Step-Down Synchronous Controller Operates from Inputs Down to 2.2V Design Note 443 Wei Gu Introduction Many telecommunications and computing applications need high efficiency step-down DC/DC converters that can operate from a very low input voltage. The high output power synchronous controller LT3740 is ideal for these applications, converting input supplies ranging from 2.2V to 22V to outputs as low as 0.8V with load currents from 2A to 20A. Applications include distributed power systems, point-of-load regulation and conversion of logic supplies. A major challenge in designing a step-down converter with low VIN is that the gate voltage for the N-channel MOSFETs is not readily available. The LT3740 solves this problem by integrating a DC/DC step-up converter for generating its own MOSFET gate drive voltage with a small inexpensive external inductor. This function permits the use of inexpensive off-the-shelf 5V gatedrive MOSFETs, offering up to 3% higher efficiency than sub-logic gate-drive MOSFETs and eliminating the need for a secondary supply. The LT3740 operates at a fixed 300kHz frequency and employs valley current mode control to deliver excellent transient response and very low on-times. Furthermore, a power good signal is available to monitor the output voltage. The tracking capability built into XFER can be used to implement control of the output voltage when ramping up and ramping down. If less than 0.8V is applied to XFER, the LT3740 uses this voltage as its reference for regulation. “Dying Gasp” Applications The LT3740’s low VIN feature makes it a good fit in “dying gasp” applications for high reliability computers. In these systems, when the power gets cut, a large bank of electrolytic capacitors at the input charged to 12V or so, serve as input to the power controller. The LT3740 can operate down to 2.2V, which allows it to run longer from the input capacitors’ charged voltage than a controller requiring a higher input voltage. This effectively reduces the input capacitance requirements, which can save significant board space. Figure 1 shows a typical application circuit. The LT3740 circuit, after its input power gets cut, is able to supply the , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 12V L2 22μH LT3740 SWB BGDP VIN CIN 3300μF CB 1μF BIAS RANGE TGATE SHDN M1 PH8230E L1 0.9μH SW VOUT 1.5V 10A 100k XREF BGATE 1μF M2 PH3830L D1 B320A 100μF ×3 SN+ RS 7mΩ PGOOD SN– VC 69.8k 12k GND 100pF FB 2200pF 78.7k DN443 F01 Figure 1. “Dying Gasp” Application Circuit 06/08/443 load for 40ms as shown in Figure 2. In comparison, the output voltage would drop out after 25ms with a controller that shuts down when the input reaches 7V. Generate a Negative Voltage from a Low Positive VIN Figure 3 shows a 2.4V to 14V input to –3.3V at 3A output converter. The LT3740 works particularly well in this application due to its wide input voltage range and ability to operate down to 2.2V. The LT3740 also operates with synchronous rectification, which allows the use of high efficiency MOSFETs, instead of less efficient switching diodes. Wide Input Voltage Range The LT3740 offers high efficiency over a wide input voltage range (2.2V to 22V) and produces output voltages as low as 0.8V. The LT3740 employs valley current mode control to deliver excellent transient response and very low on-times. Figure 4 illustrates low duty cycle waveforms for a 22V input, 0.8V output application at a fixed frequency of 300kHz. Conclusion The LT3740 can operate from low input voltages, providing a space- and cost-saving solution over a wide input voltage range. The LT3740 is a versatile platform on which to build DC/DC converter solutions that use few external components and maintain high efficiencies over wide load ranges. The integrated step-up regulator facilitates true single-supply operation with an input voltage as low as 2.2V. VSW 5V/DIV IOUT = 3A VIN 10V/DIV VOUT 1V/DIV 1μs/DIV 10ms/DIV DN443 F02 DN443 F04 Figure 4. LT3740 Low Duty Cycle Waveforms VIN: 12V VOUT: 1.5V IOUT: 4A Figure 2. Input and Output Voltages Waveforms When Input Power is Removed VIN 2.4V - 14V L2 22μH LT3740 SWB BGDP VIN CIN 100μF CB 1μF VOUT BIAS RANGE TGATE SHDN M1 PH8230E L1 0.9μH SW 100k XREF BGATE 1μF SN M2 PH3830L D1 B320A 100μF ×3 + RS 7mΩ PGOOD – SN PGND VC 267k 12k GND 100pF FB 2200pF 80.6k VOUT –3.3V 3A DN443 F03 Figure 3. A Positive to Negative Converter Data Sheet Download For applications help, call (408) 432-1900, Ext. 3565 www.linear.com Linear Technology Corporation dn443 LT/TP 0608 392K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008