LT3508 查看數據表（PDF） - Linear Technology

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LT3508

Dual Monolithic 1.4A Step-Down Switching Regulator

Linear Technology 

LT3508

PIN FUNCTIONS

BOOST1, BOOST2: The BOOST pins are used to provide

drive voltages, higher than the input voltage, to the internal

NPN power switches. Tie through a diode to a 2.8V or

higher supply, such as VOUT or VIN.

Exposed Pad: The Exposed Pad metal of the package pro-

vides 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.

FB1, FB2: The LT3508 regulates each feedback pin to

0.800V. Connect the feedback resistor divider taps to

these pins.

GND: Tie the GND pins directly to the Exposed Pad and

ground plane.

PG1, PG2: The power good pins are the open-collector

outputs of an internal comparator. PG remains low until

the FB pin is within 10% of the final regulation voltage.

As well as indicating output regulation, the PG pins can

be used to sequence the two switching regulators. These

pins can be left unconnected. The PG outputs are valid

when VIN1 is greater than 2.4V and SHDN is high. The PG

comparators are disabled in shutdown.

RT/SYNC: The RT/SYNC pin is used to set the internal

oscillator frequency. Tie a 33.2k resistor from RT/SYNC

to GND for a 1MHz switching frequency. To synchronize

the part to an external frequency, drive the RT/SYNC pin

with a logic-level signal with positive and negative pulse

widths of at least 80ns.

SHDN: The shutdown pin is used to put the LT3508 in

shutdown mode. Pull the pin below 0.3V to shut down the

LT3508. The 2.63V threshold can function as an accurate

undervoltage lockout (UVLO), preventing the regulator

from operating until the input voltage has reached the

programmed level. Do not drive SHDN more than 6V

above VIN1.

SW1, SW2: The SW pins are the outputs of the internal

power switches. Connect these pins to the inductors, catch

diodes and boost capacitors.

TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used

to soft-start the two channels, to allow one channel to

track the other output, or to allow both channels to track

another output. For tracking, tie a resistor divider to this

pin from the tracked output. For soft-start, tie a capacitor

to this pin. An internal 1.2μA soft-start current charges

the capacitor to create a voltage ramp at the pin. Leave

these pins disconnected if unused.

VC1, VC2: The VC pins are the outputs of the internal error

amps. The voltages on these pins control the peak switch

currents. These pins are normally used to compensate the

control loops, but can also be used to override the loops.

Pull these pins to ground with an open drain to shut down

each switching regulator.

VIN1: The VIN1 pin supplies current to the LT3508 internal

circuitry and to the internal power switch connected to

SW1 and must be locally bypassed. VIN1 must be greater

than 3.7V for channel 1 or channel 2 to operate.

VIN2: The VIN2 pin supplies current to the internal power

switch connected to SW2 and must be locally bypassed.

Connect this pin directly to VIN1 unless power for chan-

nel 2 is coming from a different source. VIN2 must be

greater than 3V and VIN1 must be greater than 3.7V for

channel 2 to operate.

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