AN2495 datasheet(19/36 Pages) STMICROELECTRONICS

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AN2495

Frequency response and loop compensation

Doc ID 13127 Rev 5

19/36

Figure 7.

Converter feedback network

Its transfer function, which comprises a pole at the origin and a zero-pole pair, is given by:

Equation 38

The task of the control loop design is then to determine the transfer function G2(s) to ensure

that the resulting closed-loop system is stable and performs well in terms of dynamic

response, and line and load regulation. It is well known that the characteristics of the closed-

loop system can be inferred from its open loop transfer function properties, that is

G(s) = G1(s) · G2(s).

Frequency response requirements are summarized below:

Optimum dynamic performance requires a large gain bandwidth, that is the open loop

cross-over frequency fC to be typically chosen equal to fsw/5 (fc = 10 kHz).

Phase margin

ϕm comprised between 45° and 90° is used as a design guideline. This

ensures fast transient response with very little ringing. Sometimes this is not enough

and so phase shift should be lower than 180° at any frequency below fC, because

a phase shift over 180° would result in a conditionally stable system.

Good load and line regulation implies a high DC gain (this requirement is ensured by

the feedback network, whose transfer function has a pole at the origin).

First choose a typical value for RL = R22 = 2.7 kΩ.

2 s

()

co mp s

()

out s

()

-----------------------

CTR

max

COM P

•

BRHCF

------------------------------------------------

---

•

1s R

()C

1sR

COM PCco mp

--------------------------------------------------

•

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