Datasheet

Application informations L7981
22/44 Doc ID 15182 Rev 3
Equation 21
where K is the feed forward constant and 1/K is equals to 9.
3. Calculate C
4
by placing the zero at 50% of the output filter double pole frequency (f
LC
):
Equation 22
4. Calculate C
5
by placing the second pole at four times the system bandwidth (BW):
Equation 23
5. Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
Equation 24
The suggested maximum system bandwidth is equals to the switching frequency divided by
3.5 (F
SW
/3.5), anyway lower than 100 kHz if the F
SW
is set higher than 500 kHz.
For example with V
OUT
=5 V, V
IN
=24 V, I
O
=3 A, L=18 μH, C
OUT
=22 μF, ESR<1 mΩ, the type
III compensation network is:
In Figure 12 is shown the module and phase of the open loop gain. The bandwidth is about
58 kHz and the phase margin is 50°.
R
4
BW
f
LC
--------- -
KR
1
⋅⋅=
C
4
1
π R
4
f
LC
⋅⋅
---------------------------=
C
5
C
4
2π R
4
C
4
4BW⋅ 1–⋅⋅⋅
------------------------------------------------------------- -=
R
3
R
1
4BW⋅
f
LC
----------------- 1–
---------------------------= C
3
1
2π R
3
4BW⋅⋅⋅
---------------------------------------- -=,
R
1
4.99kΩ= R
2
680Ω= R
3
200Ω= R
4
3.3kΩ= C
3
3.3nF= C
4
22nF= C
5
220pF=,,,, ,,