SC4910AITSTRT datasheet(8/19 Pages) SEMTECH

Part #	SC4910AITSTRT

Description	High Performance Secondary Side Controller with Synchronous Rectifier

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Manufacturer	SEMTECH [Semtech Corporation]

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 2005 Semtech Corp.

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SC4910A/B

POWER MANAGEMENT

Applications Information

The SC4910A/B is a secondary side PWM controller work-

ing either in current mode or voltage mode mainly for

applications of forward converters with synchronous rec-

tification. While the OUTA drives the primary MOSFET

through transformer and the secondary forward recti-

fier, the OUTB drives the secondary freewheeling recti-

fier. The switching frequency and maximum duty cycle

can be programmable with two resistors. The delay time

from the falling edge of OUTA to the rising edge of OUTB

is adaptive by monitoring the phase node voltage. The

delay time from the falling edge of OUTB to the rising

edge of OUTA is determined by a programming resistor

from the DELAY pin to ground. The ISHARE pin allows for

current sharing among the parallel operating units to

make current equally distribute load. The -SENSE pin

separated from GND pin provides true output voltage

remote sense capability. Other features include soft start,

sychronization or enable/disable by user, provided 5V

reference voltage.

Oscillator

The frequency and duty cycle of the oscillator is controlled

by placing two resistors from the RT1 and RT2 pins to

ground. The resistor at RT1 controls the maximum “on”

duty cycle and the resistor at RT2 controls the “off”

portion of a cycle. When the resistor at RT1 is equal to

that at RT2, the maximum duty cycle will be approximately

50%. The following formula is used to determine the

time duration of the “on” and “off” portions:

12

10

20

−

×

= RT

t

Current Sense and Current Limit

The CS pin has an input impedance of 20K ohms and

swings from 1.0V to 2.5V. With a 5K ohm resistor from

CS to ground, the device operates in voltage mode with

a ramp that will swing from 0.2V to 0.5V. When the 5K

resistor is connect to a voltage that is proportional to

the primary side current, the device will operate in current

mode. The cycle-by-cycle current limit is triggered when

the CS pin voltage rises above 1V. If CS exceeds 1.25V,

the faulty latch will be set and the outputs will be driven

low. The soft start capacitor is then discharged by the

internal current sink. No outputs are allowed until the

soft start capacitor is fully discharged to 0.15V. At this

point the fault latch will be reset and the SC4910 will

begin a soft start process. This results in a hiccup current

limit mode for continuous fault conditions.

SYNC/EN

The enable function looks at the SYNC/EN pin and an

internal timing capacitor. If the SYNC/EN pin is low and

the internal timing capacitor voltage is high, then the

SC4910 is disabled with OUTA, OUTB and SS pulled low.

When the SYNC/EN pin is held high, the device is enabled

and runs off of the internal oscillator. When a rising signal

is detected on the SYNC/EN pin a one-shot is triggered

and discharges the internal timing capacitor. As long as

the internal timing capacitor is below an internal reference

level, the device will synchronize with the external pulse.

If the internal timing capacitor is allowed to charge up to

the internal reference level before another SYNC pulse

is detected, the device will switch back to the internal

oscillator.

Soft Start

The SS pin is connected to the internal reference, 0.75V,

through an internal 6K ohm resistor. The SS pin is also

connected to the non-inverting input of the error amplifier.

With an external capacitor connected to this pin, the soft

start timing will be determined by this RC time constant.

During start-up, the SS pin is held low until the

undervoltage lockout threshold is reached. Once the

UVLO threshold is reach, the SS pin is released and the

device will regulate to the voltage on this pin.

Undervoltage Lockout

When the supply voltage V

CC is below the undervoltage

lockout threshold, both OUTA and OUTB are held low. The

SS pin and the COMP pin are also held low. Once the

undervoltage lockout threshold has been surpassed,

OUTA, OUTB, SS and COMP are released for normal

operation.

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