3/13
Rev. B, Sep 2005
Pin Function
RT (Pin 1): Timing Resistors to program the Oscillator
Frequency. The programming frequency range is 300KHz
to 2MHz.
6 10
10
Fosc =
RT
Hz
MODE/SYNC (Pin2): MODE/SYNC = External CLK :
Synchronization
of
the
internal
oscillator.
A
clock
frequency of twice the desired switching frequency and
with a pulse width between 100ns and 2us is applied. The
oscillator free running frequency is set slower than the
desired synchronized switching frequency to guarantee
sync. The oscillator RT component value required is given
by:
8 10
10
RT =
Fsw
where fSW = desired synchronized switching frequency.
SW1 (pin 3): Switch Pin where the Internal Switches A
and B are connected. Connect inductor form Sw1 to Sw2.
An optional Schottky diode can be connected from Sw1 to
ground. Minimize trace length to keep EMI down.
SW2 (Pin4): Switch Pin Where the Internal Switches C &
D are connected. For applications with output voltages
over 4.3V, a Schottky diode is required from SW2 to VOUT
to ensure the SW pin does not exhibit excess voltage.
GND (Pin 5): Signal and Power Ground for the IC.
VOUT (Pin 6): Output of the Synchronous Rectifier. A filter
capacitor is placed from VOUT to GND.
VIN (Pin 7): Input Supply Pin. Internal Vcc for the IC. A
ceramic bypass capacitor as close to the VIN pin and GND
(Pin 5) is required.
SHDN/SS (Pin 8): combined Soft-Start and Shutdown.
Grounding this pin shuts down the IC. Tie to >1.5V to
enable the IC and >2.5V to ensure the error amp is not
clamped from soft-start. An RC from the shutdown
command signal to this pin will provide a soft-start function
by limiting the rise time of the Vc pin.
FB (Pin 9): Feedback Pin. Connect resistor divider tap
here. The output voltage can be adjusted from 2.5V to
5.5V. The feedback reference voltage is typically 1.22V.
Vc (Pin 10): Error Amp Output. A frequency compensation
network is connected from this pin to the FB pin to
compensate the loop.
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