SC2422A

BIPHASE CURRENT MODE

CONTROLLER

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

PRELIMINARY - August 7, 2000

9

Where:

R

T

G

0.3V is the ramp added for slope compensation when

the output current is near zero.

The DC loop gain is the product of the modulator gain

and the error amplifier gain and is calculated as follows:

Refer to Application note AN00-1 for detailed treatment

of frequency compensation component selection as well

as programming the SC2422A. The application note is

available on the Semtech website or by contacting the

factory.

Programming the Under Voltage Lockout

The SC2422A may be operated from any supply in +5V

to +12V range. A pin has been dedicated to externally

selecting the voltage at which the SC2422A outputs are

active. A good typical turn-on threshold value is 4.5V for

a +5V input supply and 9V for a +12V supply. A voltage

divider connected to the UVLO pin selects this threshold.

The UVLO comparator trip point is approximately

1.475V. Sufficient hysterisis is provided to ensure

proper DC/DC converter shutdown.

The UVLO setting should consider external MOSFET

driver’s UVLO threshold. Ideally, the external MOSFET

driver should turn on before the SC2422 controller and

turn off before the controller. This assures the converter

output will rise and fall slowly using the soft start feature

and that the output voltage will not go negative at turn-

off.

PCB layout

Care must be excercised when laying out the PC board

for SC2422 or other input current mode DC/DC convert-

ers. SInce the current is delivered and sensed in pulse

packets, the inductance of the current carrying traces

and thus their length must be minimized. Ceramic by-

pass capacitors must be located near the sense resistor.

For a detailed treatment and circuit parasitic models,

consult application note:

AN00-7:“Component Selection and PC Board layout

I

RAMP

F

IN

LOOP

R

*

V

R

*

V

G

=

Considerations in Input Current Mode DC/DC Con-

verters”. This application note is available by con-

tacting the factory.

Remote Sensing Capability

The SC2422 has a single ground for error amplifier

and DAC reference and for the internal biasing of the

chip. Since the chip uses approximately 10ma of qui-

escent current, the ground pin may be connected to a

remote location without fear of ground loops. When

used as a microprocessor power supply, connecting

the ground pin directly to the ground plane may result

in undesirable voltage drops in the plane at high out-

put current. This is not entirely predictable since the

error amplifier is correcting for the DC error with ref-

erence with the ground plane and not the processor

“feedback ground”. Thus any voltage difference be-

tween the two ground will result in a DC error. This

error will obviously consume valuable static error

band tolerance. To avoid this DC error, the SC2422

ground pin (pin 9) can be connected to a copper

“Island”, to which Rref (frequency setting resistor)

and Ros (offset setting resistor) will also be con-

nected. This “Island” in turn will only be connected to

the “Processor Feedback” ground via a trace. While

the trace may be long, it should not be routed through

or near the switching sections or noisy components.

This method of remote sensing will alleviate the need

for a differential amplifier to sense the output voltage/

output return pair and the design effort and costs as-

sociated with it.

SC2422A Evaluation Board

The SC2422A based DC/DC converter utilizes the

SC1205 High Speed MOSFET drivers to achieve

VRM 9.0 output Voltage Specifications. SC2422A

Evaluation Board Schematic (Figure 1) shows the

circuit for a 40A, BiPhase DC/DC converter. The

Evaluation board is available by contacting the fac-

tory or Semtech website at WWW.Semtech.com.