SP6125 Datasheet, PDF (5/12 Pages) Sipex Corporation – High-Voltage, Step Down Controller in TSOT6

SP6125
General Overview
Vout
CP1 2pF
CZ2 130pF
RZ2 200k
Vref =0.6V
Error Amplif ier
VFB
RZ
CZ
R1
200k, 1%
R2
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/5mΩ ceramic capacitor.
Determine whether Type-III compensation is
needed.
From equation (2) f_ESRZERO = 1.45MHz. From
equation (3) f_DBPOLE = 8.4kHz. 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 95pF (use 100
pF). Following the guideline given in table 1,
a 2kΩ 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, 82mΩ Aluminum Electrolytic
capacitor. Determine whether Type-III
compensation is needed.
From equation (2) f_ESRZERO = 13kHz. From
equation (3) f_DBPOLE = 4.5kHz. 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 175pF (use
180 pF). Since f_ESRZERO ÷ f_DBPOLE is
approximately 3, RZ has to be set at 30kΩ.
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.
Mar22-07 RevC
SP6125: TSOT-6 PFET Buck Controller
5
 2007 Sipex Corporation