Datasheet

L5987 Application information
Doc ID 14972 Rev 3 23/41
Equation 22
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 23
4. Calculate C
5
by placing the second pole at four times the system bandwidth (BW):
Equation 24
5. Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
Equation 25
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
= 3.3 V, V
IN
= 12 V, I
O
= 3 A, L = 10 μH, C
OUT
= 22 μF, ESR < 1 mΩ,
the type III compensation network is:
In Figure 13 is shown the module and phase of the open loop gain. The bandwidth is about
71 kHz and the phase margin is 46°.
R
4
BW K⋅
f
LC
------------------
R
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
1.1kΩ= R
3
220Ω= R
4
3.3kΩ= C
3
3.3nF= C
4
10nF= C
5
180pF=,,,,,,