LM2745 datasheet(8/23 Pages) NSC

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Block Diagram

20137403

Application Information

The LM2745/8 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 output shutdown (SD), input undervoltage lock-out

(UVLO) mode and power good (PWGD) flag (based on

overvoltage and undervoltage detection). The overvoltage

and undervoltage signals are OR-gated to drive the power

good signal and provide a logic signal to the system if the

output voltage goes out of regulation. Current limit is

achieved by sensing the voltage V

DS across the low side

MOSFET. The LM2745/8 is also able to start-up with the

output pre-biased with a load. The LM2745 also allows for

the switching frequency to be synchronized with an external

clock source.

START UP/SOFT-START

When V

CC exceeds 2.79V and the shutdown pin (SD) sees

a logic high, the soft-start period begins. Then an internal,

fixed 10 µA source begins charging the soft-start capacitor.

During soft-start the voltage on the soft-start capacitor C

SS is

connected internally to the non-inverting input of the error

amplifier. The soft-start period lasts until the voltage on the

soft-start capacitor exceeds the LM2745/8 reference voltage

of 0.6V. At this point the reference voltage takes over at the

non-inverting error amplifier input. The capacitance of C

determines the length of the soft-start period, and can be

approximated by:

Where C

SS is in µF and tSS is in ms.

During soft start the Power Good flag is forced low and it is

released when the FB pin voltage reaches 70% of 0.6V. At

this point the chip enters normal operation mode, and the

output overvoltage and undervoltage monitoring starts.

SETTING THE OUTPUT VOLTAGE

The LM2745/8 regulates the output voltage by controlling the

duty cycle of the high side and low side MOSFETs (see

Typical Application Circuit).The equation governing output

voltage is:

SETTING THE SWITCHING FREQUENCY

During fixed-frequency mode of operation the PWM fre-

quency is adjustable between 50 kHz and 1 MHz and is set

by an external resistor, R

FADJ, between the FREQ/SYNC pin

and ground. The resistance needed for a desired frequency

LM2745 Datasheet(PDF) 8 Page - National Semiconductor (TI)

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