LINEAR Step-Down Synchronous Controller - Input to 2.2V Operation Design Note 443 by Wei Gu Owner's Manual & Specifications

Step-Down Synchronous Controller Operates from Inputs Down to 2.2V Design Note 443 Wei Gu

Introduction transient response and very low on-times. Furthermore, Many telecommunications and computing applications a power good signal is available to monitor the output need high effi ciency step-down DC/DC converters that voltage. The tracking capability built into XFER can be can operate from a very low input voltage. The high used to implement control of the output voltage when output power synchronous controller LT3740 is ideal ramping up and ramping down. If less than 0.8V is applied for these applications, converting input supplies ranging to XFER, the LT3740 uses this voltage as its reference from 2.2V to 22V to outputs as low as 0.8V with load for regulation.

currents from 2A to 20A. Applications include distributed

“Dying Gasp” Applications

power systems, point-of-load regulation and conversion

The LT3740’s low VIN feature makes it a good fi t in “dy-

of logic supplies.

ing gasp” applications for high reliability computers. In

A major challenge in designing a step-down converter these systems, when the power gets cut, a large bank of with low VIN is that the gate voltage for the N-channel electrolytic capacitors at the input charged to 12V or so, MOSFETs is not readily available. The LT3740 solves serve as input to the power controller. The LT3740 can this problem by integrating a DC/DC step-up converter operate down to 2.2V, which allows it to run longer from for generating its own MOSFET gate drive voltage with the input capacitors’ charged voltage than a controller a small inexpensive external inductor. This function requiring a higher input voltage. This effectively reduces permits the use of inexpensive off-the-shelf 5V gate- the input capacitance requirements, which can save drive MOSFETs, offering up to 3% higher effi ciency than signifi cant board space. sub-logic gate-drive MOSFETs and eliminating the need

Figure 1 shows a typical application circuit. The LT3740

for a secondary supply.

circuit, after its input power gets cut, is able to supply the

The LT3740 operates at a fi xed 300k Hz frequency and

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. employs valley current mode control to deliver excellent All other trademarks are the property of their respective owners.

VIN 12V

22μH LT3740 CIN

SWB BGDP 1μF

VIN BIAS

RANGE

TGATE PH8230E

SW L1 0.9μH VOUT

BGATE 100μF

1μF PH3830L B320A

RS PGOOD 7mΩ

VC SN– 69.8k

GND FB

DN443 F01

Figure 1. “Dying Gasp” Application Circuit