Linear Design Features: LTC3852 - Theoretical Insights by Theo Phillips

Intermediate Bus Buck Regulator Maintains 5V Gate Drive During Automobile Cold Crank Conditions

Theo Phillips and Tick Houk

DC/DC converters in today’s automobiles often take their inputs from a loosely regulated 5V intermediate bus, instead of directly from the battery voltage (which can vary from 4V during cold crank to above 24V from double-battery jump-starts and other transients). Incorporating an intermediate voltage bus has a number of advantages, one of which is the expanded range of DC/DC converter options available to power downstream electronics. Power supply designers can choose from a wide variety of low dropout (LDO)

MODE/PLLIN

and switching post-regulators that have 6V absolute maximum input voltage ratings.

LTC3852 (BUCK REGULATOR)

Because typical post-regulator outputs are substantially lower than 5V—from 3.3V down VIN2

to 1V—they can continue to operate even as their inputs drop below the nominal 5V. 5V REGMODE/SYNC

FREQ/ DETECT

With this in mind, the ideal intermediate step-down regulator would continue to provide PLLFLTR VIN 38V MAX

PLL-SYNC

power even under cold crank conditions, where the battery voltage can drop below 5V. BOOST

BURSTEN

A synchronous buck regulator often Buck regulator controllers have tradition- voltage drops below the minimum speci- OSC

PULSE SKIP M1

makes the best intermediate bus con- ally provided gate drive power through fied VGS rating of the power MOSFETs.

verter for these applications because either an external 5V supply or through ON SWITCH

BEST OF BOTH WORLDS 5k

LOGIC

of its high efficiency over a wide input an onboard LDO. Both of these supply AND SENSE+

The LTC3852 is a synchronous step-

ICMP ANTI-

IREV VOUT

range when compared to linear regula- options can only step down an input volt-

SHOOT

down DC/DC controller with a low volt- SENSE–

THROUGH

tors. In this buck topology, 40V MOSFETs age, so the gate drive potential drops with

age charge pump designed to provide

are necessary to tolerate double bat- the input voltage, limiting the operating INTVCC

5V drive to external MOSFETs even when

tery and high voltage transients, so the range of the regulator. The ideal control-

the input drops below 5V. Figure 1 OV BG

regulator should provide the required ler would require no auxiliary supply

shows the block diagram of the IC. The CINTVCC

minimum 4.5V gate drive for the power and would provide the required 5V gate SLOPE COMPENSATION

MOSFETs during cold engine cranking. drive voltage even when the input supply GND2

PGOOD

INTVCC

Figure 1. The two core circuit blocks of the LTC3852: a step-down DC/DC controller UVLO

LTC3852 (CHARGE PUMP)

block and a charge pump doubler block, which allows the controller to continue running SOFT-START 100k AND

even when inputs drop below 5V.

SWITCH CONTROL ITHB

ON/OFF

UV VFB

1.2MHz OSCILLATOR

VPUMP

FAULT LOGIC

CPUMP

RC VIN2SLEEP

CHARGE 0.88V

VIN1 C+

PUMP RUN–

2.7V to 5.5V SS

0.8V VIN1 REF 2µA 1µA

TRACK/SS

CIN C–

0.4V CSS

22 | April 2011 : LT Journal of Analog Innovation