UM0900 datasheet(17/43 Pages) STMICROELECTRONICS

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UM0900

Board description

Doc ID 17028 Rev 1

17/43

Figure 9.

Overcurrent protection

Overcurrent protection can be disabled if the W3 jumper is set to the B position. This may be

necessary and is often useful when the user decides to make the brake operate by turning

on the three low-side switches. In fact, if the motor acts as a generator, it is necessary to

protect the hardware, preventing the bus voltage from exceeding a safety threshold. In

addition to dissipating the motor energy on a brake resistor, it is possible to short the motor

phases, preventing the motor current from flowing through the bulk capacitors.

The current into the motor phases is normally limited by the motor phase impedance but

during the short-circuit transient, a high current can flow through the switches for a few ms.

In order to avoid false triggering it may be necessary to deactivate the OCP with this jumper

configuration.

3.3.6

Current sensing amplifying network

Three-shunt current reading configuration

Details of the three-shunt current-sensing reading configuration are shown in Figure 10. In

this configuration, the alternating signal on the shunt resistor, with positive and negative

values, must be converted to be compatible with the single positive input of the

microcontroller A-D converter used to read the current value.

The op-amp is used in follower mode: its gain is set by resistor r and R:

Equation 2

It is possible to calculate the voltage on the output of the op-amp OP OUT - VOUT as a sum

of a bias VBIAS and a signal VSIGN component equal to:

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