FB

VIN

VCC

SW

RT

Regulator

DRIVER

BOOT

LEVEL

SHIFT

THERMAL

SHUTDOWN

UVLO

ERROR

AMP

DIS

COMP

TRACK

SAMPLE

and

HOLD

CLK

GND

CLK

CSG

CS

CLK

RAMP GENERATOR

+

SHUTDOWN

PWM

I- LIMIT

OUT

RAMP

SS

VIN

STANDBY

5 uA

1.2

VIN

EN

Ir

HG

CLK

S

R

Q

Q

CLK

FREQUENCY

DITHERING

STANDBY

SYNC

DITHER

LM5088 -1 ONLY

OSCILLATOR

HICCUP

RESTART

LOGIC

Rs

DITHER

RES

Q

L

D

VOUT

UVLO

MINIMUM

OFF- TIME

LOGIC

7.7V

HICCUP RESTART

LM5088 - 2 ONLY

5V

A = -10

1.2V

0.9V

0.4V

5V

11 uA

LM25088

VIN (4.5V-42V)

1.205V

LM25088

SNVS609G – DECEMBER 2008 – REVISED MARCH 2011

www.ti.com

Block Diagram

Figure 15. Block Diagram

DETAILED OPERATION

The LM25088 Wide Input Range Buck Controller features all the functions necessary to implement an efficient

high voltage step-down converter using a minimum number of external components. The control method is

based on peak current mode control utilizing an emulated current ramp. Peak current mode control provides

inherent line voltage feed-forward, cycle-by-cycle current limiting and ease of loop compensation. The use of an

emulated control ramp reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable processing

of very small duty cycles necessary in high input voltage applications. The operating frequency is user

programmable from 50 kHz to 1 MHz. The LM25088-1 provides a ±5% frequency dithering function to reduce the

conducted and radiated EMI, while the LM25088-2 provides a versatile restart timer for overload protection.

Additional features include the low dropout bias regulator, tri-level enable input to control shutdown and standby

modes, soft-start, and voltage tracking and oscillator synchronization capability. The device is available in a

thermally enhanced HTSSOP-16 pin package.

The functional block diagram and typical application schematic of the LM25088 are shown in Figure 15. The

LM25088 is well suited for a wide range of applications where efficient step-down of high, unregulated input

voltage is required. The LM25088’s typical applications include Telecom, Industrial and Automotive.

High Voltage Low-Dropout Regulator

The LM25088 contains a high voltage, low-dropout regulator that provides the VCC bias supply for the controller

and the bootstrap MOSFET gate driver. The input pin (VIN) can be connected directly to an input voltage as high

as 42V. The output of the VCC regulator (7.8V) is internally current limited to 25 mA. Upon power up, the

regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds

the upper VCC UV threshold of 4.0V and the EN pin is greater than 1.2 Volts, the output (HG) is enabled and a

soft-start sequence begins. The output is terminated if VCC falls below its lower UV threshold (3.8V) or the EN

pin falls below 1.1V. When VIN is less than VCC regulation point of 7.8V, then the internal pass device acts as a

operating current of the controller. The required VCC capacitor value is dependant on system startup

characteristics with a minimum value no less than 0.1 µF.

8

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