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3507

Triple Monolithic Step-Down Regulator with LDO

Linear Technology 

LT3507

OPERATION

The LT3507 contains three independent, constant fre-

quency, current mode, switching regulators with internal

power switches plus a low dropout linear regulator. The

three regulators share common circuitry including input

source, voltage reference and oscillator, but are otherwise

independent. Operation can be best understood by refer-

ring to the Block Diagram (Figure 1).

If the RUN pins are tied to ground, the LT3507 is shut

down and draws <1μA from the input source tied to VIN1.

If any of the RUN pins are driven above 1V, the internal bias

circuits turn on, including the internal regulator, reference,

and master oscillator. Each switching regulator will only

begin to operate when its corresponding RUN pin reaches

>1.25V. The master oscillator generates three clock signals,

with the signal for Channel 1 out of phase by 180°.

The three switchers are current mode regulators. Instead

of directly modulating the duty cycle of the power switch,

the feedback loop controls the peak current in the switch

during each cycle. Compared to voltage mode control, cur-

rent mode control improves loop dynamics and provides

cycle-by-cycle current limit.

The Block Diagram shows only one of the three step-down

switching regulators. A pulse from the slave oscillator

sets the RS flip-flop and turns on the internal NPN bipo-

lar power switch. Current in the switch and the external

inductor begins to increase. When this current exceeds a

level determined by the voltage at VC, current comparator

C1 resets the flip-flop, turning off the switch. The current

in the inductor flows through the external Schottky diode

and begins to decrease. The cycle begins again at the next

pulse from the oscillator. In this way, the voltage on the

VC pin controls the current through the inductor to the

output. The internal error amplifier regulates the output

voltage by continually adjusting the VC pin voltage. The

threshold for switching on the VC pin is >1V and an active

clamp of 1.8V limits the output current.

Each switcher contains an extra, independent oscillator to

perform frequency foldback during overload conditions.

This slave oscillator is normally synchronized to the master

oscillator. A comparator senses when VFB is less than 50%

of its regulated value and switches the regulator from the

master oscillator to a slower slave oscillator. VFB is less than

50% of its regulated value during start-up, short-circuit

and overload conditions. Frequency foldback helps limit

switch current under these conditions.

The TRK/SS pins override the 0.8V reference for the FB

pins when the TRK/SS pins are below 0.8V. This allows

either coincident or ratiometric supply tracking on start-up

as well as a soft-start capability.

The switch drivers operate either from VIN or from the

BOOST pin. An external capacitor and diode are used to

generate a voltage at the BOOST pin that is higher than the

input supply. This allows the driver to saturate the internal

bipolar NPN power switch for efficient operation.

The BIAS pin allows the internal circuitry to draw its current

from a lower voltage supply than the input, also reducing

power dissipation and increasing efficiency. If the voltage

on the BIAS pin falls below 3V, then its quiescent current

will flow from VIN.

A power good comparator trips when the FB pin is at

90% of its regulated value. The PGOOD output is an

open-collector transistor that is off when the output is in

regulation, allowing an external resistor to pull the PGOOD

pin high. Power good is valid when the LT3507 is enabled

and VIN > 3.5V.

The LDO regulator uses an external NPN pass transistor to

form a linear regulator. The loop is internally compensated

to be stable with a load capacitance of 2.2μF or greater.

The LDO is disabled when all three of the RUN pins are

low.

The overvoltage and undervoltage detection shuts down

the LT3507 if the input voltage goes above or below re-

sistor programmable thresholds. The hysteresis of these

detectors is also resistor programmable.

3507f

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