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FR9886A Datasheet(PDF) 9 Page - Fitipower Integrated Technology Inc.

Part No. FR9886A
Description  21V, 2A, 1.2MHz Synchronous Step-Down 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
FR9886A-0.3-SEP-2011
FR9886A
85T
fitipower integrated technology lnc.
Function Description
The FR9886A is a high efficiency, high frequency,
internal compensation current mode synchronous
step-down
DC/DC
converter.
It
supports
2A
continuous output current and regulates output
voltage low to 0.925V.
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 turns on until the
clock period ends.
Enable
The FR9886A
S
N pin provides digital control to
turn on/turn off the regulator.
When the voltage of
S
N exceeds the threshold voltage, the regulator
will start the soft start function.
If the S
N pin
voltage is below than 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 S
N to
VIN through a 100KΩ resistor.
Soft Start
The FR9886A employs adjustable soft start function
to reduce input inrush current during start up.
When the device turns on, a 6
μA current begins
charging the capacitor which is connected from SS
pin to GND.
The equation for the soft start time is
shown as below:
 
 
 
SS
FB
SS
SS
C
nF
V
T
ms =
I
uA
The VFB voltage is 0.925V and the ISS current is 6μA.
If a 0.1μF capacitor is connected from SS pin to
GND, the soft start time will be 15ms.
Output Over Voltage Protection
When the FB pin voltage exceeds 1.5V, the output
over voltage protection function will be triggered and
turn off the high-side/low-side MOSFET.
Input Under Voltage Lockout
When the FR9886A 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 400mV (typ).
Short Circuit Protection
The FR9886A 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 0.4V, the
oscillator frequency will be reduced to 170KHz 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 FR9886A 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.
Over Temperature Protections
The FR9886A 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 50°C (typ).
Internal Compensation Function
The stability of the feedback circuit is controlled
through
internal
compensation
circuits.
This
internal compensation function is optimized for most
applications and this function can reduce external R,
C components.




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