Nov07-08 RevG

SP6126: TSOT-6 PFET Buck Controller

 2008 Exar Corporation

5

Loop Compensation Example 1- A converter

utilizing a SP6126 has a 6.8uH inductor and

a 22uF/5m

ceramic capacitor. Determine

whether Type-III compensation is needed.

condition specified in (1) is not met, Type-III

compensation has to be used by adding

external components RZ and CZ. Using

equation (4) CZ is calculated 61.2pF (use 62

pF). Following the guideline given in table 1,

a 2k

RZ should be used.

The steps followed in example 1 were used

to compensate the typical application circuit

shown on page 1. Satisfactory frequency

response of the circuit, seen in figure 2,

validates the above procedure.

Loop Compensation Example 2- A converter

utilizing a SP6126 has a 6.8uH inductor and

a

150uF,

82m

Aluminum

Electrolytic

capacitor.

Determine

whether

Type-III

compensation is needed.

= 5 kHz. Since the

condition specified in (1) is not met, Type-III

compensation has to be used by adding

external components RZ and CZ. Using

equation (4) CZ is calculated 160pF (use

150

pF).

Since

approximately 3, RZ has to be set at 30k .

Figure 2- Satisfactory frequency response of typical application circuit shown on page

1. Crossover frequency fc is 80kHz with a corresponding phase margin of 65 degrees.

GENERAL OVERVIEW