SC4808BIMSTR datasheet(18/31 Pages) SEMTECH

18 / 31 page

 2005 Semtech Corp.

www.semtech.com

SC4808A/B

POWER MANAGEMENT

The instability can be corrected by modification of the peak

current information slope. One of the methods to alter the

peak current information is to add a positive going ramp to

the output of the current sensing circuitry.

The SC4808 achieves this by using an internal slope com-

pensation circuit. The oscillator ramp is internally buffered

and an internal 25kOhms resistor in conjunction with an

external resistor at the CS pin will program the level of slope

compensation.

Rosc

15k

10k

200p

Cosc

RSlope Comp

2.2u,16V

82p

Cfilter

REF

Rsense

SC4808

REF

SYNC

GND

N = 100

25k

Current Transformer

RSlope Comp value will determine the Mode of operation (Voltage or Current)

The Peak current information is sensed and the result is

realistically summed to the buffered oscillator ramp, as

shown above. The value of the external resistor R

slope comp

will determine the percentage of the slope compensation.

As the value for R

slope comp is reduced, the current informa-

tion becomes more dominant and the mode of operation

becomes more current mode. At the same time the slope

of the current information is modified to provide the slope

compensation.

If the R

slope comp is increased, the internal ramp becomes the

dominant signal and more voltage mode of operation is

Application Information (Cont.)

achieved. As it can be calculated from the second formula

below, a 100% voltage mode operation can be achieved by

choosing R

slope comp to be greater than 6.25K ohms. Also if a

100% current mode of operation is required, R

slope comp is

reduced to zero and the contribution from the internal ramp

is completely eliminated.

()

ernal

int

sense

Comp

slope

sense

Comp

slope

Ramp

Comp

Slope

()

()









−

•

≅

Comp

slope

Comp

slope

ernal

int

external

Next page illustrates how the buffered oscillator ramp is

used to modify the sensed inductor current.

It should be noted that in order for the slope compensa-

tion to be effective, the current sensed signal slope should

be at least 50% less steeper than the oscillator positive

ramp slope. The slope will include the magnetizing current

of the transformer and the inductor output current in iso-

lated applications. In non-isolated applications, the slope

will only include the inductor output current.

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