L6258E

12/24

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.

4.2 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 refer-

ence, 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 am-

plifier 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.

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.

4.3 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 :

ACpw

dB

20

V

out

∆

V

in

∆

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

log

⋅

2V

S

⋅

Triangular Amplitude

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

==

ACpw

dB

10

224

⋅

1.6

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

log

⋅

29.5dB

==

V

sense

V

out

R

L

R

S

+

--------------------- R

S

⋅

=