SC4905AMSTR datasheet(9/20 Pages) SEMTECH

9 / 20 page

SC4905A/B

POWER MANAGEMENT

PRELIMINARY

 2002 Semtech Corp.

www.semtech.com

FEED FORWARD & MAXIMUM DUTY CYCLE

The feed forward function provided by the SC4905 will

improve the dynamic performance of the converter in

response to the changes in the input supply.

In voltage mode controllers without the voltage feed for-

ward circuitry, any changes in the input supply will cause

an error in the output voltage which is sensed by the error

amplifier and eventually is translated to an adjustment in

the duty cycle by the controller. This delay in the response

will cause the slower dynamic performance of the con-

verter.

This problem is resolved by sensing the input supply line

and making the adjustment in the duty cycle immediately

at the PWM controller.

R14

280k

C27

220p, 16V

R15

499k

Shut Down

Vin

R16

R18

8.25k

5+"'#

DMAX

VFF

REF

VDD

SYNC

ILIM

GND

OUT

The SC4905 uses the input supply line as the bias for the

oscillator circuitry, and the VFF pin. Any changes in the

line will cause the ramp peak voltage to be adjusted to

the VFF pin voltage while maintaining the oscillator fre-

quency unchanged.

The VFF pin can also be used to shut down the SC4905

if it is pulled down to GND by an open collector circuitry.

This can be useful for overvoltage protection or other

control signals.

The SC4905 also provides a programmable duty cycle,

that can be set by an external voltage divider from the

VFF pin. The ratio of the divider will determine the pro-

grammed duty cycle allowed.

VFF

VDMAX

DutyCycle =

If the application does not require an upper limit on the

duty cycle, the VFF pin should be connected to the DMAX

pin. In this mode, the duty cycle will be allowed to increase

to the maximum limit of about 100%.

SYNC/Bi-Phase operation

In noise sensitive applications where synchronization of

the oscillator frequency to a reference frequency is re-

quired, the SYNC pin can accept the external clock.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. SYNC is

a positive edge triggered input with a threshold set to

0.5*VFF.

In a single controller operation, SYNC chould be grounded

or connected to an external synchronization clock within

the SYNC frequency range (see page 3).

Rosc2

VIN

Cosc1

Cosc2

5+"'#

DMAX

VFF

REF

VDD

SYNC

ILIM

GND

OUT

Rosc1

5+"'#

DMAX

VFF

REF

VDD

SYNC

ILIM

GND

OUT

In the Bi-Phase operation mode a very unique oscillator

is utilized to allow two SC4905 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.

Application Information (Cont.)

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