LTC3355_15 Datasheet, PDF (8/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

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.
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