LT3570
7
3570fb
PIN FUNCTIONS
(QFN/TSSOP)
V
IN2
(Pins 1,2/Pin 14): Input Voltage for the Buck Regulator.
This pin also supplies the current to the internal circuitry
of the LT3570. This pin must be locally bypassed with a
capacitor.
SW2 (Pin 3/Pin15): Switch Node. This pin connects to
the emitter of an internal NPN power switch. Connect a
diode, inductor and boost capacitor to this pin to form
the buck regulator.
SW1 (Pin 4/Pin16): Switch Node. This pin connects to the
collector of an internal NPN power switch. Connect a diode
and inductor to this pin to form the boost regulator.
GND (Pins 5, 6, 16, 25/Pins 17, 21): Ground. The Exposed
Pad of the package provides both electrical contact to
ground and good thermal contact to the printed circuit
board. The Exposed Pad must be soldered to the circuit
board for proper operation.
V
IN1
(Pin 7/Pin18): Input Voltage for the Boost Regulator.
This pin supplies current to drive the boost NPN transistor
of the LT3570. This pin must be locally bypassed with a
capacitor.
SS1 (Pin 8/Pin 19): Soft-Start Pin. Place a soft-start
capacitor here. Upon start-up, a current charges the
capacitor to 2V. This pin ramps the reference voltage of
the boost switcher.
V
C1
(Pin 9/Pin 20): Control Voltage and Compensation Pin
for the Internal Error Amplifi er. Connect a series RC from
this pin to ground to compensate the switching regulator
loop for the boost regulator.
FB1 (Pin 10/Pin 1): Feedback Pin. The LT3570 regulates
this pin to 788mV. Connect the feedback resistors to
this pin to set the output voltage for the boost switching
regulator.
SHDN1 (Pin 11/Pin 2): Shutdown Pin. Tie to 1.4V or more
to enable the boost switcher. Tie all SHDN pins to 0.3V or
less to shutdown the part.
SHDN2 (Pin 12/Pin 3): Shutdown Pin. Tie to 1.4V or more
to enable the buck switcher. Tie all SHDN pins to 0.3V or
less to shutdown the part.
SHDN3 (Pin13/Pin 4): Shutdown Pin. Tie to 1.5V or more
to enable the NPN LDO. Tie all SHDN pins to 0.3V or less
to shutdown the part.
SYNC (Pin 14/Pin 5): Synchronization Pin. The SYNC pin
is used to synchronize the internal oscillator to an exter-
nal signal. The synchronizing range is equal to the initial
operating frequency set by the R
T
pin up to 1.3 times the
initial operating frequency.
R
T
(Pin 15/Pin 6): Frequency Set Pin. Place a resistor to
GND to set the internal frequency. The range of oscillation
is 500kHz to 2MHz.
SS2 (Pin 17/Pin 7): Soft-Start Pin. Place a soft-start
capacitor here. Upon start-up, a current charges the
capacitor to 2V. This pin ramps the reference voltage of
the buck switcher.
V
C2
(Pin 18/Pin 8): Control Voltage and Compensation Pin
for the Internal Error Amplifi er. Connect a series RC from
this pin to ground to compensate the switching regulator
loop for the buck regulator.
FB2 (Pin 19/Pin 9): Feedback Pin. The LT3570 regulates
this pin to 788mV. Connect the feedback resistors to
this pin to set the output voltage for the buck switching
regulator.
FB3 (Pin 20/Pin 10): Feedback Pin. The LT3570 regulates
this pin to 788mV. Connect the feedback resistors to this
pin to set the output voltage for the LDO controller.
NPN_DRIVE (Pin 21/Pin 11): Base Drive for the External
NPN. This pin provides a bias current to drive the base of
the NPN. This base current is driven from the IN3 supply
voltage.
V
IN3
(Pin 22/Pin 12): Input Voltage for the NPN LDO. This
pin supplies current to drive the base of the NPN. This pin
must be locally bypassed with a capacitor.
BIAS (Pin 23): QFN Package Only. This pin supplies current
to the internal circuitry of the LT3570 if greater than 2.5V.
This pin must be locally bypassed with a capacitor.
BOOST (Pin 24/Pin 13): Bias for the Base Drive of the
NPN Switch for the Buck Regulator. This pin provides a
bias voltage higher than V
IN2
. The voltage on this pin is
charged up through an external Schottky diode.
