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FAN4855 Rev. 1.0.1

Block Diagram

Functional Description

Boost Regulator

FAN4855 is an adjustable boost regulator that combines

variable ON and minimum OFF architecture with syn-

chronous rectification. Unique control circuitry provides

high-efficiency power conversion for both light and heavy

loads by transitioning between discontinuous and contin-

uous conduction mode based on load conditions. There

is no oscillator; a constant-peak-current limit of 0.8A in

the inductor allows the inductor current to vary between

this peak limit and some lesser value. The switching fre-

quency depends upon the load, the input and output volt-

age ranging up to 430kHz.

nals to regulator to deliver charge to the output by trig-

gering the Variable On-Time One Shot. One Shot

generates a pulse at the gate of the Power NMOS tran-

sistor Q1. This transistor will charge the Inductor L1 for

When the one–shot times out, the Q1 transistor releases

tarly charge the output through the body diode of the

transistor Q2. But, as the voltage across the Q2 changes

polarity, its gate will be driven low by the Synchronous

Rectifier Control Circuit (SRC), causing Q2 to short out

its body diode. The inductor then delivers the charge to

the load by discharging into it through Q2.

Under light load conditions, the amount of energy deliv-

ered in this single pulse satisfies the voltage-control loop,

and the converter does not command any more energy

pulses until the output drops again below the lower-volt-

age threshold. Under medium and heavy loads, a single

energy pulse is not sufficient to force the output voltage

above its upper threshold before the minimum off time

has expired and a second charge cycle is commanded.

Since the inductor current has not reached zero in this

case, the peak current is greater than the previous value

at the end of the second cycle. The result is a ratcheting

of inductor current until either the output voltage is satis-

fied, or the converter reaches its set current limit.

Variable On-Time One Shot will be turned ON again and

the process repeats.

The output capacitor of the converter filters the variable

component, limiting the output voltage ripple to a value

determined by its capacitance and its ESR.

LBO

SHDN

Start-Up

Minimum

Off-Time

Logic

Variable

On-Time

One Shot

Current

Limit

Control

Synchronous

Rectifier

Control

47

2

6

5

8

3

1

GND

N1

LBI

0.39V

A3

Control

Logic

A2

A1

–

+

–

+

–

+

SHDN

I

L PEAK

()

T

ON

V

IN

×

L

1

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