PWM current control loop

L6258

20/32

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 at 400kHz. In this case the situation of the total

Figure 8.

Aloop bode plot (uncompensated)

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

function and the resultant 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.

The transfer function of the Bx block with the compensation on the error amplifier is:

1

Zc

-------

1

Rb

--------

1

Zc

-------

Bx

Verr_out

Vsense

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

–

Zc

Rb

--------

–

==