Datasheet

LTC3520
9
3520fa
PIN FUNCTIONS
SS2 (Pin 13): Buck Converter Soft-Start Pin. This pin must
be connected to a soft-start capacitor. The value of the
capacitor determines the duration of the soft-start period.
For information on choosing the value of this capacitor, see
the Applications Information section of this datasheet.
FB2 (Pin 14): Feedback Voltage for the Buck Converter.
This pin is derived from a resistor divider on the buck
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:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 790 1
2
1
.
V
C1
(Pin 15): Buck-Boost Error Amplifi er Output. A fre-
quency compensation network is connected to FB1 to
compensate the loop. During Burst Mode operation, V
C1
is driven internally by a clamp circuit.
SGND (Pin 16): Small Signal Ground. This pin is used
as a ground reference for the internal circuitry of the
LTC3520.
SS1 (Pin 17): Buck-Boost Converter Soft-Start Pin. This
pin must be connected to a soft-start capacitor. The value
of the capacitor determines the duration of the soft-start
period. For information on choosing the value of this
capacitor, see the Applications Information section of
this datasheet.
FB1 (Pin 18): Feedback Voltage for the Buck-Boost Con-
verter. This pin is 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-boost output voltage:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 782 1
2
1
.
V
OUT1
(Pin 19): Buck-Boost Output Voltage Node. This
pin should be connected to a low ESR buck-boost output
capacitor. The capacitor should be placed as close to the
IC as possible and should have a short return path to
ground.
SW1B (Pin 20): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
PGND1 (Pin 21): High Current Ground Connection for
the Buck-Boost NMOS Power Devices. The PCB trace
connecting this pin to ground should be made as short
and wide as possible.
SW1A (Pin 22): Buck-Boost Switch Node. This pin must
be connected to one side of the buck-boost inductor.
PV
IN1
(Pin 23), PV
IN3
(Pin 24): High Current Power Sup-
ply Connections Used to Power the Buck-Boost Converter
Power Switch A. These pins should be connected together
and bypassed by a 22μF 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 PV
IN1
, PV
IN2
, PV
IN3
, and SV
IN
must be connected
together in the application circuit.
Exposed Pad (Pin 25): Ground. The Exposed Pad must be
electrically connected to ground and soldered to the PCB.
Pins PGND1, PGND2, SGND, and the Exposed Pad must
be connected together in the application circuit.