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FR9808 Datasheet(PDF) 9 Page - Fitipower Integrated Technology Inc.

Part No. FR9808
Description  21V, 4A, 500KHz Synchronous PWM-Buck DC/DC Converter
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Maker  FITIPOWER [Fitipower Integrated Technology Inc.]
Homepage  http://www.fitipower.com/en_US/index.asp
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 9 page
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9
FR9808-1.3-OCT-2012
FR9808
85T
fitipower integrated technology lnc.
Function Description
Introduction
FR9808
is
a
constant-frequency
current-mode
step-down
synchronous
DC/DC
converter.
It
regulates input voltage from 4.75V to 21V and
provides 4A of continuous load current.
To achieve bias power supply, FR9808 contains an
internal voltage regulator to support the internal
circuits.
For applications in which VIN is less than
4.5V, output decreases, and a 0.1µF ceramic
capacitor is required for decoupling.
If VIN is
greater than 4.5V, the output of the regulator is in full
regulation.
The error amplifier compares the FB voltage with the
internal 0.805V reference.
And the voltage of error
amplifier output is compared to the switch current to
control the RS flip-flop.
At the beginning of each
clock cycle, the high-side NMOS turns on when the
oscillator sets the RS flip-flop, and turns off when
current comparator resets the RS flip-flop.
Then the
low-side NMOS turns on until the clock period ends.
Internal Soft-Start
The internal soft-start function is used to eliminate
the output voltage overshooting during start-up.
When the chip initiates, the internal reference voltage
rises slowly to 0.805V and the internal COMP signal
rises slowly to achieve output voltage.
The
soft-start time is approximate
600μs.
EN/SYNC
VIN
R3
EN/SYNC
FR9808
The FR9808 EN/SYNC pin provides digital control to
turn on/turn off the regulator.
For automatic
start-up, tie EN/SYNC and VIN with a resister, as
shown in the figure.
The recommended value of R3
is
100KΩ. The FR9808 can be synchronized with
an external clock from 300KHz to 800KHz by using
the EN/SYNC pin.
Device Protection:
1. Input Under Voltage Lockout
When the power of FR9808 is on, the internal
circuits are held inactive until VIN exceeds the input
UVLO threshold voltage.
The regulator is disabled
when VIN falls below the input UVLO threshold
voltage.
The hysteretic of the UVLO comparator is
200mV.
2. Short Circuit Protection
The FR9808 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 40% of the
reference, the oscillator frequency will be reduced
to
150KHz
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.
3. Over Current Protection
The FR9808 over current protection function is
implemented 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.
4. Over Temperature Protection
The FR9808 incorporates an over temperature
protection circuit to protect itself from overheating.
When the junction temperature exceeds the thermal
shutdown threshold temperature, the regulator will
shutdown.
When the junction temperature is less
than the recovery threshold temperature, the chip
will re-enable.




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