LTC3521
7
3521fb
For more information www.linear.com/LTC3521
pin FuncTions
FB3 (Pin 1/Pin 23): Feedback Voltage for the Buck Con-
verter Derived
from a Resistor Divider Connected to the
Buck V
OUT3
Output Voltage. The buck output voltage is
given by the following equation, where R1 is a resistor
between FB3 and ground, and R2 is a resistor between
FB3 and the buck output voltage:
V
OUT3
= 0.6V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
FB2 (Pin 2/Pin 24): Feedback Voltage for the Buck Con-
verter Derived
from a Resistor Divider Connected to the
Buck V
OUT2
Output Voltage. The buck output voltage is
given by the following equation, where R1 is a resistor
between FB2 and ground, and R2 is a resistor between
FB2 and the buck output voltage:
V
OUT2
= 0.6V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
SHDN2 (Pin 3/Pin 1): Forcing this pin above 1.4V enables
the buck converter output at SW2. Forcing this pin below
0.4V disables the buck converter. This pin cannot be left
floating.
PGOOD3 (Pin 4/Pin 2): This pin is an open-drain output
which pulls low under any of the following conditions:
V
OUT3
buck output voltage is out of regulation, the part is
in overtemperature shutdown, the part is in undervoltage
lockout, or 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 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 un-
dervoltage lockout, the buck-boost converter is in current
limit, or 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+
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.
(FE/UF Packages)
