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LT3507

3507f

the TRK/SS2 voltage is substantially higher than 0.8V at

steady state and effectively turns off D1. D2 and D3 will

therefore conduct the same current and offer tight matching

between VFB2 and the internal precision 0.8V reference. In

the ratiometric mode with R6 = R2, TRK/SS2 equals 0.8V

at steady state. D1 will divert part of the bias current and

make VFB2 slightly lower than 0.8V. Although this error

is minimized by the exponential I-V characteristic of the

diodes, it does impose a ﬁnite amount of output voltage

deviation. Further, when channel 1’s output experiences

dynamic excursions (under load transient, for example),

channel 2 will be affected as well. Setting R6 to a value

that pushes the TRK/SS2 voltage to 1V at steady state will

eliminate these problems while providing near ratiometric

tracking.

The example shows channel 2 tracking channel 1, however

any channel may be set up to track any other channel.

If a capacitor is tied from the TRK/SS pin to ground, then

the internal pull-up current will generate a voltage ramp on

this pin. This results in a ramp at the output, limiting the

inductor current and therefore input current during start-up.

A good value for the soft-start capacitor is COUT/10,000,

where COUT is the value of the output capacitor.

MULTIPLE INPUT SUPPLIES

VIN1, VIN2 and VIN3 are independent and can be powered

with different voltages provided VIN1 is present when VIN2

or VIN3 is present. Each supply must be bypassed as close

to the VIN pins as possible.

For applications requiring large inductors due to high VIN

to VOUT ratios, a 2-stage step-down approach may reduce

inductor size by allowing an increase in frequency. A dual

step-down application steps down the input voltage (VIN1)

to the highest output voltage, then uses that voltage to

power the other outputs (VIN2 and VIN3). VOUT1 must be

able to provide enough current for its output plus the

input current at VIN2 and VIN3 when VOUT2 and VOUT3 are

at maximum load. The Typical Applications section shows

a 36V to 15V, 1.8V and 1.2V 2-stage converter using this

approach.

For applications with multiple voltages, the LT3507 can

accommodate input voltages as low as 3V on VIN2 and

APPLICATIONS INFORMATION

VIN3. This can be useful in applications regulating outputs

from a PCI Express bus, where the 12V input is power

limited and the 3.3V input has power available to drive

other outputs. In this case, tie the 12V input to VIN1 and

the 3.3V input to VIN2 and VIN3.

LOW DROPOUT REGULATOR

The low dropout regulator comprises an error amp, loop

compensation and a base drive amp. It uses the same

0.8V reference as the switching regulators. It requires an

external NPN pass transistor and 2.2μF of output capaci-

tance for stability.

The dropout characteristics will be determined by the pass

transistor. The collector-emitter saturation characteristics

will limit the dropout voltage. Table 4 lists some suitable

NPN transistors with their saturation speciﬁcations.

The base drive voltage has a maximum voltage of 5V.

This will limit the maximum output of the regulator to

5V – VBESAT where VBESAT is the base-emitter saturation

voltage of the pass transistor.

Table 4. NPN Pass Transistors and Saturation Characteristics

PART NUMBER

VCESAT

VBESAT

IC (mA)

IB (mA)

On Semiconductor

NSS30071

0.25

0.85

500

NSS30101

0.2

0.85

1000

Fairchild

KSC3265

0.4

500

The LDO is always on when any of the switcher channels

is on. The LDO may be shut down if it is unused by pull-

ing the FB4 pin up with a 30μA current source. The FB4

pin will clamp at about 1.25V and the LDO will shut off

reducing power consumption. This pull-up can be sourced

from one of the LT3507 outputs provided that channel is

always on when the other channels are on.

The output stage of the LDO will drive the NPN base from

the BIAS voltage if it is at least 0.8V above the LDO DRIVE

voltage.

FB Resistor Network

The output voltage of the LDO regulator is programmed

with a resistor divider (Refer to Block Diagram) between the

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