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AN4277

How to use the DAC to define thresholds

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Appendix A

How to use the DAC to define thresholds

Concerning the network shown in Figure 3 and Figure 4, it is important to properly set the

comparator inverting input voltage (V-) to define the threshold levels for over-current

protection and over-voltage protection.

As shown in Figure 7 below, in the STM32F3 Series microcontroller it is possible to set three

different sources as inverting input for the comparator:

•

An external reference (GPIO),

•

A fixed internal reference (Vref, ¾ Vref, ½ Vref, ¼ Vref),

•

A programmable internal reference (DAC).

Figure 7. Inverting input selection

Practical example: Over-current protection using the offset network

This is the case of Figure 3 when the components inside the dotted line boxes are present.

In this case, the formula to compute the over-current threshold is the following:

Equation 1

coincident with the required one. As explained in this practical example, the internal

reference can be used only when there is no need to fine-tune the over-current threshold.

Otherwise, it is necessary to use the external reference or the variable internal reference.

The latter is recommended, because it does not require any external components.

The STM32F3 Series microcontroller includes two 12-bit DAC channels that can be used for

this purpose. For three-phase motor drives, it is possible to group three comparators to

protect each leg of the inverter bridge versus over-current by setting the same DAC channel

for all three inverter inputs.

The same can be done in case of dual motor control with also having the possibility to define

two different levels of protection, one for each motor.

V

-

+

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×

⎝⎠

⎛⎞

–

+

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⎝⎠

⎛⎞

×

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=