SC2422ACSTR SEMTECH [Semtech Corporation], SC2422ACSTR Datasheet - Page 7

• Manufacturer Part Number: SC2422ACSTR
• Description: BIPHASE CURRENT MODE CONTROLLER
• Manufacturer: SEMTECH [Semtech Corporation]

PRELIMINARY - August 7, 2000
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
© 2000 SEMTECH CORP.
IL
(
V
IN
O
/V
V
OUT
IN
L
. The approximation arises from the
)
x
T
ON
(
V
IN
FxL
V
OUT
)
x
D
BIPHASE CURRENT MODE
CONTROLLER
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
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.
652 MITCHELL ROAD NEWBURY PARK CA 91320
Vid0
Vid4
VOUT
BGOUT (P15)
f
V
OSC
IN
DAC
REF
12
Ri
13
V
REF
resistor (pin 8) to ground.
k
FB(P7)
Io
(pin 8) to ground programs the
R
*
REF
500
3K
Ros
Ccomp
kHz
+
-
Rf
I
, R
Bandgap
1.5V
E/A
Rcomp
OS
and R
ERROUT(P6)
F
SC2422A
set the DAC
7