SC2449 datasheet(14/20 Pages) SEMTECH

14 / 20 page

 2002 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2449

Electrical Characteristic Curves (Cont.)

Start-up (Vin = 24V, Vout = 3.3V, Load Current = 5A/phase)

ch1: Vout

Theory of Operation

The SC2449 employs a voltage mode control with feed

forward to provide fast output response to load and line

transients.

The SC2449 has two outputs, which can be used to gen-

erate two separate supply voltages or can be combined

in bi-phase operation to generate one single supply volt-

age. The internal reference is trimmed to 1 V with +/-1%

accuracy, and the outputs voltages can be adjusted by

two external resistors. In bi-phase operation, the dual

switching regulators are operated 180° out of phase.

Load current sharing between phases is normally re-

quired, and this can be achieved by using precise feed-

back voltage divider resistors (typically 0.1%) to match

individual phase output voltage. In addition, small droop-

ing resistors ( could be PCB traces) are employed at the

output of each phase to enhance phase current balance.

PWM Control

Changes on the output voltages are fed to the inverting

input of the Error Amplifiers, by the .B1 and .B2 pins,

and compared with the internal 1 V reference. The com-

pensation to the transconductance amplifier is achieved

by connecting a capacitor in series with a resistor from

the COMP1 and COMP2 pins to AGND respectively. The

error signal from the error amplifier is compared to the

saw tooth waveform by the PWM comparator, and

Oscillator .requency Selection

The sawtooth signal is generated by charging an internal

capacitor with a current source. The charge current is

set by an external resistor connected from the RRE. pin

to AGND. The oscillator frequency and the external resis-

tance follow an inversely proportional relationship.

.eed .orward

The SC2449 incorporates a voltage feed forward scheme

to improve line transient immunity when changes of the

input voltage occur. As the input voltage changes, the

ramp valley to peak voltage of the internal oscillator fol-

lows this change instantly. As a result the output voltage

will have minimum disturbance due to the input line

change.

matched timing signal is generated to control the upper

and lower gate drives of the two phases. A single Ramp

signal is used to generate the control signals for both of

phases, hence the maximum duty cycle is less than 50%.

Synchronized Operation

The internal oscillator can be synchronized to an exter-

nal clock operating in the range of 270 kHz to 1 MHz.

SC2449 датащи(PDF) 14 Page - Semtech Corporation

SC2449 датащи(HTML) 14 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL