design features Two Wide Input Range Monolithic Switching Regulators Make it Easy to Fit Boost, Flyback, SEPIC and Inverting Topologies into Tight Spaces Bin Zhang The LT3957 and LT3958 are high power monolithic switching regulators that can generate a positive or a negative output from a wide range of input voltages. The LT3957 operates over an input range of 3V to 40V, making it suitable for everything from portable electronics to automotive and industrial applications. The LT3958 extends the input voltage to 80V for high voltage telecommunications supplies. Both produce high power from a small footprint as shown by the boost converter layout in Figure 1. FEATURES Figure 2. A high efficiency and very compact 3V–6V input, 12V output boost converter VIN 3V TO 6V By integrating the power MOSFET and LDO onto the die, the LT3957 and LT3958 simplify converter design, shrink the solution size and reduce cost when compared to non-monolithic solutions. The LT3957 has an integrated 40V/30mΩ N-MOSFET switch with an internally programmed current limit of 5.9A (typical). The LT3958 has an integrated 84V/90mΩ N-MOSFET switch with an internally programmed current limit of 4A (typical). Each IC has an internal high voltage LDO, which allows LT3957 L1 4.7µH CIN 100µF 6.3V X5R VIN 49.9k 39.2k GND LT3957 SENSE1 SYNC SENSE2 105k FBX RT CIN: MURATA GRM32ER60J107ME20L COUT: MURATA GRM32ER61C226KE20 D1: VISHAY SILICONIX MBRA120 L1: VISHAY IHLM-2525CZ-01 41.2k 300kHz INTVCC VC SS 0.33µF 10k 10nF 10Ω CVCC 4.7µF 10V X5R The LT3957 and LT3958 use a constant frequency, peak current mode control scheme, providing fast transient response and an easy to stabilize feedback loop at variable inputs and outputs. The switching frequency can be programmed over a 100kHz to 1MHz range with a single VOUT 12V COUT 800mA 22µF 16V X5R ×2 SW SGND to be powered from a supply as high as 40V, and for LT3958 as high as 80V. Figure 3. Efficiency of the converter in Figure 2 D1 EN/UVLO Figure 1. A 10V–40V input, 48V output boost converter board 100 95 EFFICIENCY (%) These versatile switching regulators can be configured as boost, flyback, SEPIC or inverting converters. The LT3957 and LT3958 feature a rugged low side N-channel power MOSFET rated for 5A/40V and 3.5A/80V respectively. A novel FBX pin architecture provides accurate regulated positive or negative output with a simple resistor divider. These ICs also include soft-start, frequency foldback, input undervoltage lockout, adjustable frequency and synchronous switching. VIN = 5V 90 VIN = 3.3V 85 80 15.8k VIN 75 70 0 200 400 ILOAD (mA) 600 800 July 2011 : LT Journal of Analog Innovation | 9 The LT3957 and LT3958 feature a rugged low side N-channel power MOSFET rated for 5A/40V and 3.5A/80V respectively. A novel FBX pin architecture provides accurate regulated positive or negative output with a simple resistor divider. 0.1µF 50V CIN 4.7µF 50V X5R • 2.4k D1 • COUT 100µF 6.3V DSN 200k VIN SW GND EN/UVLO Figure 4. A simple and compact 12V to 45V input, –5V output flyback converter takes advantage of the LT3958’s rugged 84V-rated MOSFET and the novel FBX pin architecture. 100pF 25.5k SGND SENSE1 SYNC SENSE2 RT SS 82.5k INTVCC VC 0.47µF 10k 4.7µF 10V X5R 15.8k 100pF 10nF CIN: MURATA GRM32ER71H475KA88L COUT: MURATA GRM32ER60J107ME20L At the heart of each of the LT3957 and LT3958 is a pair of feedback error amplifiers: one regulates a positive output and the other, a negative output. When the converter starts up, the output voltage ramps positive or negative depending on the topology selected. The appropriate error amplifier seamlessly takes over the feedback control, while the other becomes inactive. In this way, these parts can automatically regulate to the desired output voltage in different converter topologies. The LT3957 and LT3958 contain several features to limit the peak switch currents and output voltage overshoot during start-up or recovery from a fault 10 | July 2011 : LT Journal of Analog Innovation VIN = 12V 80 VIN = 45V 75 70 65 60 55 45 FBX external resistor, which allows designers to optimize component size and performance parameters, such as minimum/ maximum duty cycle and efficiency. 85 50 LT3958 63.4k 200kHz 90 VOUT –5V 2A EFFICIENCY (%) T1 3:1 VIN 12V TO 45V D1: VISHAY 30BQ040 DSN: VISHAY ES1C T1: COILTRONICS VP2-0066 condition. The SS pin voltage modulates the peak switch current, thereby allowing the output capacitor to charge gradually toward its final value while preventing switch current from reaching current limit. Selection of a capacitor on the SS pin determines the start-up/restart time. The frequency foldback function reduces the switching frequency when FBX voltage is close to 0V to ensure that the IC maintains control over the switch current. An overvoltage protection function protects both the positive and negative output converters from excessive output voltage overshoot. The internal power switch is turned off immediately when the FBX pin voltage exceeds the positive regulated voltage by 8% (typical), or negative regulated voltage by 11% (typical). 