Mar22-07 RevC

SP6125: TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

5

R1

200k, 1%

R2

SP6125

VFB

RZ2

200k

CZ2

130pF

CP1

2pF

Error Amplif ier

Vref =0.6V

Vout

RZ

CZ

Figure 1- RZ and CZ in conjunction with internal

compensation components form a Type-III compensation

Loop Compensation Example 1- A converter

utilizing a SP6125 has a 8.2uH inductor and

two

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 95pF (use 100

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 SP6125 has a 8.2uH inductor and

a

150uF,

82m

Aluminum

Electrolytic

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 175pF (use

180

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 about 35kHz with a corresponding phase margin of 60 degrees. The

two sets of curves, which are essentially identical, correspond to load current of 1A and 2.5A.

General Overview