sc2449iswtrt Semtech Corporation, sc2449iswtrt Datasheet - Page 16

• Manufacturer Part Number: sc2449iswtrt
• Description: Sc2449 Bi-phase/dual Controller
• Manufacturer: Semtech Corporation

Fig. 1. SC2449 control model.
The control model of SC2449 can be depicted in Fig. 1.
This model can also be used in a Spice kind of simulator
to generate loop gain Bode plots.
ence is 1 V and trimmed to +/-1% accuracy. The de-
sired output voltage can be achieved by setting the re-
sistive divider network, R1 and R2.
The error amplifier is transconductance type with fixed
gain of:
The compensation network includes a resistor and a ca-
pacitor in series, which terminates from the output of
the error amplifier to the ground.
This device uses voltage mode control with input voltage
feed forward. The peak-to-peak ramp voltage is propor-
tional to the input voltage, which results in an excellent
performance to reject input voltage variation. The PWM
gain is inversion of the ramp amplitude, and this gain is
given by:
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 fol-
lows:
T s ( ) T o
where
POWER MANAGEMENT
Control Loop Design
0
2007 Semtech Corp.
R2
+
-
0
-
+
1 s R C
T o G error V in G pwm R .
Vbg
R1
s R C
Error-Amp
. .
Verror
. .
. .
G error .
1 s R c C o
.
0
G pwm
R
C
. .
.
.
Gpwm
0.002 ⋅
V ramp
Duty
1
1 s R c C o
.
A
V
R o
R 1
-
+
L
0
R 2
The bandgap refer-
Vin
R 2
s
2
L
L C o 1
0
Co
Rc
R c
R o
Ro
16
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 accom-
plish the goal:
(1) Calculate the corner frequency of the output filter:
(2) Calculate the ESR zero frequency of the output filter
capacitor:
(3) Check that the ESR zero frequency is not too high.
If this condition is not met, the compensation structure
may not provide loop stability. The solution is to add
some electrolytic 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 adjusted to shift the filter corner fre-
quency. It is not recommended to use only high frequency
multi-layer ceramic capacitors for output filter.
(4) Choose the loop gain cross over frequency (0 dB fre-
quency). 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:
If the transient specification is not stringent, it is better
to choose a crossover frequency that is less than one
tenth of the switching frequency for good noise immu-
nity. The resistor in the compensation network can then
be calculated as:
when:
R
G pwm V in G error
F o F esr
1
F e sr
F o
<
F x_over
F e rs
2 π . L C o
2 π . R c
<
<
F esr
.
F o
F s w
.
F x_ o ver
1
F sw
5
1
.
5
2
.
C o
F x_over
F esr
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SC2449
V bg
V o