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LT3430

sn3430 3430is

BLOCK DIAGRA

The LT3430 is a constant frequency, current mode buck

converter. This means that there is an internal clock and

two feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscilla-

tor pulse which sets the RS flip-flop to turn the switch on.

When switch current reaches a level set by the inverting

input of the comparator, the flip-flop is reset and the

switch turns off. Output voltage control is obtained by

using the output of the error amplifier to set the switch

current trip point. This technique means that the error

amplifier commands current to be delivered to the output

rather than voltage. A voltage fed system will have low

phase shift up to the resonant frequency of the inductor

and output capacitor, then an abrupt 180

° shift will occur.

The current fed system will have 90

° phase shift at a much

lower frequency, but will not have the additional 90

° shift

until well beyond the LC resonant frequency. This makes

it much easier to frequency compensate the feedback loop

and also gives much quicker transient response.

Most of the circuitry of the LT3430 operates from an

internal 2.9V bias line. The bias regulator normally draws

power from the regulator input pin, but if the BIAS pin is

connected to an external voltage equal to or higher than

3V, bias power will be drawn from the external source

(typically the regulated output voltage). This will improve

efficiency if the BIAS pin voltage is lower than regulator

input voltage.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing switch to be saturated.

This boosted voltage is generated with an external

capacitor and diode. Two comparators are connected to

the shutdown pin. One has a 2.38V threshold for under-

voltage lockout and the second has a 0.4V threshold for

complete shutdown.

Figure 1. LT3430 Block Diagram

–

VIN

3, 4

2.9V BIAS

REGULATOR

200kHz

OSCILLATOR

FREQUENCY

FOLDBACK

2, 5

GND

1, 8, 9, 16

1766 F01

SLOPE COMP

ANTISLOPE COMP

BIAS

INTERNAL

VCC

SYNC

0.4V

5.5

µA

CURRENT

COMPARATOR

RLIMIT

RSENSE

ERROR

AMPLIFIER

gm = 2000µMho

FOLDBACK

CURRENT

LIMIT

CLAMP

BOOST

FLIP-FLOP

DRIVER

CIRCUITRY

POWER

SWITCH

1.22V

SHDN 15

LOCKOUT

COMPARATOR

SHUTDOWN

COMPARATOR

2.38V

×1

VC(MAX)

CLAMP

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