L6917 STMicroelectronics, L6917 Datasheet - Page 18

• Manufacturer Part Number: L6917
• Description: 5 BIT PROGRAMMABLE DUAL-PHASE CONTROLLER
• Manufacturer: STMicroelectronics

L6917B
– Z
– Z
– A(s) is the error amplifier gain;
–
Removing the dependence from the Error Amplifier gain, so assuming this gain high enough, the control loop
gain results:
With further simplifications, it results:
Considering now that in the application of interest it can be assumed that Ro>>R
R
The ACM control loop gain is designed to obtain a high DC gain to minimize static error and cross the 0dB axes
with a constant -20dB/dec slope with the desired crossover frequency
transfer function has one zero and two poles. Both the poles are fixed once the output filter is designed and the
zero is fixed by ESR and the Droop resistance. To obtain the desired shape an R
ered for the Z
tegrator minimizes the static error while placing the zero in correspondence with the L-C resonance a simple -
20dB/dec shape of the gain is assured (See Figure 12). In fact, considering the usual value for the output filter,
the LC resonance results to be at frequency lower than the above reported zero.
Figure 12. ACM Control Loop Gain Block Diagram (left) and Bode Diagram (right)
Compensation network can be simply designed placing
desired obtaining:
18/33
DROOP
G
and has a typical value of 2V
PWM
F
L
L OO P
(s) is the parallel of the two inductor impedance;
(s) is the compensation network impedance;
V
COMP
<<Ro, it results:
PWM
=
G
s
L OO P
4
-- -
5
F
=
(s) implementation. A zero at
------------------ -
–
Z
V
4
-- -
5
F
V
d V
O SC
s
C
IN
------------------ -
F
IN
=
V
V
· is the ACM PWM transfer function where DVosc is the oscillator ramp amplitude
O SC
IN
G
–
R
L/2
I
4
-- -
5
F
Cout
FB
ESR
LO O P
------------------ -
Z
-------------- -
V
R
V
REF
F
O SC
IN
FB
s
s
R
V
=
FB
OUT
R o
------------------------------------- -
Z
-------------- -
Rout
–
R
F
4
-- -
5
FB
R o
+
s
------------------ -
R
V
+
F
V
DROOP
=1/R
----------------------------------------------------------------------------------------------------------------------------------
s
OS C
R
------ -
IN
2
2
K
L
Co
=
F
C
----------------------------------- -
Z
K
F
4
-- -
5
P
----------------------------------------------------------------------------------------------------------------------------------
s
is then introduced together with an integrator. This in-
L
-- -
2
Z
2
1
s
---------------
Z
+
=
V
V
+
F
dB
C o
s
IN
o sc
+
s Co
s
LC
Z
L
1
---------------
2 Ro
----------
R
and imposing the cross-over frequency
L
-- -
2
+
s
1
FB
L
+
s Co
s
dB
R
+
Rs
------- -
Rg
LC
DROOP
---------------
2 R o
T
C o ESR
. Neglecting the effect of Z
G
+
L
LOOP
R
Z
-------------- -
R
Z
DROOP
P
FB
F
+
+
s
-C
Co ESR
ESR
F
+
series network is consid-
C o
//Ro
R
------ -
L
+
2
; ESR<<Ro and
+
L
ESR
Z
T
Co
F
(s)
+
1
R
------ -
2
F
L
(s), the
+
T
1
as