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L6919E
20/33
With further simplifications, it results:
Considering now that in the application of interest it can be assumed that Ro>>RL; ESR<<Ro and
RDROOP<<Ro, it results:
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
15). 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:
Figure 15. ACM Control Loop Gain Block Diagram (left) and Bode Diagram (right)
LAYOUT GUIDELINES
Since the device manages control functions and high-current drivers, layout is one of the most important things
G
LOOP s
()
4
5
---
V
IN
V
OS C
-------------------
Z
F s
()
Z
P s
() Z
L s
()
+
------------------------------------
Rs
Rg
--------
Z
P s
()
R
FB
---------------
+
–
=
G
LOO P s
()
4
5
---
V
IN
V
OSC
-------------------
Z
F s
()
R
FB
---------------
Ro
R
DROOP
+
Ro
R
L
2
-------
+
--------------------------------------
1s Co
R
DROOP//Ro
ESR
+
()
+
s
2
Co
L
2
---
s
L
2R o
---------------
Co ESR
Co
R
L
2
-------
+
+
1
+
+
----------------------------------------------------------------------------------------------------------------------------------
–
=
G
LOO P s
()
4
5
---
V
IN
V
OSC
-------------------
Z
F s
()
R
FB
---------------
1s Co
R
DROOP
ESR
+
()
+
s
2
Co
L
2
---
s
L
2Ro
---------------
Co ESR
Co
R
L
2
-------
+
+
1
+
+
----------------------------------------------------------------------------------------------------------------------------------
–
=
R
F
R
FB
V
OSC
V
IN
----------------------------------
5
4
---
ω
T
L
2R
DROOP
ESR
+
()
--------------------------------------------------------
=
C
F
Co
L
2
---
R
F
--------------------
=
Rout
Cout
ESR
L/2
RFB
RF
CF
REF
PWM
IFB
VCOMP
VOUT
d
VIN
ZF
dB
ω
T
ω
Z
ω
LC
GLOOP
ZF(s)
K
4
5
---
V
IN
V
osc
---------------
1
R
FB
----------
dB
=