16SP100M datasheet(8/23 Pages) NSC

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Block Diagram

20150903

Application Information

The LM2747 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 LM2747 is also able to start-up with the

output pre-biased with a load and 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 LM2747 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 LM2747 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

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