3570 datasheet(7/20 Pages) LINER

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LT3570

3570fa

PIN FUNCTIONS (QFN/TSSOP)

VIN2(Pins1,2/Pin14):InputVoltagefortheBuckRegulator.

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.

VIN1 (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.

VC1 (Pin 9/Pin 20): Control Voltage and Compensation Pin

for the Internal Error Ampliﬁer. 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 1.5V or

more to enable the boost switcher. Ground to shutdown

the part.

SHDN2 (Pin 12/Pin 3): Shutdown Pin. Tie to 1.5V or

more to enable the buck switcher. Ground to shutdown

the part.

SHDN3 (Pin13/Pin 4): Shutdown Pin. Tie to 1.5V or more

to enable the NPN LDO. Ground to shut down 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 RT pin up to 1.3 times the

initial operating frequency.

RT (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.

VC2 (Pin 18/Pin 8): Control Voltage and Compensation Pin

for the Internal Error Ampliﬁer. 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.

VIN3 (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 VIN2. The voltage on this pin is

charged up through an external Schottky diode.

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