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Feedback Loop and Inductor-Capacitor (L-C) Filter Selection

TPS5428x

COMP

11.5 kW

Error Amplifier

0.8 V

REF

BOOT

COMP

Offset

f(I

DRAIN

)

PWM to

Switch

SLOPE

COMP

TPS54283

TPS54286

COMP

(kW)

COMP

(pF)

700

UDG-07012

COMP

- I

SLOPE

TPS54283,, TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

In the feedback signal path, the output voltage setting divider is followed by an internal gM-type error amplifier

with a typical transconductance of 30

µS. An internal series connected R-C circuit from the g

M amplifier output to

ground serves as the compensation network for the converter. The signal from the error amplifier output is then

buffered and combined with a slope compensation signal before it is mirrored to be referenced to the SW node.

Here, it is compared with the current feedback signal to create a pulse-width-modulated (PWM) signal-fed to

drive the upper MOSFET switch. A simplified equivalent circuit of the signal control path is depicted in Figure 21.

NOTE:

Noise coupling from the SWx node to internal circuitry of BOOTx may impact narrow

pulse width operation, especially at load currents less than 1 A. See SW Node

Ringing for further information on reducing noise on the SWx node.

Figure 21. Feedback Loop Equivalent Circuit

A more conventional small signal equivalent block diagram is shown in Figure 22. Here, the full closed loop

signal path is shown. Because the TPS5428x contains internal slope compensation and loop compensation

components, the external L-C filter must be selected appropriately so that the resulting control loop meets criteria

for stability. This approach differs from an externally-compensated controller, where the L-C filter is generally

selected first, and the compensation network is found afterwards. To find the appropriate L and C filter

combination, the Output-to-Vc signal path plots (see the next section) of gain and phase are used along with

other design criterial to aid in finding the combinations that best results in a stable feedback loop.

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