COILTRONICSDR125-221-R,or datasheet(6/16 Pages) TI1

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F =

VOUT

2 x L x 1.04 x 1020

RL x (RT)

VIN

VCC

RTN

ON TIMER

DRIVER

Vin

BST

LEVEL

SHIFT

THERMAL

SHUTDOWN

FINISH

START

RCL

FINISH

START

VCC

UVLO

REGULATION

COMPARATOR

FINISH

START

REGULATOR

CURRENT LIMIT

OFF TIMER

SHUTDOWN

BUCK

SWITCH

CURRENT

SENSE

GND

SM74301

7V BIAS

BYPASS

SWITCH

2.875V

VINSENSE

6V to 95V

Input

RT/SD

0.7V

300 ns MIN

OFF TIMER

CLR

SET

0.51A

VOUT

RFB1

FB2

RCL

2.5V

OVER-VOLTAGE

COMPARATOR

CHARGE

PRE -

SM74301

SNVS719A – JULY 2011

www.ti.com

Block Diagram

FUNCTIONAL DESCRIPTION

The SM74301 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-8 and the thermally enhanced WSON-8 packages. The

regulator is based on a control scheme using an on-time inversely proportional to VIN. The control scheme

requires no loop compensation. Current limit is implemented with forced off-time, which is inversely proportional

to VOUT. This scheme ensures short circuit control while providing minimum foldback.

The SM74301 can be applied in numerous applications to efficiently regulate down higher voltages. This

regulator is well suited for high voltage PV panels, 48 Volt Telecom, and the 42V Automotive power bus ranges.

Features include: Thermal Shutdown, VCC under-voltage lockout, Gate drive under-voltage lockout, Max Duty

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

Control Circuit Overview

The SM74301 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with

line voltage (VIN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback

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

for a fixed time determined by the line voltage and a programming resistor (RT). Following the ON period the

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 SM74301 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:

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