Datasheet

Closing the loop B5973D
20/44 DocID14117 Rev 4
The poles of the transfer function can be calculated through the following expression:
Equation 9
In the denominator of A
LC
the typical second order system equation can be recognized:
Equation 10
If the damping coefficient  is very close to zero, the roots of the equation become a double
root whose value is 
n
.
Similarly for A
LC
the poles can usually be defined as a double pole whose value is:
Equation 11
7.3 PWM comparator
The PWM gain is given by following equation:
Equation 12
where V
OSCMAX
is the maximum value of a sawtooth waveform and V
OSCMIN
is the
minimum value. A voltage feed forward is implemented to ensure a constant GPWM. This is
obtained by generating a sawtooth waveform directly proportional to the input voltage V
CC
.
Equation 13
Where K is equal to 0.076. Therefore the PWM gain is also equal to:
Equation 14
This means that even if the input voltage changes, the error amplifier does not change its
value to keep the loop in regulation, thus ensuring a better line regulation and line transient
response.
F
PLC1 2
ESR C
OUT
ESR C
OUT

2
4L C
OUT
––
2L C
OUT

------------------------------------------------------------------------------------------------------------------------------------------=
s
2
2 
n
 s 
2
n
++
F
PLC
1
2  LC
OUT

----------------------------------------------=
G
PWM
s
V
cc
V
OSCMAX
V
OSCMIN
–
-------------------------------------------------------------=
V
OSCMAX
V
OSCMIN
– KV
CC
=
G
PWM
s
1
K
---- const==