Datasheet

VSENSE
COMP
VOUT
R8
R4
C4
C6
R9
Coea
Roea
gm
1300 mA/V
0.8 V
Power Stage
PH
R
ESR
C
O
R
L
b
a
c
12 A/V
TPS54120
SBVS180C –JANUARY 2012–REVISED JUNE 2012 www.ti.com
SMALL-SIGNAL MODEL FOR LOOP RESPONSE
Figure 33 shows an equivalent model for the device control loop. This model can be run in a circuit simulation
program to check frequency and transient responses. The error amplifier is a transconductance amplifier with a
g
M
of 1300 mA/V, and can be modeled using an ideal voltage-controlled current source. Resistor Roea (2.38
MΩ) and capacitor Coea (20.7 pF) model the open-loop gain and frequency response of thSLVA497e error
amplifier.
Figure 33. Small-Signal Model for Loop Response
The 1-mV ac voltage source between nodes a and b effectively breaks the control loop for the frequency-
response measurements. Plotting a/c and c/b show the small-signal responses of the power stage and frequency
compensation, respectively. Plotting a/b shows the small-signal response of the overall loop. The dynamic loop
response can be checked by replacing R
L
with a current source that has the appropriate load step amplitude and
step rate in a time-domain analysis. Refer to Application Report SLVA503, Understanding Compensation
Network for the TPS54120, for a more detailed treatment of the small-signal model and compensation for the
TPS54120.
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