LM2731XMF datasheet(4/16 Pages) NSC

4 / 16 page

Block Diagram

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Theory of Operation

The LM2731 is a switching converter IC that operates at a

fixed frequency (0.6 or 1.6 MHz) for fast transient response

over a wide input voltage range and incorporates pulse-by-

pulse current limiting protection. Because this is current

mode control, a 33 m

Ω sense resistor in series with the

switch FET is used to provide a voltage (which is propor-

tional to the FET current) to both the input of the pulse width

modulation (PWM) comparator and the current limit ampli-

fier.

At the beginning of each cycle, the S-R latch turns on the

FET. As the current through the FET increases, a voltage

(proportional to this current) is summed with the ramp com-

ing from the ramp generator and then fed into the input of the

PWM comparator. When this voltage exceeds the voltage on

the other input (coming from the Gm amplifier), the latch

resets and turns the FET off. Since the signal coming from

the Gm amplifier is derived from the feedback (which

samples the voltage at the output), the action of the PWM

comparator constantly sets the correct peak current through

the FET to keep the output voltage in regulation.

Q1 and Q2 along with R3 - R6 form a bandgap voltage

reference used by the IC to hold the output in regulation. The

currents flowing through Q1 and Q2 will be equal, and the

feedback loop will adjust the regulated output to maintain

this. Because of this, the regulated output is always main-

tained at a voltage level equal to the voltage at the FB node

"multiplied up" by the ratio of the output resistive divider.

The current limit comparator feeds directly into the flip-flop

that drives the switch FET. If the FET current reaches the

limit threshold, the FET is turned off and the cycle terminated

until the next clock pulse. The current limit input terminates

the pulse regardless of the status of the output of the PWM

comparator.

LM2731XMF Datasheet(PDF) 4 Page - National Semiconductor (TI)