skip Mode The LT3972 is capable of operating in either Low-Ripple Burst Mode or Pulse-Skip Mode which are selected using the SYNC pin. See the Synchronization section for details. To enhance efficiency at light loads, the LT3972 can be operated in Low-Ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3972 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3972 delivers power to the output with single, low current pulses, the output ripple is kept below 15mV for a typical application. In addition, VIN and BD quiescent currents are reduced to typically 30μA and 80μA respectively during the sleep time. As the load current decreases towards a no load condition, the percentage of time that the LT3972 operates in sleep mode increases and the average input current is greatly reduced resulting in high efficiency even at very low loads. See Figure 4. At higher output loads (above 140mA for the front page application) the LT3972 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and Low-Ripple Burst Mode is seamless, and will not disturb the output voltage. If low quiescent current is not required the LT3972 can operate in Pulse-Skip mode. The benefit of this mode is that the LT3972 will enter full frequency standard PWM operation at a lower output load current than when in Burst Mode. The front page application circuit will switch at full frequency at output loads higher than about 60mA. BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μF capacitor will work well. Figure 2 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 5a) is best. For outputs between 2.8V and 3V, use a 1μF boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1μF boost capacitor (see Figure 5b). For lower output voltages the boost diode can be tied to the input (Figure 5c), or to another supply greater than 2.8V. Tying BD to VIN reduces the maximum input voltage to 28V. The circuit in Figure 5a is more efficient because the BOOST pin current and BD pin quiescent current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. The minimum operating voltage of an LT3972 application is limited by the minimum input voltage (3.6V) and by the maximum duty cycle as outlined in a previous section. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3972 is turned on with its RUN/SS pin when the output is already in regulation, then the boost capacitor may 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 will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 6 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher, which will allow it to 3972f VSW 5V/DIV IL 0.2A/DIV VOUT 10mV/DIV VIN = 12V VOUT = 3.3V ILOAD = 10mA 5μs/DIV 3680 F04 Figure 4. Burst Mode Operation 14 LT3972 APPLICATIONS INFORMATION VOUT BD BOOST C3 SW INPUT VOLTAGE (V) VIN VIN LT3972 5.0 4.5 4.0 TO RUN 3.5 3.0 VOUT = 3.3V TA = 25°C L = 8.2μH f = 700kHz 1 10 100 1000 LOAD CURRENT (mA) 10000 6.0 5.5 TO START (WORST CASE) 4.7μF GND (5a) For VOUT > 2.8V VOUT BD BOOST VIN VIN LT3972 SW INPUT VOLTAGE (V) C3 D2 2.5 2.0 8.0 7.0 TO START (WORST CASE) 4.7μF GND 6.0 5.0 4.0 3.0 2.0 1 VOUT = 5V TA = 25°C L = 8.2μH f = 700kHz 10 100 1000 LOAD CURRENT (mA) 10000 3680 F06 TO RUN (5b) For 2.5V < VOUT < 2.8V VOUT BD BOOST VIN VIN LT3972 SW C3 4.7μF GND Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit 3680 FO5 Soft-Start (5c) For VOUT < 2.5V; VIN(MAX) = 30V Figure 5. Three Circuits For Generating The Boost Voltage start. The plots show the worst-case situation where VIN is ramping very slowly. For lower start-up voltage, the boost diode can be tied to VIN ; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3972, requiring a higher input voltage to maintain regulation. The RUN/SS pin can be used to soft-start the LT3972, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 7 shows the startup and shut-down waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20μA when the RUN/SS pin reaches 2.5V. Synchronization To select Low-Ripple Burst Mode operation, tie the SYNC pin below 0.5V (this can be ground or a logic output). 