11/24

L6258E

4

PWM CURRENT CONTROL LOOP

4.1 Open Loop Transfer Function Analysis

Block diagram : refer to Fig. 6.

Table 7. Application data:

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 L6258E internal current control loop working in PWM mode; the

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 .

Gain and bandwidth must be chosen depending on many parameters of the application, like the characteristics

of the load, power supply etc..., and most important is the stability of the system that must always be guaran-

teed.

To have a very flexible system and to have the possibility to adapt the system to any application, the error am-

plifier must be compensated using an RC network connected between the output and the negative input of the

same.

For the evaluation of the stability of the system, we have to consider the open loop gain of the current control

loop:

Gs transconductance gain = 1/Rb

Gin transconductance gain = 1/Ra

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

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

()

⋅

==