l6712a STMicroelectronics, l6712a Datasheet - Page 21

• Manufacturer Part Number: l6712a
• Description: Two-phase Interleaved Dc/dc Controller
• Manufacturer: STMicroelectronics

Considering now that in the application of interest it can be assumed that R
R
Figure 14. ACM Control Loop Gain Block Diagram (left) and Bode Diagram (right).
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 ω
ZF(s), the 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
at ω
while placing the zero in correspondence with the L-C resonance a simple -20dB/dec shape of the gain is
assured (See Figure 14). In fact, considering the usual value for the output filter, the LC resonance results
to be at frequency lower than the above reported zero.Compensation network can be simply designed
placing ω
Voltage Mode (VM) Control Loop (DROOP = SGND)
Disconnecting the DROOP pin from the Control Loop, the system topology becomes a Voltage Mode. The
simplest way to compensate this loop still keeping the same compensation network consists in placing the
RF-CF zero in correspondence with the L-C filter resonance.
The loop gain becomes now:
DROOP
V
G
COMP
LOOP
PWM
F
=1/R
<<R
Z
s ( )
COMP
F
= ω
Z
C
o
F
d•V
, it results:
F
=
C
LC
F
is then introduced together with an integrator. This integrator minimizes the static error
R
IN
–
I
and imposing the cross-over frequency ω
F
DROOP
4
-- -
5
R
L/2
⋅
=
F
Cout
ESR
------------------ -
∆ V
FB
R
---------------------------------- -
VID
V
G
OSC
FB
IN
LOOP
DROOP
⋅
V
R
V
∆ V
IN
⋅
OUT
FB
Z
-------------- -
R
Rout
s ( )
OSC
F
FB
s ( )
F
-C
=
⋅
⋅
RA_Gain
F
5
-- - ω
4
–
------------------------------------------------------------------------------------------------------------------------------------------ - RA_Gain
s
2
series network is considered for the Z
------------------ -
∆
⋅
V
⋅
V
Co
OSC
T
IN
⋅
--------------------------------------------------------- -
2
⋅
L
-- -
2
⋅
⋅
§
©
Z
-------------- -
+
R
1 s Co
------------------------ -
RA_Gain
R
F
K
s
FB
+
s ( )
DROOP
K
⋅
=
---------------
2 Ro
⋅
dB
⋅
ª
«
¬
4
5
L
----------------------------------- - RA_Gain
Z
⋅
L
⋅
P
ûV
s ( )
⋅
V
+
T
Z
§
©
IN
OSC
+
as desired obtaining:
ESR
P
------------------------ -
RA_Gain
R
+
s ( )
Co ESR Co
DROOP
⋅
Z
R
1
L
FB
·
¹
ω
⋅
s ( )
º
»
¼
LC
dB
⋅
G
+
LOOP
ω
+
ESR
Z
C
F
F
(s) implementation. A zero
·
¹
⋅
T
=
---------------
2 Ro
. Neglecting the effect of
------------------- -
⋅
o
L
Co
>>R
R
F
⋅
L6712A L6712
L
L
-- -
2
+
; ESR<<R
1
ω
Z
⋅
T
F
(s)
o
21/27
ω
and