LM2746MXAX datasheet(9/24 Pages) NSC

9 / 24 page

Block Diagram

20147703

Application Information

THEORY OF OPERATION

The LM2746 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 logically OR’ed 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.

START UP/SOFT-START

When V

CC exceeds 2.76V 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 LM2746 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.

NORMAL OPERATION

While in normal operation mode, the LM2746 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:

The PWM frequency is adjustable between 50 kHz and 1

MHz and is set by an external resistor, R

FADJ, between the

FREQ pin and ground. The resistance needed for a desired

frequency is approximately:

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