LT1576CS8 datasheet(7/28 Pages) LINER

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LT1576/LT1576-5

BLOCK DIAGRAM

it much easier to frequency compensate the feedback loop

and also gives much quicker transient response.

Most of the circuitry of the LT1576 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 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 the switch to saturate. This

boosted voltage is generated with an external capacitor

and diode. Two comparators are connected to the shut-

down pin. One has a 2.44V threshold for undervoltage

lockout and the second has a 0.4V threshold for complete

shutdown.

The LT1576 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

PIN FUNCTIONS

This pin sits at about 1V for very light loads and 2V at

maximum load. It can be driven to ground to shut off the

regulator, but if driven high, current must be limited to

4mA.

FB/SENSE (Pin 7): The feedback pin is the input to the

error amplifier which is referenced to an internal 1.21V

source. An external resistive divider is used to set the

output voltage. Three additional functions are performed

by the FB pin. The fixed voltage (-5) parts have the divider

resistors included on-chip and the FB pin is used as a

SENSE pin, connected directly to the 5V output. When the

pin voltage drops below 0.7V, the switch current limit and

the switching frequency are reduced and the external sync

function is disabled. See Feedback Pin Function section in

Applications Information for details.

SYNC (Pin 8): The SYNC pin is used to synchronize the

internal oscillator to an external signal. It is directly logic

compatible and can be driven with any signal between

10% and 90% duty cycle. The synchronizing range is

equal to

initial operating frequency, up to 400kHz. This pin

replaces SHDN on -SYNC option parts. See Synchronizing

section in Applications Information for details.

SHDN (Pin 8): The shutdown pin is used to turn off the

regulator and to reduce input drain current to a few

microamperes. Actually, this pin has two separate thresh-

olds, one at 2.44V to disable switching, and a second at

0.4V to force complete micropower shutdown. The 2.44V

threshold functions as an accurate undervoltage lockout

(UVLO). This can be used to prevent the regulator from

operating until the input voltage has reached a predeter-

mined level.

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