3972f 15 LT3972 APPLICATIONS INFORMATION RUN 15k RUN/SS 0.22μF GND VRUN/SS 2V/DIV IL 1A/DIV VOUT 2V/DIV 2ms/DIV 3680 F07 Figure 7. To Soft-Start the LT3972, Add a Resisitor and Capacitor to the RUN/SS Pin Synchronizing the LT3972 oscillator to an external frequency can be done by connecting a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.3V and peaks that are above 0.8V (up to 6V). The LT3972 will not enter Burst Mode at low output loads while synchronized to an external clock, but instead will skip pulses to maintain regulation. The LT3972 may be synchronized over a 250kHz to 2MHz range. The RT resistor should be chosen to set the LT3972 switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal will be 250kHz and higher, the RT should be chosen for 200kHz. To assure reliable and safe operation the LT3972 will only synchronize when the output voltage is near regulation as indicated by the PG flag. It is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the RT resistor. See Inductor Selection section. It is also important to note that slope compensation is set by the RT value: When the sync frequency is much higher than the one set by RT, the slope compensation will be significantly reduced which may require a larger inductor value to prevent subharmonic oscillation. Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, an LT3972 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3972 is absent. This may occur in battery charging ap- plications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3972’s output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LT3972’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the RUN/SS pin, 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 LT3972 can pull large currents from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. D4 MBRS340 VIN VIN RUN/SS VC GND FB BACKUP BOOST LT3972 SW VOUT 3680 F08 Figure 8. 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 LT3972 Runs Only When the Input is Present PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3972’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. 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. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and VC nodes small so that the ground 3972f 16 LT3972 APPLICATIONS INFORMATION L1 VOUT C2 RRT CC traces will shield them from the SW and BOOST nodes. The Exposed Pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3972 to additional ground planes within the circuit board and on the bottom side. R2 RC Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3972 circuits. However, these capacitors can cause problems if the LT3972 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the R1 D1 C1 GND RPG 3680 F09 VIAS TO LOCAL GROUND PLANE VIAS TO VOUT VIAS TO SYNC VIAS TO RUN/SS VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT3972 DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING + 4.7μF LOW IMPEDANCE ENERGIZED 24V SUPPLY STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR IIN 10A/DIV 20μs/DIV (10a) 0.7W LT3972 VIN 20V/DIV + 0.1μF 4.7μF IIN 10A/DIV (10b) 20μs/DIV LT3972 VIN 20V/DIV + 22μF 35V AI.EI. + 4.7μF IIN 10A/DIV (10c) 20μs/DIV 3680 F10 Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT3972 is Connected to a Live Supply 3972f 17 LT3972 APPLICATIONS INFORMATION voltage at the VIN pin of the LT3972 can ring to twice the nominal input voltage, possibly exceeding the LT3972’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3972 into an energized supply, the input network should be designed to prevent this overshoot. Figure 10 shows the waveforms that result when an LT3972 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 4.7μF ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 10b. A 0.7Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1μF capacitor improves high frequency filtering. For high input voltages its impact on efficiency is minor, reducing efficiency by 1.5 percent for a 5V output at full load operating from 24V. High Temperature Considerations The PCB must provide heat sinking to keep the LT3972 cool. