SC1175CSW datasheet(6/20 Pages) SEMTECH

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SC1175

POWER MANAGEMENT

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

functions. If held below the Soft Start Enable thresh-

old, both channels are inhibited. DH1 and DH2 will be

low, turning off the top FETs. Between the Soft Start

Enable threshold 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 before 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.

Theory of Operation (Cont.)

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.

The formula is:

100W being the value of the resistors connecting the

pins 2 and 3 to the two output sense resistors.

The estimated voltage drop across the MOSFETs is

0.1V.

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

If R (pull-up) = +20KW then place a 20WK resistor on

pin 3.

If R (pull-up) = -20KW then place a 20KW 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:

(mohms)

SENSE 1 = 72mV / I max master

Selection of R

SENSE 2 for the slave channel: (mohms)

SENSE 1 = 48mV / I max master

The errors will be minimized if the power compo-

nents have been sized proportionately to the maxi-

mum 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: (mohms)

SENSE 1 = 72mV / I max ch 1

Selection of R

SENSE 2 for the channel 2: (mohms)

SENSE 1 = 72mV / I max ch 2













−

Ω

−

SLAVE

OUT

762

100

)

pull

(

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