SLUS593B 鈭?DECEMBER 2003 鈭?REVISED APRIL 2004
TPS40054
TPS40055
TPS40057
A
PPLICATION INFORMATION
C2
(optional)
鈭?
R3
C1
R2
0 dB
C3
R1
VFB
7
8
VOUT
RBIAS
+
鈭?70擄
VREF
UDG鈭?2189
+1
鈭?
鈭?0擄
COMP
180擄
GAIN
PHASE
Figure 12. Type III Compensation Configuration
Figure 13. Type III Compensation Gain and
Phase
The poles and zeros for a Type III network are described in equations (25).
f
Z1
+
f
P1
+
2p
2p
1
R2
1
R2
C1
C2
(Hertz)
(Hertz)
f
Z2
+
f
P2
+
2p
2p
1
R1
1
R3
C3
C3
(Hertz)
(Hertz)
(25)
The value of R1 is somewhat arbitraty, but influences other component values. A value between 50k鈩?and
100k鈩?usually yields reasonable values.
The unity gain frequency is described in equation (26)
f
C
+
1
2p
R1
C2
G
(Hertz)
(26)
where G is the reciprocal of the modulator gain at f
C
.
The modulator gain as a function of frequency at f
C
, is described in equation (27).
AMOD(f)
+
AMOD
Minimum Load Resistance
Care must be taken not to load down the output of the error amplifier with the feedback resistor, R2, that is too
small. The error amplifier has a finite output source and sink current which must be considered when sizing R2.
Too small a value does not allow the output to swing over its full range.
R2
(MIN)
+
V
C (max)
I
SOURCE (min)
+
3.5 V
+
1750
W
2 mA
f
LC
f
C
2
and
G
+
1
AMOD(f)
(27)
(28)
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