LINEAR 60V, 3A Step-Down DC DC Converter with Low Dropout and 100µA Quiescent Current – Design Note 352

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60V, 3A Step-Down DC/DC Converter Has Low Dropout and

POW ER LOSS (W

100µA Quiescent Current – Design Note 352 Keith Szolusha

Introduction cycle, the low dropout of the switch maintains a regulated 3.3V output down to 4V input at all load currents. This is

High voltage bipolar monolithic step-down converters are

important for automotive cold-crank operation.

usually optimized for high efficiency at high output cur- rents, often at the expense of light load efficiency and

The LT3434’s 3.3V to 60V input voltage range makes it

operation near dropout. The problem is that a 2m A

EF FI CI EN CY ideal for 14V and 42V automotive battery-fed applications

quiescent current at zero load drains batteries in applica-

with both 4V cold crank and high input voltage transients

tions that spend long periods of time at minimum load

(up to 60V). The 3A switch current rating provides maxi-

current. One common solution for reducing quiescent

mum load currents of up to 2.5A. The LT3434 maintains

current and improving battery run-time is a shutdown

output regulation down to 4V input for 3.3V output and

function, but shutdown drops the output voltage to zero.

down to 3.3V input for 2.5V or lower output voltages.

Shutdown is not acceptable in systems where a constant

The high input voltage and low quiescent current make

regulated output voltage is required for light load applica-

this an ideal choice for many 48V nonisolated telecom

tions, system diagnostics and ready-to-use load tran-

applications, 40V Fire Wire® peripherals and multisource

sients. In some systems, a regulated output voltage is

battery-powered applications with autoplug adaptors.

needed at very low input voltage (low dropout). Simply

The LT3434 can survive load-dump input transients up to

shutting down the converter to zero output to avoid output

60V that are common in these systems.

droop is not always an option. SU PP LY UR RE NT (µ A)

It also includes other important features to shrink solu-

The LT®3434, 3A monolithic buck switching regulator is

tion size, simplify configuration and improve system

designed to optimize efficiency over all current and volt-

robustness:

age levels, both high and low. Micropower bias current and Burst Mode® operation enable it to consume merely

Fixed 200k Hz switching frequency provides low output

100µA at zero load and 12V input. The high efficiency

ripple, high efficiency and the ability to provide wide

bipolar NPN power switch (0.1Ω) provides up to 85%

, LTC, LT and Burst Mode are registered trademarks of Linear Technology efficiency at a 2A load current. Combined with high duty Corporation. Fire Wire is a registered trademark of Apple Computer, Inc.

DIODES INC. 1N4448W VOUT VIN = 12V VOUT = 3.3V

VIN VOUT = 5V

VIN BOOST 3.3V L = 33µH

4V TO 60V 33µH COUT = 100µF

0.68µF4.7µF 100V COOPER TDK C5750X7R2A475M UP3B-330

EFFICIENCY

CERAMIC SHDN SW

DIODES INC.

B360 VOUT = 3.3V

LT3434

TYPICAL POWER

VC VBIAS LOSS

FB 24k

4.7k 1% 6.3V ×2 10p F +CT SYNC PGFB KEMET

A700D107M006ATE

PG ALUMINUM

0.01µF0.01µF GND 100k 0.001

ORGANIC 0.0001 0.001 0.01 0.1

LOAD CURRENT (A) INPUT VOLTAGE (V)

DN352 F01

DN352 F02 DN352 F03

Figure 1. LT3434 Wide Input Voltage Range Figure 2. The Efficiency of Figure 1 Figure 3. With Zero Load DC/DC Converter Application to 3.3V Output at is Typically Greater Than 75%. At Current, the Supply Current to 2A Load Current Featuring Burst Mode Light Loads, Suppy Current is the LT3434 is Extremely Low, Operation for Light Load Operation and 4V Low Minimized with Burst Mode Typically Below 100µA (VIN ≥ Dropout Operation Operation 12V)