SC2621A Semtech Corporation, SC2621A Datasheet - Page 9

• Manufacturer Part Number: SC2621A
• Description: Voltage Mode PWM Controller
• Manufacturer: Semtech Corporation

Fig. 4. Bode plots for control loop design
Fig. 5. Compensation network
The top resistor R
be chosen from 1k to 5k. Then the bottom resistor R
is found from:
where 0.5V is the internal reference voltage of the
SC2621A.
The other components of the compensator can be cal-
culated using following design procedure:
(1). Plot the converter gain, including LC filter and PWM
modulator.
POWER MANAGEMENT
Applications Information (Cont.)
R
bot
2007 Semtech Corp.
Vc
-30
-30
-60
-60
www.DataSheet4U.com
60
60
30
30
=
0
0
100
100
V
O
0
-
5 .
Fz1
Fz1
0
V
1
C1
5 .
V
C2
×
1K
1K
top
R
Fo
Fo
top
VREF
0.5V
-
+
R2
Fz2
Fz2
of the voltage divider in Fig. 5 can
2
3
FR EQ UENCY (Hz )
FR EQ UENCY (Hz )
Fp1
Fp1
Fz
Fz
10K
10K
COM PENSATOR GAI N
COM PENSATOR GAI N
C3
Rbot
Rtop
100 K
100 K
R3
Fc
Fc
Fp2
Fp2
Vo
1M
1M
bot
9
(2). Select the open loop crossover frequency Fc located
at 10% to 20% of the switching frequency. At Fc, find the
required DC gain.
(3). Use the first compensator pole Fp1 to cancel the
ESR zero Fz.
(4). Have the second compensator pole Fp2 at half the
switching frequency to attenuate the switching ripple and
high frequency noise.
(5). Place the first compensator zero Fz1 at or below
50% of the power stage resonant frequency Fo.
(6). Place the second compensator zero Fz2 at or below
the power stage resonant frequency Fo.
A MathCAD program is available upon request for the
calculation of the compensation parameters.
LA
LAY Y Y Y Y OUT GUIDELINES
LA
The switching regulator is a high di/dt power circuit. Its
Printed Circuit Board (PCB) layout is critical. A good lay-
out can achieve an optimum circuit performance while
minimizing the component stress, resulting in better sys-
tem reliability. During PCB layout, the SC2621A control-
ler, MOSFETs, inductor, and power decoupling capacitors
have to be considered as a unit.
The following guidelines are typically recommended for
using the SC2621A controller.
(1). Place a 4.7uF to 10uF ceramic capacitor close to
the drain of top MOSFET for the high frequency and high
current decoupling. The loop formed by the capacitor,
the top and bottom MOSFETs must be as small as pos-
sible. Keep the input bulk capacitors close to the drain
of the top MOSFETs.
(2). Place the SC2621A over a quiet ground plane to avoid
pulsing current noise. Keep the ground return of the gate
drive short.
(3). Connect bypass capacitors as close as possible to
the decoupling pins (DRV and Vcc) to the ground pin GND.
The trace length of the decoupling capasitor on DRV pin
should be no more than 0.2” (5mm).
(4). Locate the components of the bootstrap circuit close
to the SC2621A.
LA
LA
OUT GUIDELINES
OUT GUIDELINES
OUT GUIDELINES
OUT GUIDELINES
SC2621A
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