L6918A STMicroelectronics, L6918A Datasheet - Page 23

• Manufacturer Part Number: L6918A
• Description: 5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER
• Manufacturer: STMicroelectronics

AVERAGE CURRENT MODE (ACM) CONTROL LOOP
The average current mode control loop is reported in figure 15. The current information I
pin flows into R
The ACM control loop gain results (obtained opening the loop after the COMP pin):
where:
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 T. Neglecting the effect of Z
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
A zero at
DROOP
–
– Z
– Z
– Z
– A(s) is the error amplifier gain;
–
load Ro;
and has a typical value of 2V
R
PWM
P
F
L
DRO OP
(s) is the parallel of the two inductor impedance;
(s) is the compensation network impedance;
(s) is the impedance resulting by the parallel of the output capacitor (and its ESR) and the applied
<<Ro, it results:
G
LOO P
=
f
=1/R
4
-- -
5
=
FB
G
Rsense
--------------------- - R
------------------ -
s
F
LOO P
C
implementing the dependence of the output voltage from the read current.
V
V
Rg
OS C
=
F
IN
is then introduced together with an integrator. This integrator minimizes the static error
–
4
-- -
5
s
is the ACM PWM transfer function where V
------------------ -
=
G
V
F B
V
–
O SC
G
LOO P
I N
4
-- -
5
LOO P
is the equivalent output resistance determined by the droop function;
------------------ -
F
V
Z
-------------- -
-C
V
s
R
O SC
F
IN
s
FB
F
s
=
series network is considered for the Z
=
–
–
Z
-------------- -
Ro
-------------------------------------
------------------------------------------------------------------------------------------------------------------- -
R
Z
4
-- -
5
F
F B
P
Ro
s
+
------------------ -
s
P WM Z
R
V
V
+
DRO OP
------------------------------------------------------------------------------------------------------------------------------- -- -
s
OS C
+
IN
2
R
------ -
Z
2
L
L
Co
s
----------------------------------- -
Z
F
------------------------------------------------------------------------------------------------------------------------------- -- -
s
P
2
L
-- -
2
s
s
1
Z
+
Z
-------------- -
Co
F
A s
+
+
s
F
s Co
s
R
Z
s
1
DRO OP
L
L
-- -
2
---------------
2 Ro
+
+
s
+
s C o
L
s
1
R
+
------- -
Rg
Rs
+
+
---------------
2 Ro
DRO OP
----------- -
A s
C o ESR
Z
osc
1
L
+
P
R
Z
-------------- -
s
R
DRO OP
is the oscillator ramp amplitude
P
+
F
F B
R
+
s
(s) implementation.
Co ESR
F B
ESR
+
//Ro
Co
+
R
------ -
+
2
E SR
L
FB
Co
+
L
sourced by the FB
L6918 L6918A
; ESR<<Ro and
1
R
------ -
2
L
+
1
F
(s), the
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