Datasheet

www.ti.com
K
COMP(co)
+
1
Ť
K
CO
(fc)
Ť
(Hz)
(30)
f − Frequency − kHz
Gain − dB
f
Z
f
C
f
P
K
COMP(co)
R
2
+ K
LF
R
1
(W)
(31)
TPS40100
SLUS601–MAY 2005
APPLICATION INFORMATION (continued)
Using either the analytical model or the simulated model, determine the control to output gain at the chosen loop
crossover frequency. The gain of the compensator is the reciprocal of this gain:
where
• K
COMP(CO)
is the required compensator gain at the crossover frequency
• K
CO
(f
C
) is the value of the control to output transfer function at the crossover frequency
If simulating the response using the model, the control to output gain is V
X
/V
OUT
. Sweep the AC voltage source
over the range of interest and plot V
X
/V
OUT
.
Depending on the chosen loop crossover frequency and the characteristics of the output capacitor, either a Type
II or a Type III compensator could be required. If the output capacitance has sufficient ESR, phase shift from the
ESR zero may by used to eliminate the need for a Type III compensator. The model in Figure 6 uses a Type II
compensator. In this case the compensator response looks like Figure 7.
COMPENSATOR GAIN
vs
FREQUENCY
Figure 7.
First select R
1
. The choice is somewhat arbitrary but affects the rest of the components once chosen. The
suggested value is 10 kΩ.
R
2
is found from the gain required from the compensator at the crossover frequency.
It is generally recommended to place the pole frequency one decade above the crossover frequency and the
zero frequency one decade below the crossover frequency.
17