STMicroelectronics 12 V, 150 mA non-isolated buck converter using VIPer™ Plus - VIPER06XS STEVAL-ISA115V1 STEVAL-ISA115V1 Data Sheet
Product codes
STEVAL-ISA115V1
DocID024275 Rev 2
19/29
AN4260
Feedback loop calculation guidelines
7.2
Compensation procedure for a DCM buck
The first step is to choose the pole and zero of the compensator and the crossing frequency.
In this case C(f) has only a zero (fzc) and a pole at the origin, thus a possible setting is:
•
fzc=k*fp
•
fcross = fcross_sel_
≤
fsw
/
10
where k is chosen arbitrarily. A starting point could be k=5
and, since by
definition it is
⏐
C(fcross_sel)*G1(fcross_sel)
⏐
= 1, C
0
can be calculated as follows:
Equation 11
At this point the Bode diagram of G1(f)*C(f) can be plotted, in order to check the phase
margin for the stability.
margin for the stability.
If the margin is not high enough, another choice should be made for k and fcross_sel, and
the procedure is repeated.
the procedure is repeated.
When the stability is ensured, the next step is to find the values of the schematic
components, which can be calculated as follows:
components, which can be calculated as follows:
Equation 12
and from
Equation 13
are suggested values. Commercial
values are chosen, let us call them C7_act, R7_act, resulting into fzc_act.
Equation 14
C
0
Equation 15
C
0
j 2
π
fcross_sel
⋅ ⋅ ⋅
1
j fcross_sel
⋅
fzc
----------------------------------
+
-----------------------------------------------------
H
C OM P
G1 fcross_sel
(
)
---------------------------------------------
⋅
=
C7
L fsw
⋅
V
IN
V
OU T
–
-------------------------
Gm
–
C
0
----------------
R4
R4
R5
+
----------------------
⋅
⋅
=
R3
1
2
π
fzc C7
⋅ ⋅
⋅
------------------------------------
=
fzc_act
1
2
π
R3_act C7_act
⋅ ⋅
⋅
-----------------------------------------------------------
=
C
0
_act
L fsw
⋅
V
IN
V
O UT
–
-------------------------
Gm
–
C7_act
-------------------
R4_act
R4_act
R5 4
( )
_act
+
------------------------------------------------------
⋅
⋅
=