LT3641
8
3641fa
PIN FUNCTIONS
FB2 (Pin 1/Pin 26): The low voltage converter regulates the
FB2 pin to 600mV. Connect the feedback resistor divider
tap to this pin to set output voltage.
PGOOD2 (Pin 2/Pin 27): Open-drain logic output that starts
to sink current when FB2 is in regulation.
EN/UVLO (Pin 3/Pin 28): Pull this pin below 0.3V to shut
down the LT3641. The 1.26V threshold can function as an
accurate undervoltage lockout, preventing the LT3641 from
operating until V
IN
voltage has reached the programmed
level.
SYNC (Pin 4/Pin 1): Driving the SYNC pin with an external
clock signal synchronizes both converters to the applied
frequency. The lowest external clock frequency should be
20% higher than the internal oscillator frequency.
SS1 (Pin 5/Pin 2): The SS1 pin sets the FB1 voltage ex-
ternally between 0V and 1.265V, providing soft-start and
tracking. Tie this pin 1.5V or higher to use the internal
1.265V reference. A capacitor to ground at this pin sets the
ramp time to regulated output voltage for the high voltage
converter. Use a resistor divider to track another supply.
FB1 (Pin 6/Pin 3): The high voltage converter regulates the
FB1 pin to 1.265V. Connect the feedback resistor divider
tap to this pin to set output voltage.
RT (Pin 7/Pin 4): Oscillator Resistor Input. Connecting a
resistor to ground from this pin sets the internal oscillator
frequency.
RST2 (Pin 8/Pin 5): Open-drain logic output that remains
asserted for the period set by the CPOR pin capacitor after
FB2 goes above 550mV.
RST1 (Pin 9/Pin 6): Open-drain logic output that remains
asserted for the period set by the CPOR pin capacitor after
FB1 goes above 1.165V.
WDO (Pin 10/Pin 7): Open-drain logic output that remains
asserted for the period set by the CPOR pin capacitor if
WDE is enabled and WDI pin is not driven by an appropri-
ate signal.
CWDT (Pin 11/Pin 8): Connect a capacitor to ground at
this pin to set watchdog timer.
CPOR (Pin 12/Pin 9): Connect a capacitor to ground at this
pin to set the power-on reset timer and WDO output timer.
WDE (Pin 13/Pin 10): Watchdog Enable Pin.
WDI (Pin 14/Pin 11): The WDI pin receives watchdog
signals from a microprocessor.
GND (Pins 15, 16, 23, 26, Exposed Pad Pin 29/Pins 12,
13, 20, 23, Exposed Pad Pin 29): Ground. These pins
must be soldered to PCB ground.
NC (Pin 17/Pin 14): Not Connected. This pin can be con-
nected to ground.
DA (Pin 18/Pin 15): The DA pin is used to sense the catch
diode current for current limit and protection. Connect this
pin to catch diode anode.
SW1 (Pin 19/Pin 16): Output of the High Voltage Internal
Power Switch. Connect this pin to the inductor and catch
diode cathode.
SW (Pin 20/Pin 17): The SW pin is used to charge the
boost capacitor. Connect this pin to the boost capacitor.
BST (Pin 21/Pin 18): The BST pin is used to provide a drive
voltage, higher than V
IN
pin voltage, to the high voltage
channel internal power switch. Connect an external boost
diode to this pin.
V
IN
(Pin 22/Pin 19): The V
IN
pin supplies current to
the LT3641’s internal circuitry and to the high voltage
channel internal power switch. This pin must be locally
bypassed.
V
IN2
(Pin 24/Pin 21): The V
IN2
pin supplies current to the
internal power MOSFET of the low voltage converter and
to the LT3641’s internal circuitry when V
IN2
is above 3V.
SW2 (Pin 25/Pin 22): Switch Node of the Low Voltage
Converter. Connect this pin to an inductor.
EN2 (Pin 27/Pin 24): Low Voltage Converter Enable Pin. The
enable threshold is 100mV below FB1 target voltage. The
disable threshold has 50mV hysteresis. The accurate threshold
can function as an accurate undervoltage lockout.
SS2 (Pin 28/Pin 25): The SS2 pin sets the FB2 voltage
externally between 0V and 0.6V, providing soft-start and
tracking. Tie this pin 0.8V or higher to use the internal
0.6V reference. A capacitor to ground at this pin sets the
ramp time to regulated output voltage for the low voltage
converter. Use a resistor divider to track another supply.
(FE/QFN)
