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3507

Triple Monolithic Step-Down Regulator with LDO

Linear Technology 

LT3507

PIN FUNCTIONS

BOOST1, BOOST2, BOOST3 (Pins 1, 27, 32): The BOOST

pins are used to provide drive voltages, higher than the

input voltage, to the internal bipolar NPN power switches.

These pins must be tied through a diode from VOUT, VIN

or another supply greater than 2.5V.

VIN1 (Pins 2, 3): The VIN1 pins supply power to the internal

switch of the 2.4A regulator and to the LT3507’s internal

reference and start-up circuitry. These pins must be locally

bypassed (Note 6).

VINSW (Pin 4): The VINSW pin is a switched VIN1 for the

user programmable undervoltage and overvoltage detec-

tion. It is connected to VIN1 when any of the RUN pins

are pulled high, and high impedance when all RUN pins

are low or open.

OVLO (Pin 5): The LT3507 goes into overvoltage shutdown

when this pin goes above 1.2V. If unused, the OVLO pin

should be tied to GND.

UVLO (Pin 6): The LT3507 goes into undervoltage shutdown

when this pin drops below 1.2V. If unused, the UVLO pin

should be tied to VINSW.

VC1, VC2, VC3 (Pins 7, 23, 19): 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. Each switching

regulator can be shut down by pulling its respective VC

pin to ground with an NMOS or NPN transistor.

TRK/SS1, TRK/SS2, TRK/SS3, TRK/SS4 (Pins 8, 21, 18,

26): The TRK/SS pins allow a regulator to track the output

of another regulator. When the TRK/SS pin is below 0.8V,

the FB pin regulates to the TRK/SS voltage. This pin can

also be used as a soft-start by connecting a capacitor from

TRK/SS to ground. The TRK/SS pins should be left open

if neither feature is used.

FB1, FB2, FB3 (Pins 9, 22, 20): The FB pins are the nega-

tive inputs of the error amplifiers. The LT3507 regulates

each feedback pin to the lesser of 0.8V or the TRK/SS

pin voltage. Connect the feedback resistor divider taps

to these pins.

PGOOD1, PGOOD2, PGOOD3 (Pins 10, 11, 12): The

PGOOD pins are the open-collector outputs of an internal

comparator. PGOOD remains low until the FB pin is within

10% of the final regulation voltage. As well as indicating

output regulation, the PGOOD pins can sequence the

switching regulators. These pins must be left unconnected

if unused. The PGOOD outputs are valid when VIN is greater

than 3.5V and any of the RUN pins are high. They are not

valid when all RUN pins are low.

RT/SYNC (Pin 13): The RT/SYNC pin requires a resistor

to ground or a clock signal to set the operating frequency

of the LT3507.

RUN1, RUN2, RUN3 (Pins 14, 15, 16): The RUN pins are

used to shut down the individual switching regulators.

When all three RUN pins are low, the LT3507 shuts down

and draws less than 1μA from VIN1.

BIAS (Pin 17): The BIAS pin supplies the current to the

LT3507’s internal regulator. This pin should be tied to the

lowest available voltage source above 3V (either VIN, VOUT

or any other available supply). The LDO pass transistor’s

base current is supplied from the BIAS pin if it is at least

0.8V above the LDO DRIVE output.

DRIVE (Pin 24): The DRIVE pin provides the base drive for

an external NPN transistor used for the LDO regulator.

FB4 (Pin 25): The FB4 pin is the negative input to the LDO

error amplifier. It is regulated to 0.8V through the LDO

feedback resistor divider.

VIN2 (Pins 30, 31)/VIN3 (Pins 35, 36 ): The VIN2 and VIN3

pins supply power to the internal switches of the 1.5A con-

verters. These pins must be locally bypassed (Note 6).

SW1 (Pins 37, 38)/SW2 (Pins 28, 29)/SW3 (Pins 33,

34): The SW pins are the outputs of the internal power

switches. Connect these pins to the inductors and switch-

ing diodes.

Exposed Pad (Pin 39): Ground. The underside Exposed

Pad metal 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 a grounded

pad on the circuit board for proper operation.

Note 6: VINX pins that are connected together may share a bypass capacitor.

3507f

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