LM2742 datasheet(9/22 Pages) NSC

9 / 22 page

Block Diagram

20087501

Application Information

THEORY OF OPERATION

The LM2742 is a voltage-mode, high-speed synchronous

buck regulator with a PWM control scheme. It is designed for

use in set-top boxes, thin clients, DSL/Cable modems, and

other applications that require high efficiency buck convert-

ers. It has power good (PWRGD), and output shutdown

(SD). Current limit is achieved by sensing the voltage V

across the low side FET. During current limit the high side

gate is turned off and the low side gate turned on. The soft

start capacitor is discharged by a 95µA source (reducing the

maximum duty cycle) until the current is under control.

START UP

When V

CC exceeds 4.2V and the shutdown pin SD sees a

logic high the soft start capacitor begins charging through an

internal fixed 10µA source. During this time the output of the

error amplifier is allowed to rise with the voltage of the soft

start capacitor. This capacitor, C

SS, determines soft start

time, and can be determined approximately by:

An application for a microprocessor might need a delay of

3ms, in which case C

SS would be 12nF. For a different

device, a 100ms delay might be more appropriate, in which

case C

SS would be 400nF. (390 10%) During soft start the

PWRGD flag is forced low and is released when the voltage

reaches a set value. At this point this chip enters normal

operation mode and the Power Good flag is released.

Since the output is floating when the LM2742 is turned off, it

is possible that the output capacitor may be precharged to

some positive value. During start-up, the LM2742 operates

fully synchronous and will discharge the output capacitor to

some extent depending on the output voltage, soft start

capacitance, and the size of the output capacitor.

NORMAL OPERATION

While in normal operation mode, the LM2742 regulates the

output voltage by controlling the duty cycle of the high side

and low side FETs. The equation governing output voltage

is:

O =0.6x(RFB1 +RFB2)/RFB1

The PWM frequency is adjustable between 50kHz and

2MHz and is set by an external resistor, R

FADJ, between the

FREQ pin and ground. The resistance needed for a desired

frequency is approximately:

LM2742 Datasheet(PDF) 9 Page - National Semiconductor (TI)

LM2742 Datasheet(HTML) 9 Page - National Semiconductor (TI)

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