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3972. Place additional vias can reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to JA = 35°C/W or less. With 100 LFPM airflow, this resistance can fall by another 25%. Further increases in airflow will lead to lower thermal resistance. Because of the large output current capability of the LT3972, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125°C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125°C. Power dissipation within the LT3972 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT3972 power dissipation by the thermal resistance from junction to ambient. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator. TYPICAL APPLICATIONS 5V Step-Down Converter VIN 6.3V TO 33V TRANSIENT TO 62V ON OFF VOUT 5V 3.5A VIN RUN/SS BD BOOST 0.47μF 10μF 15k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MPLC0730L4R7 3972f L 4.7μH VC RT PG SYNC LT3972 SW D 536k GND FB 100k 3680 TA02 47μF 18 LT3972 TYPICAL APPLICATIONS 3.3V Step-Down Converter VIN 4.4V TO 33V TRANSIENT TO 62V ON OFF VOUT 3.3V 3.5A VIN RUN/SS BD BOOST 0.47μF 4.7μF 19k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 VC RT PG SYNC 316k GND FB 100k 3680 TA03 L 3.3μH LT3972 SW D 22μF 2.5V Step-Down Converter VIN 4V TO 33V TRANSIENT TO 62V ON OFF VOUT 2.5V 3.5A VIN RUN/SS BD BOOST 1μF 4.7μF 15.4k 63.4k 680pF f = 600kHz D1: ON SEMI MBRA340 D2: MBR0540 L: NEC MPLC0730L3R3 VC RT PG 215k SYNC GND FB 100k 3680 TA04 D2 L 3.3μH LT3972 SW D1 47μF 3972f 19 LT3972 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 62V ON OFF VOUT 5V 2.5A VIN RUN/SS BD BOOST 0.47μF 4.7μF 15k 12.7k 680pF f = 2MHz D: ON SEMI MBRA340 L: NEC MPLC0730L2R2 VC RT PG 536k SYNC GND FB 100k 3680 TA05 L 2.2μH LT3972 SW D 22μF 12V Step-Down Converter VIN 15V TO 33V TRANSIENT TO 62V ON OFF VOUT 12V 3.5A VIN RUN/SS BD BOOST 0.47μF 10μF 17.4k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MBP107558R2P VC RT PG SYNC GND 715k FB 50k 3680 TA06 L 8.2μH LT3972 SW D 47μF 3972f 20 LT3972 TYPICAL APPLICATIONS 1.8V Step-Down Converter VIN 3.5V TO 27V VIN ON OFF RUN/SS BD BOOST 0.47μF 4.7μF 16.9k 78.7k 680pF f = 500kHz D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 VC RT PG SYNC 127k GND FB 100k 3680 TA08 VOUT 1.8V 3.5A L 3.3μH LT3972 SW D 47μF 3972f 21 LT3972 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 0.05 3.50 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 6 0.38 10 0.10 3.00 0.10 (4 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 1.65 0.10 (2 SIDES) (DD) DFN 1103 5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES) 1 0.25 0.05 0.50 BSC 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 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 3972f 22 LT3972 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP Exposed Die Pad , (Reference LTC DWG # 05-08-1664 Rev B) BOTTOM VIEW OF EXPOSED PAD OPTION 2.794 (.110 0.102 .004) 0.889 (.035 0.127 .005) 1 2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004) 5.23 (.206) MIN 2.083 (.082 0.102 3.20 – 3.45 .004) (.126 – .136) 10 0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 0.102 (.118 .004) (NOTE 3) 10 9 8 7 6 0.497 0.076 (.0196 .003) REF 4.90 0.152 (.193 .006) 0.254 (.010) GAUGE PLANE 0.53 0.152 (.021 .006) DETAIL “A” 0.18 (.007) SEATING PLANE 1.10 (.043) MAX DETAIL “A” 0 – 6 TYP 12345 3.00 0.102 (.118 .004) (NOTE 4) 0.86 (.034) REF 0.17 – 0.27 (.007 – .011) TYP 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 0.50 (.0197) BSC 0.1016 (.004 0.0508 .002) MSOP (MSE) 0307 REV B 3972f 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 LT3972 TYPICAL APPLICATION 1.2V Step-Down Converter VIN 3.6V TO 27V VIN ON OFF RUN/SS BD BOOST 0.47μF 4.7μF 17k 78.7k 470pF f = 500kHz 3680 TA09 VOUT 1.2V 3.5A L 3.3μH VC RT PG SYNC LT3972 SW D 52.3k GND FB 100k 100μF D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 RELATED PARTS PART NUMBER LT1933 LT1936 LT1940 LT1976/LT1967 LT3434/LT3435 LT3437 LT3480 LT3481 LT3493 LT3505 LT3508 LT3680 LT3684 LT3685 LT3693 DESCRIPTION 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 36V, 1.4A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converters with Burst Mode Operation 60V, 2.4A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converters with Burst Mode Operation 60V, 400mA (IOUT), Micropower 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 with Burst Mode Operation 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 40V, 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter 36V, 3.5A, 2.4MHz, Low Quiescent Current (