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LT3045-1

30451fa

For more information www.linear.com/LT3045-1

High Efficiency Linear Regulator: Voltage Input-to-

Output Control (VIOC)

The VIOC pin is used to control an upstream switching

converter (e.g. buck, boost, buck-boost, etc) to maintain

a constant voltage across the LT3045-1, regardless of

the LDO’s output voltage. This maximizes efficiency

while maintaining PSRR performance. The VIOC pin is

the output of a fast unity-gain amplifier that measures

the difference between IN and OUT or 1.5V, whichever is

higher. As shown in Figure 9, the VIOC feature is simple

to use. Just tie the VIOC pin to the upstream switching

converter’s feedback (FB) pin, and this will regulate the

LT3045-1’s input-to-output differential to the switching

converter’s feedback voltage. When paralleling multiple

LT3045-1s, tie the VIOC pin of one of the LT3045-1 to the

upstream switching converter’s feedback pin and float the

remaining VIOC pins.

While the VIOC buffer is inside the switching converter’s

feedback loop, given the VIOC buffer’s high bandwidth,

theswitchingconverter’sfrequencycompensationdoesn’t

need to be adjusted. Phase delay through the VIOC buffer

is typically less than 2° for frequencies as high as 100kHz;

hence,withintheswitchingconverter’sbandwidth(usually

much less than 100kHz), the VIOC buffer will be transpar-

ent and just act like an ideal wire.

For example, for a switching converter with less than

100kHz bandwidth and a phase margin of 50°, using the

VIOC buffer, the phase margin will degrade by at most 2°.

Hence,thephasemarginfortheswitchingconverter(using

the VIOC pin) will be at least 48°. Given the VIOC buffer is

inside the switching converter’s feedback loop, the total

capacitance on the VIOC pin is required to be below 20pF.

As shown in Figure 10, the input-to-output differential

voltage is easily programmable to support different ap-

APPLICATIONS INFORMATION

Figure 9. VIOC Basic Operation

LT3045-1

VLDOOUT: VARIABLE

IOUT(MAX): 500mA

30451 F09

–

EN/UV

PGFB

GND

OUT

VIOC

ILIM

10µF

VIN

SET

OUTS

UPSTREAM

DC/DC

CONVERTER

0.47µF

1×

VLDOIN

VFBSWITCHER

Figure 10. Programming Input-to-Output Differential

LT3045-1

VOUT: VARIABLE

IOUT(MAX): 500mA

30451 F10

–

EN/UV

PGFB

GND

OUT

VIOC

ILIM

10µF

VIN

SET

OUTS

UPSTREAM

DC/DC

CONVERTER

0.47µF

1×

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