FR9801 datasheet(10/14 Pages) FITIPOWER

10 / 14 page

FR9801-0.5-JUL-2013

FR9801

85T

fitipower integrated technology lnc.

Function Description

The

FR9801

high

efficiency,

internal

compensation and constant frequency current mode

synchronous

step-down

DC/DC

converter.

regulates input voltage from 4.5V to 21V and down to

output voltage as low as 0.8V.

Control Loop

Under normal operation, the output voltage is sensed

by FB pin through a resistive voltage divider and

amplified through the error amplifier.

The voltage of

error amplifier output is compared to the switch

current to control the RS latch.

At the beginning of

each clock cycle, the high-side NMOS turns on when

the oscillator sets the RS latch, and turns off when

current comparator resets the RS latch.

Then the

low-side NMOS will turn on until the clock period

ends.

Enable

The FR9801

pin provides digital control to

turn on/off the regulator.

When the voltage of

exceeds the threshold voltage, the regulator

will start the soft start function.

If the

voltage is below the shutdown threshold voltage, the

regulator will turn into the shutdown mode and the

shutdown current will be

smaller than 1μA.

For

auto start-up operation, connect

to VIN

through a 100KΩ resistor.

Soft Start

The FR9801 employs internal soft start function to

reduce input inrush current during start up.

The

typical value of internal soft start time is 1ms.

Output Over Voltage Protection

When the FB pin voltage exceeds 1.4V, the output

over voltage protection function will be triggered and

turn off the high-side/low-side MOSFET.

Input Under Voltage Lockout

When the FR9801 is power on, the internal circuits will

be held inactive until VIN voltage exceeds the input

UVLO threshold voltage.

And the regulator will be

disabled when VIN is below the input UVLO threshold

voltage.

The hysteretic of the UVLO comparator is

400mV (typ).

Short Circuit Protection

The FR9801 provides short circuit protection

function to prevent the device damaged from short

condition.

When the short condition occurs and

the feedback voltage drops lower than 0.4V, the

oscillator frequency will be reduced to 160KHz to

prevent the inductor current increasing beyond the

current limit.

In the meantime, the current limit will

also be reduced to lower the short current.

Once

the short condition is removed, the frequency and

current limit will return to normal.

Over Current Protection

The FR9801 over current protection function is

implemented by using cycle-by-cycle current limit

architecture.

The inductor current is monitored by

measuring the high-side MOSFET series sense

resistor voltage.

When the load current increases,

the inductor current will also increase.

When the

peak inductor current reaches the current limit

threshold, the output voltage will start to drop.

When the over current condition is removed, the

output voltage will return to the regulated value.

Over Temperature Protection

The FR9801 incorporates an over temperature

protection circuit to protect itself from overheating.

When the junction

temperature

exceeds

the

thermal

shutdown

threshold

temperature,

the

regulator will be shutdown.

And the hysteretic of

the over temperature protection is 60°C (typ).

Internal Compensation Function

The stability of the feedback circuit is controlled by

internal compensation circuits.

This internal

compensation function is optimized for most

applications, and this function can reduce external

R, C components.

FR9801 Datasheet(PDF) 10 Page - Fitipower Integrated Technology Inc.

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