LTC3355
8
3355fb
For more information www.linear.com/LTC3355
PIN FUNCTIONS
PFI (Pin 1): Input to the Power-Fail Comparator. The input
voltage below which the PFOB pin indicates a power-fail
condition can be programmed by connecting this pin to
an external resistor divider between V
IN
and ground.
FB (Pin 2): Sets the V
OUT
voltage for both the buck and
boost voltage control loops via an external resistor divider.
The reference voltage is 0.8V.
MODE (Pin
3): This pin sets the buck and boost switch-
ing modes. A low is PWM mode, a high is Burst Mode
operation.
V
INS
(Pin 4): Input Current Limit Sense Voltage Pin. Con-
nect a sense resistor from V
INS
to V
IN
. Must be locally
bypassed with a low ESR ceramic capacitor. Connect to
V
IN
if input current limit is not needed.
V
IN
(Pin 5): Input Power Pin Supplies Current to the In-
ternal Regulator and Buck Power Switch. Must be locally
bypassed with a low ESR ceramic capacitor.
V
INM5
(Pin 6): This pin is used to filter an internal supply
regulator which generates a voltage of V
IN
– 4.65V. Con-
nect a 1µF ceramic capacitor from V
INM5
to V
IN
.
SW1 (Pin 7): Buck Output of the Internal Power Switch.
Connect this pin to
the catch diode and inductor. Minimize
trace area at this pin to reduce EMI.
EN_CHG (Pin 8): A high on this pin enables the superca-
pacitor charger.
CPGOOD (Pin 9): Open-drain output is high impedance
when the V
CAP
voltage is higher than 92.5% of the pro-
grammed voltage.
PFOB (Pin 10): Open Drain of the Power-Fail Comparator.
Pulled low and enables the boost converter when the
PFI
input has determined that the input supply has dropped out.
CFB (Pin 11): This pin is used to program the V
CAP
volt-
age via an external resistor divider. The reference voltage
is 0.8V.
I
CHG
(Pin 12): This pin programs the V
CAP
charge current
by connecting a resistor to ground.
RSTB (Pin 13): Open-drain reset output is high imped-
ance when the V
OUT
voltage is higher than 92.5% of the
programmed regulation voltage.
V
CAP
(Pin 14): This pin is the constant current, constant
voltage linear charger output and connects to the super-
capacitor.
V
OUT
(Pin 15): The Output Voltage Supply. The buck powers
this supply from V
IN
when the input voltage is present and
the boost powers this supply from V
CAP
when the input
voltage has dropped out.
SW2 (Pin 16, 17): Boost Output
of the Internal Power
Switch. Connect these pins to the rectifier diode and
inductor. Minimize trace area at these pins to reduce EMI.
INTV
CC
(Pin 18): This pin is used to filter an internal supply.
Connect a 1µF ceramic capacitor from this pin to ground.
INTV
CC
is 2.5V during start-up until V
OUT
exceeds 2.5V
then INTV
CC
follows V
OUT
.
V
CBST
(Pin 19): This pin is the output of the boost internal
error amplifier. The voltage on this pin controls the peak
switch current for the boost regulator. Connect an RC
series network from this pin to ground to compensate
the boost control loop.
I
BSTPK
(Pin 20): This pin programs the boost peak current
limit by connecting a resistor to ground.
GND (Exposed Pad Pin 21): Ground. The exposed pad must
be connected to
a continuous ground plane on the second
layer of the printed circuit board by several vias directly
under the part to achieve optimum thermal conduction.
