FR9855 datasheet(11/16 Pages) FITIPOWER

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FR9855-Preliminary 0.3-JAN-2016

FR9855

85T

Function Description

The FR9855 is a synchronous step-down DC/DC

converter with fast constant on time (FCOT) mode

control. It has integrated high-side (70

mΩ, typ and

low-side (38

mΩ, typ) power switches, and provides

5.5A continuous load current. It regulates input

voltage from 4.5V to 18V, and down to an output

voltage as low as 0.765V. Using FCOT control

scheme provides fast transient response, which can

minimize the component size without additional

external compensation network.

Enable/Mode

The FR9855

H N pin includes enable and mode

function. Enable function provides digital control to

turn on/off the converter. When the voltage of

H N

exceeds the threshold voltage, the converter starts

the soft start function. If the

H N pin voltage is

below than the shutdown threshold voltage, the

converter will turn into the shutdown mode and the

shutdown current will be smaller than 1μA. The

mode function can be selected in PWM or power

saving mode. The mode setting can refer to

following table.

Mode

>3.5V

Power Saving Mode

1.5V~2.5V

PWM

Shutdown

For auto start-up operation, connect

H N to VIN

through a 100k

Ω resistor, and the converter can

automatically enter power saving mode.

Soft Start

The FR9855 employs adjustable soft start function

to reduce input inrush current during start up.

When the device turns on, a 6μA current begins

charging the capacitor which is connected from SS

pin to GND. The equation for the soft start time is

shown as below:

μA

The VFB voltage is 0.765V and the ISS current is 6μA.

If a 100nF capacitor is connected from SS pin to

GND, the soft-start time will be 12.75ms.

Input Under Voltage Lockout

When the FR9855 is power on, the internal circuits

are 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 350mV (typ).

Over Current Protection

The FR9855 over current protection function is

implemented

using

cycle-by-cycle

current

limit

architecture. The inductor current is monitored by

Low-side

MOSFET.

When

the

load

current

increases, the inductor current also increases.

When the valley inductor current reaches the current

limit threshold, the output voltage starts to drop.

When the over current condition is removed, the

output voltage returns to the regulated value.

Short Circuit Protection

The

FR9855

provides

short

circuit

protection

function to prevent the device damage from short

condition. When the short condition occurs and the

feedback voltage drops lower than 0.33V, the

oscillator frequency will be reduced naturally and

hiccup mode will be triggered to prevent the inductor

current increasing beyond the current limit. Once the

short condition is removed, the frequency will return

to normal.

Over Temperature Protection

The FR9855 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 30°C (typ).

Power Good Signal Output (PG)

PG pin is an open-drain output and requires a pull

up resistor. PG is actively held low in soft-start,

standby and shutdown. It is released when the

output

voltage

rises

above

90%

nominal

regulation point.

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