SC4808BIMSTR datasheet(13/31 Pages) SEMTECH

13 / 31 page

 2005 Semtech Corp.

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SC4808A/B

POWER MANAGEMENT

overcurrent comparators. An internal 1 to 3 feedback volt-

age divider provides a 3X amplification of the CS signal.

This is used for comparison to the external error amplifier

signal. If an external resistor is connected from CS to the

current sense resistor, the internal current source will pro-

vide a programmable slope compensation. The value of

the resistor will determine the level of compensation. At

higher compensation levels, voltage mode of operation can

be achieved. The error amplifier signal at the FB pin will be

used in conjunction with the CS signal to achieve regula-

tion.

Two levels of undervoltage lockout are also available. The

LUVLO (line under voltage lockout) pin via an external re-

sistive divider will program the undervoltage lockout level.

During the LUVLO, the driver outputs are disabled and the

softstart is reset.

Once VCC has exceeded the UVLO (VCC under voltage lock-

out) limit, the internal reference, oscillator, drivers and logic

are powered up.

SYNC is a positive edge triggered input with a threshold

set to 1.6V (SC4808A), and 1.0V (SC4808B).

By connecting an external control signal to the SYNC pin,

the internal oscillator frequency will be synchronized to the

positive edge of the external control signal. In a single con-

troller operation, SYNC should be grounded or connected

to an external synchronization clock within the SYNC fre-

quency range (see page 3).

In the Bi-phase operation mode a very unique oscillator

is utilized to allow two SC4808 to be synchronized

together and work out of phase. This feature is setup by

simple connection of the SYNC input to the RC pin of the

other part. The fastest oscillator automatically becomes

the master, forcing the two PWMs to operate out of

phase. This feature minimizes the input and output

ripples, and reduces stress on the capacitors.

THEORY OF OPERATION

The SC4808 is a versatile double ended, high speed, low

power, pulse width modulator that is optimized for applica-

tions requiring minimum space.

The device contains all of the control and drive circuity re-

quired for isolated or non isolated power supplies where

an external error amplifier is used. A fixed oscillator fre-

quency (up to 1MHz) can be programmed by an external

RC network.

The SC4808 is a peak current or voltage mode controller,

depending on the amount of slope compensation,

programmable with only one external resistor. The cycle by

cycle peak current limit prevents core saturation when a

transformer is used for isolation while the overcurrent

circuitry initiates the softstart cycle.

The SC4808 dual output drive stages are arranged in a

push-pull configuration. Both outputs switch at half the

oscillator frequency using a toggle flip flop. The dead time

between the two outputs is programmable depending on

the values of the timing capacitor and resistors, thus limiting

each output stage duty cycle to less than 50%.

The SC4808 also provides flexibility with programmable

LUVLO thresholds, with built-in hysteresis.

SUPPLY

A single supply, VCC is used to provide the bias for the

internal reference, oscillator, drivers, and logic circuitry of

SC4808.

PWM CONTROLLER

SC4808 is a double ended PWM controller that can be

used in voltage or current mode applications. Two voltage

options are available for the SC4808. The SC4808A ver-

sion has a typical VCC under voltage of 12.0V and a 5V

reference. The SC4808B version provides a 4.4V VCC UVLO,

and a 3.125V reference. The oscillator frequency is pro-

grammed by a resistor and a capacitor network connected

to an external reference provided by the SC4808. The two

outputs, OUTA and OUTB, are 180 degrees out of phase

and run at half of the oscillator frequency.

An external error amplifier will provide the error signal to

the FB pin of the SC4808.

The current sense input and internal slope compensation

are both provided via the CS pin. The current sense input

from a sense resistor is used for the peak current and

Application Information

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