LINEAR 60V Step-Down DC DC Converter with Minimal 100µA Quiescent Current - Design Note 1007

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60V Step-Down DC/DC Converter Has Only 100µA Quiescent Current – Design Note 1007 Keith Szolusha

Introduction ideal for 14V and 42V (future) automotive applications High voltage bipolar monolithic step-down converters are which must meet both sub-4V cold crank and high input voltage transients (up to 60V). Additionally, high input

usually optimized for high efficiency at high output cur-

voltage and low quiescent current make this an ideal

rents. But the consumption of over 1m A quiescent current

solution for many 48V nonisolated telecom applications,

at zero load can be undesirable in battery-powered appli-

40V Fire Wire® peripherals and multiple source handheld

cations which operate with long periods of light load cur-

battery-powered applications with autoplug adaptors. The

rent. Although a shutdown function may reduce the

quiescent current in order to save battery life, it also drops LT1976 can survive load-dump input transients up to 60V that are common in these systems designed to run at a

the output voltage to zero. In systems where a regulated

fixed frequency of 200k Hz, the LT1976 can also be syn-

output voltage is always needed for light load applications

chronized at frequencies up to 700k Hz. The shutdown pin

such as system diagnostics (and ready-to-use load tran-

provides a 2.4V undervoltage lockout threshold as well as

sients), a solution with high efficiency at light load is needed.

a 1.3V threshold for micropower shutdown (<1µA). A

The LT®1976 is designed to optimize efficiency over all

single capacitor provides soft-start capability and limits

current levels, both high and low, with an extremely wide

inrush current and output voltage overshoot. A power

input voltage range. Micropower bias current and Burst

good flag and power good comparator provides the sys-

Mode® operation enable it to consume a mere 100µA at

tem with an indication that either the output voltage, the

zero load and 12V input. The high efficiency bipolar NPN

input voltage or some other line is good or bad. The LT1976

power switch (0.2Ω) provides up to 90% efficiency at load

is provided in a small 16-pin TSSOP thermally enhanced

current up to 1.25A.

package to provide optimal thermal performance.

The LT1976 is a 1.5A monolithic buck switching regula-

, LTC, LT and Burst Mode are registered trademarks of Linear Technology tor. Its 3.3V to 60V input voltage range makes the LT1976 Corporation. Fire Wire is a registered trademark of Apple Corporation.

VIN BOOST 3.3V

4V TO 60V

100V SW CER LT1976 0.1µF

10MQ100N

VC CSS

VBIAS

12.4k FB 1%

CT 100µF

1µF SYNC 6.3V

GND PG TANT 100k 1%

DN1007 F01

Figure 1. Typical LT1976 Automotive DC/DC Converter Delivers 1A at 3.3V