Datasheet

A5973D Closing the loop
19/41
If the damping coefficient δ is very close to zero, the roots of the equation become a double
root whose value is ω
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 the following formula:
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.
In summary, the open loop gain can be expressed as:
Equation 15
F
PLC
1
2 π• LC
OUT
••
----------------------------------------------=
G
PWM
s()
V
cc
V
OSCMAX
V
OSCMIN
–()
-------------------------------------------------------------=
V
OSCMAX
V
OSCMIN
– KV
CC
•=
G
PWM
s()
1
K
--- - const==
Gs() G
PWM
s()
R
2
R
1
R
2
+
--------------------
• A
O
s()• A
LC
• s()=