L6258EA PWM current control loop
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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:
Aloop
dB
= ACerr
dB
+ ACpw
dB
+ ACload
dB
+ ACsense
dB
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.
Vout = 2 · V
S
· (0.5 - DutyCycle)
ACpw
dB
20
V
out
Δ
V
in
Δ
----------------
log⋅
2V
S
⋅
Triangular Amplitude
-------------------------------------------------------==
