Datasheet

DocID14117 Rev 4 19/44
B5973D Closing the loop
44
The poles of this transfer function are (if C
c
>> C
0
+C
P
):
Equation 3
Equation 4
whereas the zero is defined as:
Equation 5
F
P1
is the low frequency which sets the bandwidth, while the zero F
Z1
is usually put near to
the frequency of the double pole of the L-C filter (see Section 7.2 below). F
P2
is usually at a
very high frequency.
7.2 LC filter
The transfer function of the LC filter is given by:
Equation 6
where R
LOAD
is defined as the ratio between V
OUT
and I
OUT
.
If R
LOAD
>>ESR, the previous expression of A
LC
can be simplified and becomes:
Equation 7
The zero of this transfer function is given by:
Equation 8
F
0
is the zero introduced by the ESR of the output capacitor and it is very important to
increase the phase margin of the loop.
F
P1
1
2  R
0
 C
c

-------------------------------------=
F
P2
1
2  R
c
 C
0
C
p
+
--------------------------------------------------------=
F
Z1
1
2  R
c
 C
c

-------------------------------------=
A
LC
s
R
LOAD
1 ESR C
OUT
s+
s
2
LC
OUT
ESR R
LOAD
+s ESR C
OUT
 R
LOAD
L+R
LOAD
++
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=
A
LC
s
1 ESR C
OUT
 s+
LC
OUT
 s
2
ESR C
OUT
 s1++
----------------------------------------------------------------------------------------------=
F
O
1
2  ESR C
OUT

----------------------------------------------------=