Datasheet

L7981 Application informations
Doc ID 15182 Rev 3 19/44
6.4 Compensation network
The compensation network has to assure stability and good dynamic performance. The loop
of the L7981 is based on the voltage mode control. The error amplifier is a voltage
operational amplifier with high bandwidth. So selecting the compensation network the E/A
will be considered as ideal, that is, its bandwidth is much larger than the system one.
The transfer functions of PWM modulator and the output LC filter are studied (see Figure
10.). The transfer function of the PWM modulator, from the error amplifier output (COMP
pin) to the OUT pin, results:
Equation 13
where V
S
is the sawtooth amplitude. As seen in Chapter 5.1, the voltage feed forward
generates a sawtooth amplitude directly proportional to the input voltage, that is:
Equation 14
In this way the PWM modulator gain results constant and equals to:
Equation 15
The synchronization of the device with an external clock provided trough SYNCH pin can
modifies the PWM modulator gain (see Chapter 5.1 to understand how this gain changes
and how to keep it constant in spite of the external synchronization).
Figure 9. The error amplifier, the PWM modulation and the LC output filter
The transfer function on the LC filter is given by:
G
PW0
V
IN
V
s
---------=
V
S
KV
IN
⋅=
G
PW0
V
IN
V
s
---------
1
K
--- - 13===
FB
COMP
V
REF
E/A
PWM
V
S
OUT
V
CC
C
OUT
ESR
L
G
PW0
G
LC
FB
COMP
V
REF
E/A
PWM
V
S
OUT
V
CC
C
OUT
ESR
L
G
PW0
G
LC