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L6258EP

The Bode plot of the Ax|dB function shows a DC gain of -1.9dB and a pole at 163Hz.

It is clear now that (because of the negative gain of the Ax function), Bx function must have an high DC gain in

order to increment the total open loop gain increasing the bandwidth too.

4.4 Error Amplifier and Sense Amplifier

As explained before the gain of these two blocks is :

Being the voltage across the sense resistor the input of the Bx block and the error amplifier voltage the output

of the same, the voltage gain is given by :

Verr_out = -(ic · Zc) so ic = -(Verr_out ·

)

because ib = icwe have:

Vsense ·

= -(Verr_out ·

)

In the case of no external RC network is used to compensate the error amplifier, the typical open loop transfer

function of the error plus the sense amplifier is something with a gain around 80dB and a unity gain bandwidth

Figure 8.

The BODE diagram shows together the error amplifier open loop transfer function, the Ax function and the re-

sultant total Aloop given by the following equation :

The total Aloop has an high DC gain of 78.1dB with a bandwidth of 15KHz, but the problem in this case is the

stability of the system; in fact the total Aloop cross the zero dB axis with a slope of -40dB/decade.

Now it is necessary to compensate the error amplifier in order to obtain a total Aloop with an high DC gain and

a large bandwidth. Aloop must have enough phase margin to guarantee the stability of the system.

A method to reach the stability of the system, using the RC network showed in the block diagram, is to cancel

the load pole with the zero given by the compensation of the error amplifier.

ib

Vsense Gs

⋅

Vsense

1

Rb

--------

⋅

==

1

Zc

-------

1

Rb

--------

1

Zc

-------

Bx

Verr_out

Vsense

------------------------

–

Zc

Rb

--------

–

==