SC1176 datasheet(7/23 Pages) SEMTECH

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 2006 Semtech Corp.

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POWER MANAGEMENT

SC1176/SC1176A

ing is not 1:1. The Master and Slave channels still

have their own current limits, identical to the

independent channel case.

Power Good

The controller provides a power good signal. This is an

open collector output, which is pulled low if the output

voltage is outside of the power good window.

Soft Start/Enable

The Soft Start/Enable (SS/ENA) pin serves several func-

tions. If held below the Soft Start Enable threshold, both

channels are inhibited. DH1 and DH2 will be low, turning

off the top FETs. Between the Soft Start Enable thresh-

old and the Soft Start End threshold, the duty cycle is

allowed to increase. At the Soft Start End threshold,

maximum duty cycle is reached. In practical applications

the error amplifier will be controlling the duty cycle be-

fore the Soft Start End threshold is reached. To avoid

boost problems during startup in current share mode,

both channels start up in asynchronous mode, and the

bottom FET body diode is used for recirculating current

during the FET off time. When the SS/ENA pin reaches

the Soft Start Transition threshold, the channels begin

operating in synchronous mode for improved efficiency.

The soft start pin sources approximately 25uA and soft

start timing can be set by selection of an appropriate

soft start capacitor value.

SENSE RESISTOR SELECTION

Current Sharing Mode

Calculation of the three programming resistors to achieve

sharing.

Three resistors will determine the current sharing load

line.

First the offset resistor will ensure that the load line

crosses the origin (0 Amp on each channel) for sharing

at light current. A pull up resistor from the 5V bias (V

CC of

the chip) will be used. For low duty cycle on the slave

channel (below 50%), the pull up will be on pin 3. For

high duty cycle on the slave channel (above 50%), the

pull up will be on pin 2.

Applications Information - Theory of Operation

The formula is:

SLAVE

OUT

pull

)

(

−

Ω

−

100

Ω being the value of the resistors connecting the

pins 2 and 3 to the two output sense resistors.

.1 V is an estimated voltage drop across the

MOSFETs.

Positive values go to pin 3, negative to pin 2.

A +20K will be a 20K on pin 3.

A -20K will be a 20K on pin 2.

Now that the offset resistor has been fixed, we need to

set up the maximum current for each channel.

Selection of R

SENSE 1 for the master channel: (in m

ohm)

SENSE 1 = 72mV / I max master

Selection of R

SENSE 2 for the slave channel: (in m ohm)

SENSE 2 = 72mV / I max slave

The errors will be minimized if the power components

have been sized proportionately to the maximum

currents.

Independent Channels

Calculation of the two current limiting resistors.

There is no need for an offset resistor in the indepen-

dent channels mode, only the two sense resistors are

used:

Selection of R

SENSE 1 for the channel 1: (in m ohm)

SENSE 1 = 72mV / I max ch 1

Selection of R

SENSE 2 for the channel 2: (in m ohm)

SENSE 1 = 72mV / I max ch 2

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