LTC3521 Datasheet, PDF (7/20 Pages) Linear Technology – Wide VIN, 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters

LTC3521
Pin Functions
in overtemperature shutdown, the part is in undervoltage
lockout, or if the SHDN3 pin is pulled low.
PGOOD2 (Pin 5/Pin 3): This pin is an open-drain output
which pulls low under any of the following conditions:
V_OUT2 buck output voltage is out of regulation, the part is
in overtemperature shutdown, the part is in undervoltage
lockout, or if the SHDN2 pin is pulled low.
PGOOD1 (Pin 6/Pin 4): This pin is an open-drain output
which pulls low under any of the following conditions:
V_OUT1 buck-boost output voltage is out of regulation,
the part is in overtemperature shutdown, the part is in
undervoltage lockout, the buck-boost converter is in current
limit, or if the SHDN1 pin is pulled low. See the Operation
section of this data sheet for details on the functionality
of this pin in PWM mode.
V_IN (Pin 7/Pin 5): Low Current Power Supply Connection
Used to Power the Internal Circuitry of the LTC3521. This
pin should be bypassed by a 4.7µF, or larger, ceramic
capacitor. The bypass capacitor should be placed as close
to the pin as possible and should have a short return path
to ground. Pins V_IN , PV_IN1, and PV_IN2 must be connected
together in the application circuit.
GND (Pin 8/Pin 6): Small Signal Ground. This pin is used as
a ground reference for the internal circuitry of the LTC3521.
PWM (Pin 9/Pin 7): Logic Input Used to Choose Between
Burst Mode Operation and PWM Mode for All Three Con-
verters. This pin cannot be left floating.
PWM = Low: Burst Mode operation is enabled on all
three converters. The buck converters will operate in
Burst Mode operation at light current but will automati-
cally transition to PWM operation at high currents. The
buck converters can supply maximum output current
(600mA) in this mode. The buck-boost converter will
operate in variable frequency mode and can only supply
a reduced load current (typically 50mA).
PWM = High: All three converters are forced into PWM
mode operation. The buck converters will remain at
constant-frequency operation until their minimum on-
time is reached. The buck-boost converter will remain
in PWM mode at all load currents.
FB1 (Pin 10/Pin 8): Feedback Voltage for the Buck-Boost
Converter Derived from a Resistor Divider on the Buck-
Boost Output Voltage. The buck-boost output voltage is
given by the following equation, where R1 is a resistor
between FB1 and ground, and R2 is a resistor between
FB1 and the buck output voltage:
V_OUT1
=
0.6V
₁₊
R2
R1
SHDN3 (Pin 11/Pin 9): Forcing this pin above 1.4V en-
ables the buck converter output at SW3. Forcing this pin
below 0.4V disables the buck converter. This pin cannot
be left floating.
SHDN1 (Pin 12/Pin 10): Forcing this pin above 1.4V
enables the buck-boost converter. Forcing this pin below
0.4V disables the buck-boost converter. This pin cannot
be left floating.
PV_IN1 (Pin 13/Pin 11): High current power supply connec-
tion used to supply switch A of the buck-boost converter.
This pin should be bypassed by a 4.7µF, or larger, ceramic
cap. The bypass capacitor should be placed as close to
the pin as possible and should have a short return path
to ground. Pins V_IN, PV_IN1, and PV_IN2 must be connected
together in the application circuit.
NC (Pin 13, UF Package Only): No Internal Connection.
SW1B (Pin 14/Pin 14): Buck-Boost Switch Node. This pin
must be connected to one side of the buck-boost inductor.
SW1A (Pin 15/Pin 15): Buck-Boost Switch Node. This pin
must be connected to one side of the buck-boost inductor.
V_OUT1 (Pin 16/Pin 16): Buck-Boost Output Voltage Node.
This pin should be connected to a low ESR ceramic capaci-
tor. The capacitor should be placed as close to the IC as
possible and should have a short return to ground.
SW3 (Pin 17/Pin 17): Buck converter Switch Node. This
pin must be connected to the opposite side of the inductor
connected to V_OUT3.
PGND2 (Pin 18/Pin 18): High Current Ground Connec-
tion for Both Buck Converters. The PCB trace connecting
this pin to ground should be made as short and wide as
possible.
SW2 (Pin 19/Pin 20): Buck Converter Switch Node. This
pin must be connected to the opposite side of the inductor
connected to V_OUT2.
3521f