SC2450 datasheet(14/20 Pages) SEMTECH

14 / 20 page

 2001 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2450

Electrical Characteristic Curves (Cont.)

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

ch1: Vout

Theory of Operation

The SC2450 employs a voltage mode control with feed forward

to provide fast output response to load and line transients.

The SC2450 has two outputs, which can be used to generate

two separate supply voltages or can be combined in bi-phase

operation to generate one single supply voltage. The internal

reference is trimmed to 1 V with +/-1% accuracy, and the out-

puts 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 required, and this can be achieved by using precise

feedback voltage divider resistors (typically 0.1%) to match

individual phase output voltage. In addition, small drooping re-

sistors ( 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 FB1 and FB2 pins, and compared

with the internal 1 V reference. The compensation to the

transconductance amplifier is achieved by connecting a ca-

pacitor in series with a resistor from the COMP1 and COMP2

pins to AGND respectively. The error signal from the error am-

plifier is compared to the saw tooth waveform by the PWM

comparator, and 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%.

Oscillator Frequency Selection

The sawtooth signal is generated by charging an internal ca-

pacitor with a current source. The charge current is set by an

external resistor connected from the RREF pin to AGND. The

oscillator frequency and the external resistance follow an in-

versely proportional relationship.

Feed Forward

The SC2450 incorporates a voltage feed forward scheme to

improve line transient immunity when changes of the input volt-

age occur. As the input voltage changes, the ramp valley to

peak voltage of the internal oscillator follows this change in-

stantly. As a result the output voltage will have minimum dis-

turbance due to the input line change.

Synchronized Operation

The internal oscillator can be synchronized to an external clock

operating in the range of 300 kHz to 1 MHz. The switching

frequency of each channel is one half of the oscillator frequency.

The oscillator clock is also available externally through the

CLKOUT pin and can be used to provide synchronization for

other converters.

SC2450 датащи(PDF) 14 Page - Semtech Corporation

SC2450 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