LTC3440
6
3440fd
For more information www.linear.com/LTC3440
pin Functions
R
T
(Pin 1): Timing Resistor to Program the Oscillator
Frequency. The programming frequency range is 300kHz
to 2MHz.
f
OSC
=
6• 10
R
T
Hz
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
T
=
8 • 10
f
SW
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 voltages
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
OUT
= 1.22V • 1+
R2
⎛
⎝
⎜
⎞
⎠
⎟
V
C
(Pin 10): Error Amp Output. A frequency compensa-
tion 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.
