SC4910AITSTRT datasheet(9/19 Pages) SEMTECH

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 2005 Semtech Corp.

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SC4910A/B

POWER MANAGEMENT

Programmable Delay

SC4910 is for single ended topologies with secondary

side synchronous rectification. It provides outputs to drive

the primary MOSFET through a small pulse transformer

and the secondary synchronous rectifiers directly. To avoid

cross conduction and optimize performance, adjustable

delay is necessary between forwarding and freewheeling

switches. The delay from falling edge of OUTB to rising

edge of OUTA is determined by a resistor from the DELAY

pin to ground. The following formula is used to calculate

the delay time:

DELAY

−

•

where, R is the delay time setting resistor.

R should be between 20K and 200K.

The delay time from falling edge of OUTA to rising edge of

OUTB is adaptive and is triggered when the PHASE node

falls below 1.5V. If after 220nS the PHASE node has still

not fallen, the device will automatically switch.

Operation Mode

SC4910 could be configured either current mode or

voltage mode operation. In current mode, the current

sense signal comes to the CS pin while an external resistor

could configure slope compensation. In voltage mode,

an external resistor forms sawtooth with the internal 20K

resistor for voltage mode operation while current limit

signal comes to the same pin.

In current mode, which is preferred for application of

SC4910, current is sensed by a current transformer for

current feedback and over current protection. The current

in the primary switch is sensed and controlled by

developing a voltage proportional to current across a

sense resistor on the secondary. The sensed voltage is

then fed into the CS pin of SC4910. The typical current

limit threshold in the current sense pin of the SC4910 is

1.0V. The over current limit is assumed typical 120% of

full load current. Then the current sense resistor can be

calculated by the following equation:

)

(

120

•

Applications Information (Cont.)

where

n – Power transformer primary to secondary turns ratio

S – Secondary turns of current sense transformer

Io(pk) – Peak inductor current

An example of choosing a current sense resistor is given

below. Assume the converter full load current is 20A and

peak inductor current is 23A, the power transformer

primary to secondary turns ratio is 6:1 and the current

sense transformer primary to secondary turns ratio is

1:100, then,

Ω

≈

•

120

100

Slope Compensation

Slope compensation is needed to prevent sub-harmonic

oscillation at duty cycle higher than 50% and to

compensate the peak to average difference in peak

current mode control. The following equation can be used

to calculate the external slope. If negative Se is obtained

by the equation, no slope compensation is needed.

)

(

•

∆

•

−

≥

where

Se – External slope magnitude

Vin – Low input line voltage

Vo – Output voltage

n – Power transformer primary to secondary turns ratio

S – Secondary turns of current sense transformer

∆I

L - Peak-to-peak Inductor current ripple

For example, if the low input line voltage is 36V, output

voltage is 3.3V, power transformer primary to secondary

turns ratio is 6:1; the peak-to-peak inductor current ripple

is 6A, and current sense gain R

S is 21W, then the external

slope needed is:

100

)

(

≈

•

−

•

≥

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