FR9886 데이터 시트보기 (PDF) - Unspecified
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FR9886 Datasheet PDF : 14 Pages
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fitipower integrated technology lnc.
Function Description
FR9886
85T
The FR9886 is a high efficiency, internal
compensation, and constant frequency current mode
step-down synchronous DC/DC converter. It has
integrated high-side (120mΩ, typ) and low-side
(100mΩ, typ) power switches, and provides 2.5A
continuous load current. It regulates input voltage
from 4.5V to 23V, and down to an output voltage as
low as 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 FR9886 SHDN pin provides digital control to
turn on/turn off the regulator. When the voltage of
SHDN exceeds the threshold voltage, the regulator
starts the soft start function. If the SHDN 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 1uA. For
auto start-up operation, connect EN to VIN through a
100KΩ resistor.
Soft Start
The FR9886 employs adjustable soft start function to
reduce input inrush current during start up. When
the device turns on, a 6uA current begins charging
the capacitor which is connected from SS pin to
GND. The equation for the soft start time is shown
as below:
T SS
ms
=
CSS nF VFB
ISS uA
The VFB voltage is 0.925V and the ISS current is 6uA.
If a 0.1uF 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 FR9886 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 250mV
(typ).
Short Circuit Protection
The FR9886 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 110KHz 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 FR9886 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
also increases. When the peak 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.
Over Temperature Protection
The FR9886 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℃ (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.
FR9886- 1.0-APR-2011
9
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