
L6712A L6712
22/29
Figure 18. 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
ωT. Neglecting the effect of
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 RF-CF series network is considered for the ZF(s) implementation. A zero
at
ωF=1/RFCF is then introduced together with an integrator. This integrator 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 18). 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
ωZ= ωLC and imposing the cross-over frequency ωT as desired obtaining:
3.12.3Voltage 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:
3.13 LAYOUT GUIDELINES
Since the device manages control functions and high-current drivers, layout is one of the most important
things to consider when designing such high current applications.
A good layout solution can generate a benefit in lowering power dissipation on the power paths, reducing
radiation and a proper connection between signal and power ground can optimize the performance of the
control loops.
Rout
Cout
ESR
L/2
RFB
RF
CF
VID
PWM
IDROOP
VCOMP
VOUT
d
VIN
ZF
FB
DROOP
COMP
RA_Gain
dB
ω
ωT
ωZ
ωLC
GLOOP
ZF(s)
K
dB
FB
OSC
IN
R
1
V
5
4
K
=
R
F
R
FB
V
OSC
V
IN
-----------------------------------
5
4
---
ω
T
L
2
R
DROOP
RA_Gain
-------------------------
ESR
+
----------------------------------------------------------
C
F
Co
L
2
---
R
F
--------------------
=
=
G
LOOP s
()
V
IN
V
OSC
-------------------
Z
F s
()
R
FB
---------------
Z
P s
()
Z
P s
() Z
L s
()
+
------------------------------------ RA_Gain
–
=