FR9765 datasheet(8/11 Pages) FITIPOWER

8 / 11 page

FR9765-1.0-MAR-2012

FR9765

85T

fitipower integrated technology lnc.

Function Description

The FR9765 is a constant frequency current mode

step-down

asynchronous

DC/DC

converter.

regulates input voltage from 4.75V to 24V, down to

output voltage as low as 0.925V, and provides 3A of

continuous load current.

Control Loop

During normal operation, the output voltage is sensed

at FB pin by a resistive voltage divider and amplified

through the error amplifier.

The voltage of error

amplifier output pin

– COMP is compared to the

switch current to control the RS latch.

At each cycle,

the high side NMOS will be turned on when the

oscillator sets the RS latch and turned off when

current comparator resets the RS latch.

When the

load current increases, the FB pin voltage will drop

below 0.925V, and it will cause the COMP voltage

increasing until average inductor current arrives at

new load current.

Enable

The FR9765 EN pin provides digital control to turn

on/off the regulator.

When the voltage of EN

exceeds the threshold voltage, the regulator will start

the soft start function.

If the EN pin voltage is below

the threshold voltage, only the bandgap voltage will

be alive.

If the EN pin voltage is below the

shutdown threshold voltage, the regulator will be

disabled and turn into the shutdown mode.

Maximum Load Current

The maximum load current decreases at lower input

voltage because of large IR drop on the high side

switch

and

low

side

switch.

The

slope

compensation signal reduces the peak inductor

current as a function of the duty cycle to prevent

sub-harmonic oscillations when duty cycles are

greater than 50%.

Input Under Voltage Lockout

When the FR9765 is power on, the internal circuits

will be held inactive until VIN 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 400 mV.

Short Circuit Protection

The

FR9765

provides

short

circuit

protection

function to prevent the device damaged from short

condition.

When the output is short to ground, the

oscillator frequency will be reduced 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 Temperature Protection

The FR9765 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 shut down.

Compensation

The stability of the feedback circuit is controlled by

COMP pin.

The compensation value of the

application

circuit

optimized

for

particular

requirements.

If different conversions are required,

some of the components may need to be changed to

ensure stability.

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