SLMA002 datasheet(20/55 Pages) TI1

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OUTmax

REF

CLx

- REF(1 +

)

REF

(1 +

) ´ TS

´ L

LOAD

(1 -

)

(3)

Minimum Output Capacitance

Modifying The Feedback Loop

PVDD2

BOOT2

SW2

PVDD1

BOOT1

SW1

GND

TPS5428x

SEQ

ILIM2

FB2

EN1

EN2

FB1

OUTPUT1

UDG-07013

TPS54283,, TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

Ensure the value of capacitance selected for closed loop stability is compatible with the requirements of Soft

Start.

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

increasing the required value 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 29 ) 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.

Figure 29. 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 29). 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.

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