Copyright © 2016, Texas Instruments Incorporated

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

LM5008A

7V BIAS

BYPASS

SWITCH

RT

2.875V

VINSENSE

Q2

C1

C5

6V to 95V

Input

RT/SD

0.7V

300 ns MIN

OFF TIMER

CLR

R

SET

S

0.51A

C2

C3

VOUT

R3

RFB1

FB2

RCL

RCL

2.5V

OVER-VOLTAGE

COMPARATOR

CHARGE

PRE -

C4

8

LM5008A

SNVS583H – MARCH 2009 – REVISED OCTOBER 2018

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Product Folder Links: LM5008A

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7 Detailed Description

7.1 Overview

The LM5008A regulator is an easy-to-use buck DC-DC converter that operates from 6-V to 95-V supply voltage.

The device is intended for step-down conversions from 12-V, 24-V, and 48-V unregulated, semi-regulated and

fully-regulated supply rails. With integrated buck power MOSFET, the LM5008A delivers up to 350-mA DC load

current with exceptional efficiency and low input quiescent current in a very small solution size.

Designed for simple implementation, a nearly fixed-frequency, constant on-time (COT) operation with

discontinuous conduction mode (DCM) at light loads is ideal for low-noise, high current, fast transient load

requirements. Control loop compensation is not required reducing design time and external component count.

The LM5008A incorporates other features for comprehensive system requirements, including VCC undervoltage

lockout (UVLO), gate drive undervoltage lockout, maximum duty cycle limiter, intelligent current limit off-timer, a

pre-charge switch, and thermal shutdown with automatic recovery. These features enable a flexible and easy-to-

use platform for a wide range of applications. The pin arrangement is designed for simple and optimized PCB

layout, requiring only a few external components.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Control Circuit Overview

The LM5008A 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 (2.5 V). 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 remains off for at least the minimum off-timer period of 300 ns. If FB is still below the reference at that

time, the switch turns on again for another on-time period. This continues until regulation is achieved.