Datasheet

6
LTC3440
3440fb
R
T
(Pin 1): Timing Resistor to Program the Oscillator
Frequency. The programming frequency range is 300kHz
to 2MHz.
f
R
Hz
OSC
T
=
610
10
•
MODE/SYNC (Pin 2): MODE/SYNC = External CLK : Syn-
chronization of the internal oscillator. A clock frequency of
twice the desired switching frequency and with a pulse
width between 100ns and 2μs is applied. The oscillator
free running frequency is set slower than the desired
synchronized switching frequency to guarantee sync. The
oscillator R
T
component value required is given by:
R
f
T
SW
=
810
10
•
where f
SW
= desired synchronized switching frequency.
SW1 (Pin 3): Switch Pin Where the Internal Switches A
and B are Connected. Connect inductor from SW1 to SW2.
An optional Schottky diode can be connected from SW1 to
ground. Minimize trace length to keep EMI down.
SW2 (Pin 4): Switch Pin Where the Internal Switches C
and D are Connected. For applications with output volt-
UU
U
PI FU CTIO S
ages over 4.3V, a Schottky diode is required from SW2 to
V
OUT
to ensure the SW pin does not exhibit excess voltage.
GND (Pin 5): Signal and Power Ground for the IC.
V
OUT
(Pin 6): Output of the Synchronous Rectifier. A filter
capacitor is placed from V
OUT
to GND.
V
IN
(Pin 7): Input Supply Pin. Internal V
CC
for the IC. A
ceramic bypass capacitor as close to the V
IN
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 com-
mand signal to this pin will provide a soft-start function by
limiting the rise time of the V
C
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.
V
C
(Pin 10): Error Amp Output. A frequency compensation
network is connected from this pin to the FB pin to
compensate the loop. See the section “Compensating the
Feedback Loop” for guidelines.
Exposed Pad (Pin 11, DFN Package Only): Ground. This
pin must be soldered to the PCB and electrically connected
to ground.