40 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 ILOAD (A) 2 Figure 5. Efficiency of the converter in Figure 4 A 3V–6V INPUT, 12V OUTPUT BOOST CONVERTER Figure 2 shows a 3V–6V input, 12V output boost power supply using the LT3957. To ensure the 3V minimum operation voltage, the voltage drop across the internal LDO is eliminated by connecting the INTVCC pin directly to VIN pin through a 10Ω resistor (given that the maximum VIN operating voltage of 6V is below INTVCC ’s maximum voltage rating of 8V). Figure 3 shows the efficiency of this converter. A 12V–45V INPUT, –5V OUTPUT FLYBACK CONVERTER The LT3957 or LT3958 can be configured as a flyback converter for applications where the converters have multiple outputs, isolated outputs or high input/output voltage conversion ratios. The flyback converter has a very low parts count for multiple outputs, and with prudent selection of transformer turns ratio it can have high output/input voltage conversion ratios with a desirable duty cycle. The frequency foldback feature of the LT3957 and LT3958 provides robust design features D1 VOUT 24V COUT 700mA 4.7µF 50V ×2 VIN 200k SW GND EN/UVLO LT3958 36.5k 100pF SENSE1 SGND SENSE2 SYNC FBX RT SS 41.2k 300kHz Figure 6. A simple and compact 9V–36V input, 24V output SEPIC converter with short-circuit protection • L1B 0.47µF 280k INTVCC VC 10k Figure 6 shows the LT3958 in a 9V to 36V input, 24V output SEPIC converter. This topology allows for the input to be higher, equal or lower than the desired output voltage. In a SEPIC converter, no DC path exists between the input and output, which An inverting converter is similar to the SEPIC converter in that it can step up or step down the input, but with a negative output. It also offers output disconnect and short-circuit protection. Figure 8 shows a 4V–15V input, –5V output inverting L1B D1 SW 100 200 300 400 ILOAD (mA) 500 600 700 CONCLUSION The LT3957 and LT3958 are versatile monolithic switching regulators that can be configured as boost, flyback, SEPIC or inverting converters accepting a wide range of input voltages. By integrating the power MOSFET and LDO, they greatly simplify converter design, shrink solution size and reduce costs. These ICs also offer low shutdown current, soft-start, frequency foldback, input UVLO, adjustable frequency and synchronization. The combination of these features makes the LT3957 and LT3958 suitable for a variety of applications ranging from battery portable electronics to automotive, industrial and telecommunications power supplies. n Figure 9. Efficiency of the converter in Figure 8 95 SGND SENSE1 SYNC SENSE2 82.5k INTVCC VC SS 0.47µF 10k 10nF VIN = 4V 90 FBX RT 0 converter using LT3957 as the switching regulator. Figure 9 shows the efficiency of this converter at different input voltages. LT3957 41.2k 300kHz 65 GND EN/UVLO 37.4k VOUT –5V COUT 1.6A 100µF 6.3V • • VIN 70 Figure 7. Efficiency of the converter in Figure 6. A 4V–15V INPUT, –5V OUTPUT INVERTING CONVERTER 75k 75 50 A 9V–36V INPUT, 24V OUTPUT SEPIC CONVERTER Figure 8. A simple and compact 4V–15V input, –5V output inverting converter with shortcircuit protection 80 55 is an advantage for applications requiring the output to be disconnected from the input source when the circuit is shut down. This design can sustain an indefinite output short-circuit condition due to its topology and the frequency foldback feature. Figure 7 shows the efficiency of this converter at different input voltages. CIN 10µF 25V X5R VIN = 36V 85 60 output short-circuit protection. Each IC’s FBX feature allows the flyback converter to easily produce either positive or negative outputs. Figure 4 shows a LT3958 in a flyback converter that operates from a 12V to 45V input and delivers 2A to a –5V output. Figure 5 shows the efficiency of this converter at different input voltages. CDC 10µF, 25V X5R VIN = 24V VIN = 9V 90 20k CIN, COUT, CDC: MURATA GRM32ER71H475KA88L D1: VISHAY 10BQ100 L1A, L1B: COILTRONICS DRQ127-330 L1A 95 CVCC 4.7µF 10V X5R 10nF VIN 4V TO 15V 100 CVCC 4.7µF 10V X5R 15.8k 85 EFFICIENCY (%) CIN 4.7µF 50V X5R EFFICIENCY (%) L1A • VIN 9V TO 36V CDC 4.7µF, 50V X5R VIN = 15V 80 75 70 65 60 CIN, CDC: MURATA GRM31ER61E106KA12 COUT: MURATA GRM32ER60J107ME20L D1: VISHAY 20BQ030 L1A, L1B: COILTRONICS DRQ127-100 55 0 0.2 0.4 0.6 0.8 1 ILOAD (A) 1.2 1.4 1.6 July 2011 : LT Journal of Analog Innovation | 11