
______________________________________________________ GENERAL OVERVIEW
SP6127
Vout
CP1
2pF
RZ
CZ2
130pF
RZ2
200k
CZ
R1
200k, 1%
VFB
Vref =0.6V
R2
Error Amplif ier
Figure 1- RZ and CZ in conjunction with internal compensation components form a Type-
III compensation network
Loop Compensation Example 1 - A converter
utilizing a SP6127 has a 3.3 μ H inductor and a
22 μ F/5m ceramic capacitor. Determine
whether Type-III compensation is needed.
response of the circuit, seen in figure 2,
validates the above procedure.
Loop Compensation Example 2 - A converter
utilizing the SP6127 has a 3.3 μ H inductor and
From equation (2)
f ESRZERO
= 1.45MHz. From
a 220 μ F, 82m Aluminum Electrolytic
capacitor. Determine whether Type-III
equation (3) f DBPOLE = 18.4 kHz. Since the
compensation is needed.
condition specified in (1) is not met, Type-III
compensation must be used by adding
From equation (2)
f ESRZERO
= 8.8kHz. From
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
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 ESRZERO / f DBPOLE is approximately 2,
RZ must be set at 40k .
Figure 2- Satisfactory frequency response of typical application circuit shown on page 1.
Crossover frequency f c is 100kHz with a corresponding phase margin of 60 degrees.
Jan28-08 RevE
SP6127 TSOT-6 PFET Buck Controller
Page 5
? 2007 Exar Corporation