TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
R1
(1 +
) ´ TS
VREF
tSS
R2
R1
1
+
(1 +
I
CLx - VREF
)(1 -
)
COUTmax
=
R2
RLOAD
VREF
2 ´ VIN ´ L
(3)
Minimum Output Capacitance
Ensure the value of capacitance selected for closed loop stability is compatible with the requirements of Soft
Start.
Modifying The Feedback Loop
Within the limits of the internal compensation, there is flexibility in the selection of the inductor and output
capacitor values. A smaller inductor increases ripple current, and raises the resonant frequency, thereby
incerasing the required amount of output capacitance. A smaller capacitor could also be used, increasing the
resonant frequency, and increasing the overall loop bandwidth—perhaps at the expense of adequate phase
margin.
The internal compensation of the TPS54x8x is designed for capacitors with an ESR zero frequency between
20kHz and 60kHz. It is possible, with additional feedback compensation components, to use capacitors with
higher or lower ESR zero frequencies. For either case, the components C1 and R3 (ref.Figure 30 ) are added to
re-compensate the feedback loop for stability. In this configuration a low frequency pole is followed by a higher
frequency zero. The placement of this pole-zero pair is dependent on the type of output capacitor used, and the
desired closed loop frequency response.
TPS5438x
1
2
3
4
5
6
7
PVDD1 PVDD2 14
BOOT1 BOOT2 13
OUTPUT1
SW1
GND
EN1
EN2
FB1
SW2 12
BP 11
C2
R1
SEQ 10
C1
R3
ILIM2
FB2
9
8
R2
UDG-07013
Figure 30. Optional Loop Compensation Components
NOTE:
Once the filter and compensation components have been established, laboratory
measurements of the physical design should be performed to confirm converter
stability.
Using High-ESR Output Capacitors
If a high ESR capacitor is used in the output filter, a zero appears in the loop response that could lead to
instability. To compensate, a small R-C series connected network is placed in parallel with the lower voltage
setting divider resistor (Ref Figure 30). The values of the components are determined such that a pole is placed
at the same frequency as the ESR zero and a new zero is placed at a frequency location conducive to good loop
stability.
20
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