LTC3520
8
3520fa
PIN FUNCTIONS
SV
IN
(Pin 1): Small Signal Power Supply Connection.
This pin is used to power the internal circuitry of the
LTC3520. This pin should be bypassed using a 0.1μF or
larger ceramic capacitor placed as close as possible to
the pin with a short return path to ground. Pins PV
IN1
,
PV
IN2
, PV
IN3
, and SV
IN
must be connected together in
the application circuit.
A
OUT
(Pin 2): Uncommitted Amplifi er Output. This pin
should be connected to the base of an external PNP
transistor for use as an LDO regulator. If used as a
battery-good indicator or for supply sequencing, this pin
is the comparator output.
A
IN
(Pin 3): Non-Inverting Input to the Uncommitted
Amplifi er. In LDO applications, this pin is connected to
the LDO feedback voltage.
R
T
(Pin 4): Programs the Frequency of the Internal Oscilla-
tor. This pin must be tied to ground via an external resistor.
The value of the resistor controls the oscillator frequency.
For details on choosing the value of this resistor see the
Applications Information section of this datasheet.
PWM1 (Pin 5): Logic Input Used to Choose Between Burst
and PWM Mode for the Buck-Boost Converter. This pin
cannot be left fl oating.
PWM1 = Low: The buck-boost converter will operate in
variable frequency mode to improve effi ciency at light
loads. In this mode, the LTC3520 can only supply a
reduced load current (typically 50mA).
PWM1 = High: The buck-boost converter will remain
in low noise, fi xed frequency PWM mode at all load
currents.
SD1 (Pin 6): Buck-Boost Active-Low Shutdown Pin. Forc-
ing 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 fl oating.
SD2 (Pin 7): Buck Active-Low Shutdown Pin. Forcing
this pin above 1.4V enables the buck converter. Forcing
this pin below 0.4V disables the buck converter. This pin
cannot be left fl oating.
SD3 (Pin 8): Uncommitted Amplifi er Active-Low Shutdown
Pin. Forcing this pin above 1.4V enables the uncommitted
amplifi er. Forcing this pin below 0.4V disables the uncom-
mitted amplifi er. This pin cannot be left fl oating.
PV
IN2
(Pin 9): High Current Power Supply Connection
Used to Supply the Buck Converter PMOS Power Device.
This pin should be 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.
SW2 (Pin 10): Buck Converter Switch Node. This pin must
be connected to one side of the buck inductor.
PGND2 (Pin 11): High Current Ground Connection for the
Buck Converter N-Channel MOSFET Power Device. The
PCB trace connecting this pin to ground should be made
as short and wide as possible.
PWM2 (Pin 12): Burst/PWM Mode Control Pin for the Buck
Converter. This pin can be used in the following ways:
PWM2 forced high: With PWM2 forced high, the buck
converter will be forced into low noise fi xed frequency
operation. The buck converter will remain in this mode
unless the load current is low enough that the minimum
on-time is reached at which point the converter will
begin pulse-skipping to maintain regulation.
PWM2 connected to ground via resistor: PWM2 can be
connected to ground through a resistor to control the
load current at which Burst Mode operation is entered
and exited. Larger resistor values will cause the buck
converter to enter Burst Mode operation at lower load
currents and will result in lower output voltage ripple
in Burst Mode operation. Smaller resistor values will
cause Burst Mode operation to be entered at higher load
currents and the Burst Mode ripple will be larger.
PWM2 forced low: With PWM2 forced to ground, the
buck converter will operate in Burst Mode operation for
all but the highest load currents. Generally, this mode of
operation is utilized to force the buck converter into Burst
Mode operation when it is known that the load current
will be relatively low (under 75mA) or in applications
that are not sensitive to output voltage ripple.
