SC2422A

BIPHASE CURRENT MODE

CONTROLLER

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

PRELIMINARY - August 7, 2000

7

Applications Information

The SC2422A is an Input Current Mode Controller de-

signed for High Current, High performance two phase

DC/DC converters. The Current mode control is imple-

mented by generating the PWM ramp from the Input

Current, rather than the output current. This has the

advantage of eliminating the output current sense re-

sistors, and the power loss associated with output cur-

rent sensing. Eliminating the output current sense re-

sistors has the added advantage of improving the tran-

sient response by reducing the output impedance.

The output voltage is programmed via a 5-bit DAC in

32 steps. A novel technique allows programmable

DAC step size and output offset, allowing the SC2422A

based DC/DC converters to work in VRM9.0, VRM 8.3,

VRM8.4, VRM8.5 or future specified voltage ranges.

Theory of Operation

Pulse by Pulse Current Matching

The operation of the Input Current Mode, ICM, is as

follows:

The SC2422A Oscillator generates the OUT1 and

OUT2 logic output drives. OUT1 and OUT2 are non-

overlapping and sequentially command an external,

power MOSFET driver to turn on the Top MOSFETs.

When the Top MOSFET is enhanced (each phase),

the input voltage is impressed across the MOSFET

and the output Inductor. The AC current in the inductor

is:

Where F is the frequency (per phase) and L is the out-

put inductor. D is the duty cycle and is approximately

equal to V

fact that the Duty cycle extends slightly to compensate

for losses in the current path. These losses include

RDS_ON of the MOSFET, the Equivalent Series Re-

sistance of the Inductors and the PCB trace resis-

tances.

The inductor current flows in the input current sense

resistor, generating a PWM ramp, same as in all cur-

rent mode controllers. The ramp is compared with an

amplified, level shifted and filtered version of the output

voltage at the PWM comparator. The comparator then

outputs a gate drive pulse that terminates when the

ramp voltage equals the error amplifier output signal.

The current mode control is inherently immune to input

voltage changes because the ramp amplitude reflects

the input voltage changes.

Since the input current sense resistor is the same for

both phases, the inherent inaccuracy due to mismatch

between output current sense resistors is avoided.

Also, since the comparator threshold is the same for

both phases, accurate current matching is achieved

between phases. This implements a pulse by pulse

current matching with a faster response to changes in

output current by monitoring the input current for each

phase.

Programming the SC2422A

Figure 2 below, is the connection schematic for the In-

ternal Error Amplifier.

Figure 2: Error amplifier connections

The external components, R

step size, output voltage offset and droop, accordingly.

A resistor from R

to ground programs the

frequency as well as the DAC current step size.

Programming the Switching Frequency

The oscillator frequency can be selected first by setting

the value of R

The switching frequency per phase is 1/2 of the above

oscillator frequency.

FxL

D

x

)

V

V

(

L

T

x

)

V

V

(

IL

OUT

IN

ON

OUT

IN

−

=

−

=

V

12

V

R

kHz

500

*

k

13

f

IN

REF

OSC

=

Ω

=

BGOUT (P15)

ERROUT(P6)

VOUT

FB(P7)

Ccomp

+

-

Ri

3K

Ros

Rf

Rcomp

E/A

Vid0

Io

Vid4

Bandgap

1.5V

DAC