L6258E_07 Datasheet, PDF (18/31 Pages) STMicroelectronics – PWM controlled high current DMOS universal motor driver

ACpw _dB
=
20 ⋅ log ²-----^â‹---²----⁴-
1.6
=
29.5dB
L6258E
Moreover, having the two references Tri_0 and Tri_180 a triangular shape it is clear that the
transfer function of this block is a linear constant gain without poles and zeros.
3.3
Load attenuation
The load block is composed by the equivalent circuit of the motor winding (resistance and
inductance) plus the sense resistor.
We will considered the effect of the Bemf voltage of the motor in the next chapter.
The input of this block is the PWM voltage of the power amplifier and as output we have the
voltage across the sense resistor produced by the current flowing into the motor winding.
The relation between the two variable is:
Vsense
=
-----^V-----^o---^u----^t-----
RL + RS
⋅
RS
so the gain of this block is:
ACload = ^V-----^s---^e----ⁿ---^s----^e-- = --------^R----^S----------
vout
RL + RS
ACloaddB
=
20 ⋅ log --------^R----^S----------
RL + RS
AloaddB
=
20 ⋅ log -------⁰---^.--³---³---------
12 + 0.33
=
–31.4dB
where:
R_L = equivalent resistance of the motor winding
R_S = sense resistor
Because of the inductance of the motor L_L, the load has a pole at the frequency:
Fpole = ---------------¹-----------------
2π ⋅ --------^L----^L--------
R_L + R_S
Fpole = -------------------¹--------------------- = 163Hz
6.28
⋅
-¹---²-----^â‹---¹---⁰----–---³--
12 + 0.33
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