June 26, 2007

SP6127 TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

Page 5

Loop Compensation Example 1- A converter

utilizing a SP6127 has a 3.3µH inductor and a

22µF/5m

ceramic

capacitor.

Determine

whether Type-III compensation is needed.

From equation (2)

f

equation (3)

f

DBPOLE

= 18.4 kHz. Since the

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

compensation

must

be

used

by

adding

external components RZ and CZ. Using

equation (4) CZ is calculated to be 34pF (use

33pF). 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 the SP6127 has a 3.3µH inductor and

a

220µF,

82m

Aluminum

Electrolytic

capacitor.

Determine

whether

Type-III

compensation is needed.

From equation (2)

f

equation (3)

f

DBPOLE

= 5.9kHz. Since the

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

compensation needs to be used by adding

external components RZ and CZ. Using

equation (4) CZ is calculated 108pF (use 100

pF). Since

f

f

RZ must be set at 40k .

R1

200k, 1%

R2

SP6127

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 network

Figure 2- Satisfactory frequency response of typical application circuit shown on page 1.

Crossover frequency