SC2450ISWTR(2001) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC2450ISWTR Datasheet PDF : 20 Pages
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SC2450
POWER MANAGEMENT
Control Loop Design
R2
R1
0
Gpwm
Error-Amp
-
Verror
Duty
L
+
R
+
Vbg
-
C
+
Vin
-
Rc
Ro
Co
0
0
0
0
Fig. 1. SC2450 control model.
The control model of SC2450 can be depicted in Fig. 1. This
model can also be used in Spice kind of simulator to generate
loop gain Bode plots. The bandgap reference is 1 V and
trimmed to +/-1% accuracy. The desired output voltage can
be achieved by setting the resistive divider network, R1 and
R2.
The task here is to properly choose the compensation network
for a nicely shaped loop-gain Bode plot. The following design
procedures are recommended to accomplish the goal:
(1) Calculate the corner frequency of the output filter:
.o
1
2.π. L .C o
(2) Calculate the ESR zero frequency of the output filter ca-
pacitor:
. esr
1
2 .π.R c .C o
(3) Check that the ESR zero frequency is not too high.
.
.
e r s<
sw
5
The error amplifier is transconductance type with fixed gain of:
..
G
..
error
..
A
0.002 ⋅
V
The compensation network includes a resistor and a capacitor
in series, which terminates from the output of the error ampli-
fier to the ground.
If this condition is not met, the compensation structure may
not provide loop stability. The solution is to add some electro-
lytic capacitors to the output capacitor bank to correct the
output filter corner frequency and the ESR zero frequency. In
some cases, the filter inductance may also need to be ad-
justed to shift the filter corner frequency. It is not recommended
to use only high frequency multi-layer ceramic capacitors for
output filter.
This device uses voltage mode control with input voltage feed
forward. The peak-to-peak ramp voltage is proportional to the
input voltage, which results in an excellent performance to re-
ject input voltage variation. The PWM gain is inversion of the
ramp amplitude, and this gain is given by:
1
G pwm V ramp
where the ramp amplitude (peak-to-peak) is 3 volts when input
voltage is 24 volts.
The total control loop-gain can then be derived as follows:
T( s)
T o.
1 s.R.C
s. R. C
.
1
1 s.R c.C o
s. R c.C o
L
Ro
s2.L.C o. 1
Rc
Ro
where
(4) Choose the loop gain cross over frequency (0 dB frequency).
It is recommended that the crossover frequency is always less
than one fifth of the switching frequency or the output ripple
frequency in bi-phase mode operation:
.x_over
. sw
5
If the transient specification is not stringent, it is better to choose
a crossover frequency that is less than one tenth of the switch-
ing frequency for good noise immunity. The resistor in the
compensation network can then be calculated as:
R
when:
1
.
F esr
2
.
F x_over
.
Vo
G pwm.V in.G error F o
F esr V bg
To
..
G
..
error.Vin.G pwm .R .
R
R
1
2
R
2
. o < . esr< . x_over
2001 Semtech Corp.
16
www.semtech.com
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