L6246 Просмотр технического описания (PDF) - STMicroelectronics
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L6246 Datasheet PDF : 12 Pages
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If Now We Define:
#3
Aloop
=
ACPW ⋅
ACERR
⋅
ACSENSE
⋅
RS
RS + ZL
we obtain:
#4
IL
VIN
=
Aloop
⋅
ACINP
ACSENSE
1 + Aloop
⋅
1
RS
A t lo wf req u en cyis :
Aloop
=
32
⋅
R2
R1
⋅
10
⋅
RS
(RS + ZL)
if R2 = 1M, R1 = 1K, RS = 0.2, RL = 7
then Aloop = 8889 = 80dB.
Being Aloop very high we can simplify the #4 in
this way:
IL
VIN
=
ACINP
ACSENSE
⋅
1
RS
=
10
1
⋅ 0.2
=
1
2
For the stability we have to study the stability of
Aloop, that as we can see from the #3 is a multi-
plication, so in dB is a sum:
Aloop | dB = ACPW | dB+ACERR | dB+ACSENSE |
dB
+
RS
RS + ZL
dB
So we can take in consideration the BODE dia-
grams of the each operational amplifier, with par-
ticular attention to the Error amplifier.
1)The Power amplifier is actually composed by
two operational amplifiers in the way to have
a gain of +16 and -16 (in voltage) respec-
tevely, for a total of 32 = 30dB.
The point at -3dB is around 130KHz.
2)The Sense amplifier has a gain of 20dB with
the point at -3dB around 210KHz.
3)The load introduce an attenuationof:
20log
RS
RS + RL
=
-31dB
with
RS
=
0.2
and
RL
=
7
L6246
and its pole is at frequency
1
2π L
(RS + RL)
so around 1KHz if L = 1.2mH.
So considering:
Ax | dB = Aloop | dB ACERR | dB ACPW |
dB+
ACSENSE
|
dB
+
RS
RS + RL
dB
we have these Bode diagrams:
ACPW
30dB
ACSENSE
20dB
130KHz
210KHz
-31dB
LOAD
AX
19dB
1K
10K
100K
D95IN270A
As can be easily see the bandwith is narrow and
the gain is low. It is possible to increase both
choosing an appropriate compensation of the Er-
ror amplifier.
The total bandwith should be, of course, at least a
decade lower of the 130KHz to avoid instability
problem. The bandwith guaranteed by the Error
amplifier has a Gmax of 80dB and a gain of 0dB
at 1MHz approximately, the real is some dB more
with a larger bandwith.
9/12
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