Datasheet

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f
P
+ f
C
10 +
1
2 p R
2
C
1
(Hz)
(32)
f
Z
+
f
C
10
+
1
2 p R
2
C
2
(Hz)
(33)
Compensating the Loop (Type III)
+
VX
COMP
Error Amplifier
FB
UDG−04143
R
2
R
1
R
BIAS
C
2
C
1
R
3
C
3
f − Frequency − kHz
Gain − dB
f
Z
f
Z3
f
P3
K
HF
K
COMP(co)
K
LF
f
C
f
P
TPS40100
SLUS601–MAY 2005
APPLICATION INFORMATION (continued)
If the output capacitor does not have sufficient ESR to use the phase shift from the ESR zero, a Type III
compensator is required. This is the case for most designs with ceramic output capacitors only. A series R-C
circuit is added in parallel to R
1
as shown in Figure 8.
This is the same compensator as in Figure 6 except for the addition of C
3
and R
3
. A typical response of this
circuit is shown in Figure 9.
COMPENSATOR GAIN
vs
FREQUENCY
Figure 9. Figure 8. Type III Compensator Schematic
The reason for using the Type III compensator is to take advantage of the phase lead associated with the
upward slope of the gain between f
Z3
and f
P3
. The crossover frequency should be located between these two
frequencies. The amount of phase lead generated is dependent on the separation of the f
Z3
and f
P3
. In general, if
f
Z3
is one half of f
C
and f
P3
is twice f
C
, the amount of phase lead at f
C
generated is sufficient for most applications.
Certainly more or less may be used depending on the situation.
As an example of selecting the extra required extra phase lead, suppose that the control to output gain phase
evaluates to -145° at f
C
. The Type II compensator has approximately 11.5° of phase lag at f
C
due to the origin
pole, the zero at f
C
/10 and the pole at 10xf
C
. This would give only 23.5° of phase margin, which while stable is
not ideal. Placing f
Z3
and f
P3
at one half and twice the crossover frequency respectively adds approximately 36°
of phase lead at f
C
for a new phase margin of 59.5°.
To calculate the values for this type of compensator, first select R1. Again the choice is somewhat arbitrary. 10
kΩ is a suggested value.
18