SC2542TETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2542TETRT Datasheet PDF : 27 Pages
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SC2542
POWER MANAGEMENT
Applications Information (Cont.)
FO =
1
2π LCO
FZ
=
2π
1
R C ESR O
The compensator in Figure 10 includes an error ampli-
fier and compensation networks Zf and Zs. It is imple-
mented by the circuit in Figure 12. The compensator pro-
vides an integrator, double poles, and double zeros. As
shown in Figure 11, the integrator is used to boost the
gain at low frequency. Two zeros are introduced to com-
pensate excessive phase lag at the loop gain crossover
due to the integrator (-90deg) and the complex pole pair
(-180deg). Two high frequency poles are designed to com-
pensate the ESR zero and to attenuate high frequency
noise.
A resistive divider is used to program the output voltage.
The top resistor, Rtop, of the divider in Figure 12 can be
chosen from 20kΩ to 30kΩ. Then the bottom resistor,
Rbot, is found from:
RBOT
=
0.75V
VO − 0.75V
∗ RTOP
where 0.75V is the internal reference voltage of the
SC2542.
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.
(2). Select the open loop crossover frequency Fc located
at 10% to 20% of the switching frequency.
(3). Use the first compensator pole Fp1 to cancel the
ESR zero.
(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.
Design Procedure for a Step-down Power Converter
Figure 11. Bode plots for control loop design.
C2
C1
R2
C3
R3
Vc
-
Out
Rtop
Vo
+
E/A
0.75V
Rbot
Selection criteria and design procedures for the follow-
ing parameters are described:
1) Output inductor (L) type and value
2) Output capacitor (Co) type and value
3) Input capacitor (Cin) type and value
4) Power MOSFETs
5) Current sensing and limiting circuit
6) Voltage sensing circuit
7) Loop compensation network
The following step-down converter specifications are
needed:
Figure 12. Compensation network.
2005 Semtech Corp.
14
www.semtech.com
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