L6258EX

PWM current control loop

17/32

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 guaranteed.

To have a very flexible system and to have the possibility to adapt the system to any

application, the error amplifier 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:

Aloop = ACerr · ACpw · ACload · ACsense

where AC... is the gain of the blocks that refers to the error, power and sense amplifier plus

the attenuation of the load block.

The same formula in dB can be written in this way:

So now we can start to analyse the dynamic characteristics of each single block, with

particular attention to the error amplifier.

3.2

Power amplifier

The power amplifier is not a linear amplifier, but is a circuit driving in PWM mode the output

stage in full bridge configuration.

The output duty cycle variation is given by the comparison between the voltage of the error

amplifier and two triangular wave references Tri_0 and Tri_180. Because all the current

control loop is referred to the Vr reference, the result is that when the output voltage of the

error amplifier is equal to the Vr voltage the two output Out_A and Out_B have the same

phase and duty cycle at 50%; increasing the output voltage of the error amplifier above the

Vr voltage, the duty cycle of the Out_A increases and the duty cycle of the Out_B decreases

of the same percentage; on the contrary decreasing the voltage of the error amplifier below

the Vr voltage, the duty cycle of the Out_A decreases and the duty cycle of the Out_B

increases of the same percentage.

The gain of this block is defined by the amplitude of the two triangular wave references;

more precisely the gain of the power amplifier block is a reversed proportion of the

amplitude of the two references.

In fact a variation of the error amplifier output voltage produces a larger variation in duty

cycle of the two outputs Out_A and Out_B in case of low amplitude of the two triangular

wave references.

The duty cycle has the max value of 100% when the input voltage is equal to the amplitude

of the two triangular references.

The transfer function of this block consist in the relation between the output duty cycle and

the amplitude of the triangular references.

ACpw

dB

20

V

out

Δ

V

in

Δ

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

log

⋅

2V

S

⋅

Triangular Amplitude

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