TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
VIN
VC
VOUT
+
+
VREF
Modulator
_
_
Filter
Current
Feedback
Network
Compensation
Network
Figure 23. Small Signal Equivalent Block Diagram
Inductor-Capacitor (L-C) Selection
The following figures plot the TPS5438x Output-to-Vc gain and phase versus frequency for various duty cycles
(10%, 30%, 50%, 70%, 90%) at three (200 mA, 400 mA, 600 mA) peak-to-peak ripple current levels. The loop
response curve selected to compensate the loop is based on the duty cycle of the application and the ripple
current in the inductor. Once the curve has been selected and the inductor value has been calculated, the output
capacitor is found by calculating the L-C resonant frequency required to compensate the feedback loop. A brief
example follows the curves.
Note that the internal error amplifier compensation is optimized for output capacitors with an ESR zero frequency
between 20 kHz and 60 kHz. See the following sections for further details.
GAIN AND PHASE
vs
FREQUENCY
GAIN AND PHASE
vs
FREQUENCY
100
80
270
225
180
135
90
100
80
270
225
180
135
90
Duty Cycle %
Duty Cycle %
Gain Phase
Gain Phase
10
30
50
70
90
10
30
50
70
90
60
40
60
40
45
45
20
0
20
0
0
0
-45
-45
-20
-90
-20
-90
100
1 k
10 k
f - Frequency -Hz
100 k
1 M
100
1 k
10 k
100 k
1 M
Figure 24. TPS54383 at 200-mApp Ripple Current
Figure 25. TPS54383 at 400-mApp Ripple Current
18
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