Q

Q

FB

VIN

VCC

SW

RTN

ON TIMER

DRIVER

Vin

BST

LEVEL

SHIFT

THERMAL

SHUTDOWN

FINISH

START

RCL

FINISH

START

RT

L1

R

VCC

UVLO

FB

D1

UVLO

REGULATION

COMPARATOR

SD

FINISH

START

REGULATOR

CURRENT LIMIT

OFF TIMER

SHUTDOWN

GD

BUCK

SWITCH

CURRENT

SENSE

GND

7V BIAS

BYPASS

SWITCH

2.875V

Q2

C1

C5

6V to 95V

Input

RT/SD

0.7V

300 ns MIN

OFF TIMER

CLR

R

SET

S

C2

C3

R3

FB2

2.5A

OVER-VOLTAGE

COMPARATOR

CHARGE

C4

SM72485

0.3A

SM72485

SNVS697D – JANUARY 2011 – REVISED APRIL 2013

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BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

The SM72485 Step Down Switching Regulator features all the functions needed to implement a low cost,

efficient, Buck bias power converter. This high voltage regulator contains a 100 V N-Channel Buck Switch, is

easy to implement and is provided in the VSSOP and the thermally enhanced WSON package. The regulator is

scheme ensures short circuit control while providing minimum foldback.

The SM72485 can be applied in numerous applications to efficiently regulate down higher voltages. This

regulator is well suited for high voltage PV panel junction boxes, 48 Volt Telecom and the new 42V Automotive

lockout, Max Duty Cycle limit timer, intelligent current limit off timer, and a pre-charge switch.

Control Circuit Overview

The SM72485 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with

(FB) compared to an internal reference (2.5V). If the FB level is below the reference the buck switch is turned on

switch will remain off for at least the minimum off-timer period of 300ns. If FB is still below the reference at that

time the switch will turn on again for another on-time period. This will continue until regulation is achieved.

The SM72485 operates in discontinuous conduction mode at light load currents, and continuous conduction

mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero

and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-

time period starts when the voltage at FB falls below the internal reference - until then the inductor current

remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with

load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce

with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows:

6

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