SC2422 Semtech Corporation, SC2422 Datasheet - Page 9

• Manufacturer Part Number: SC2422
• Description: BIPHASE CURRENT MODE CONTROLLER
• Manufacturer: Semtech Corporation

G
PRELIMINARY - August 7, 2000
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
© 2000 SEMTECH CORP.
OSC
LOOP
S
CA
= Input current sense resistor
= Current Amplifier Gain
= Oscillator period
V
V
RAMP
IN
*
R
*
R
F
I
BIPHASE CURRENT MODE
CONTROLLER
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.
652 MITCHELL ROAD NEWBURY PARK CA 91320
SC2422A
9