If Now We Define:

#3

Aloop

we obtain:

#4

=

Aloop

⋅

⋅

1

1

Atlowfrequencyis:

Aloop

= 32 ⋅

R2

R1

⋅ 10 ⋅

then Aloop = 8889 = 80dB.

Being Aloop very high we can simplify the #4 in

this way:

=

⋅

1

=

1

10

=

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:

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

and its pole is at frequency

1

2

)

so around 1KHz if L = 1.2mH.

So considering:

we have these Bode diagrams:

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.

ACPW

30dB

130KHz

ACSENSE

20dB

210KHz

-31dB

LOAD

AX

19dB

1K

10K

100K

D95IN270A

L6246

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