PWM current control loop

L6258

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3

PWM current control loop

3.1

Open loop transfer function analysis

Block diagram: refer to Figure 5.

Input parameters:

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Gs transconductance gain = 1/Rb

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Gin transconductance gain = 1/Ra

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Ampl. of the Tria_0_180 ref. = 1.6V (peak to peak)

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these data refer to a typical application, and will be used as an example during the analysis

of the stability of the current control loop.

The block diagram shows the schematics of the L6258 internal current control loop working

the relation between the two variables is given by the following formula:

where:

is the control voltage defining the load current value

is the gain of the input transconductance amplifier ( 1/Ra )

is the gain of the sense transconductance amplifier ( 1/Rb )

is the resistor connected in series to the output to sense the load current

In this configuration the input voltage is compared with the feedback voltage coming from

the sense resistor, then the difference between this two signals is amplified by the error

amplifier in order to have an error signal controlling the duty cycle of the output stage

keeping the load current under control.

It is clear that to have a good performance of the current control loop, the error amplifier

must have an high DC gain and a large bandwidth.

I

LOAD

R

S

1

R

b

-------

⋅⋅

V

DAC

1

R

a

-------

⋅

=

I

LOAD

V

DAC

R

b

R

a

R

s

⋅

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

⋅

0.5

V

DAC

R

S

--------------- A

()

⋅

==