SC2545 datasheet(8/24 Pages) SEMTECH

8 / 24 page

www.semtech.com

SC2545

POWER MANAGEMENT

Overview

The SC2545 is a constant frequency 2-phase voltage

mode step-down PWM switching controller driving all N-

channel MOSFETs. The two channels of the controller

operate at 180 degree out of phase from each other.

Since input currents are interleaved in a two-phase

converter, input ripple current is lower and smaller input

capacitance can be used for filtering. Also, with lower

inductor current and smaller inductor ripple current per

phase, overall I2 R losses are reduced.

FFFFFreq

req

requency Se

uency Se

uency Setting

tting

The frequency of the SC2545 is user- programmable.

The oscillator of SC2545 can be programmed with an

external resistor from the Rosc pin to the ground. The

step-down controller is capable of operating up to

300kHz. The relationship between oscillation frequency

versus oscillation resistor is shown in Figure 1.

The advantages of using constant frequency operation

are simple passive component selection and ease of

feedback compensation. Before setting the operating

frequency, the following trade-offs should be considered.

1) Passive component size

2) Circuitry efficiency

3) EMI condition

4) Minimum switch on time

5) Maximum duty ratio

For a given output power, the sizes of the passive

components are inversely proportional to the switching

frequency, whereas MOSFETs/Diodes switching losses

are proportional to the operating frequency. Other issues

such as heat dissipation, packaging and cost issues are

also to be considered. The frequency bands for signal

transmission should be avoided because of EM

interference.

100

150

200

250

300

350

Ros c (KOHM)

Figure 1. Switching frequency versus Rosc.

This soft start scheme will ensure the duty cycle to

increase slowly, therefore limiting the charging current

into the output capacitor and also ensuring the

inductor does not saturate. The soft start capacitor

will eventually be charged up to 2.5V.

The soft-start sequence is initiated when EN pin is high

and Vcc >4.5V or during recovery from a fault condition

( OCP, OVP, or UVLO).

The period of start up can be programed by the soft

start capacitor:

Css

Tss

Sof

Soft Star

t Star

t Start and

t and

t and ”P

PPeel of

eel of

eel offffff ”TTTTTracking

racking

During start-up, the reference voltage of the error

amplifier equals 30% of the voltage on Css (soft-start

capacitor) which is connected between the SS pin and

ground. When the controller is enabled (by pulling EN

pin high), one internal 84uA current source, I

SS, (soft

start current) will charge the soft-start capacitor

gradually. The PWM output starts pulsing when the

soft start voltage reaches 1V.

Shutdown

When the EN pin is pulled low, an internal 15uA current

source discharges the soft-start capacitor and DRVH/

DRVL signals stop pulsing. The output voltage ramps

down at a rate determined by the load condition.

The SC2545 can also be shutdown by pulling down

directly on the SS pin. The designer needs to consider

the slope of the SS pin voltage and choose a suitable

pull down resistor to prevent the output from

undershooting.

Shutdown can also be triggered when an OCP condition

occurs. When an OCP condition is detected, DRVH and

DRVL will stop pulsing and enter a “tri-state shutdown”

with the output voltage ramping down at a rate deter-

mined by the load condition. The internal 15uA current

source will begin discharging the soft-start capacitor

and when the soft-start voltage reaches 0.65V, DRVL

will go high.

Applications Information

By tieing the two soft start pins together, the two

output rails track each other at start up till the rail with

lower set level reaching regulation. Please refer the

waveform on page 20.

SC2545 датащи(PDF) 8 Page - Semtech Corporation

SC2545 датащи(HTML) 8 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 21 22 23 24

датащи скачать

ссылки URL