UM0971 datasheet(16/45 Pages) STMICROELECTRONICS

16 / 45 page

Board description

UM0971

16/45

Doc ID 17701 Rev 1

2.5.1

Zero-crossing methods for BEMF reading

Putting the switch on Pos 1, the motor phase voltage is directly input to the voltage divider

block.

When the patented ST zero-crossing method is used, the voltage divider is simply made up

of a 10 k

Ω series resistor for limiting the current to the control unit that processes the signal.

When the “classic” (industry standard) method is used, the voltage divider must scale and

filter the back-EMF voltage before it is input to the control unit. The partition ratio

determination depends on motor bus voltage. Therefore, the voltage divider resistor and

filtering capacitor values are calculated by the user.

2.5.2

Low amplitude BEMF signal amplification

When the back-EMF signal is very low (low speed) or for low voltage applications, the back-

EMF zero-crossing detection can become difficult due to the very weak signal. The

application note AN1103; improved B-EMF detection for low-speed and low-voltage

applications with ST72141, offers a circuit solution for improving back-EMF zero-crossing

detection at very low speeds or for low voltage applications.

This circuit can greatly improve the performance of sensorless BLDC drives in low voltage

applications, especially for automotive applications. With this technique, the sensorless

drive can be used in much wider speed ranges.

With reference to Figure 9, by setting Pos 2, an amplification block is inserted in back-EMF

signal processing, therefore allowing all the cases listed above to be covered.

For the actual amplification network, please see the circuit schematic in Figure 10:

Figure 10.

Low back-EMF amplification network

The output voltage Vout can be expressed in function of generic back-EMF phase voltage

Vbemf_x in this way:

Equation 1

With the resistor values actually used in the circuit schematic:

Vout

Vbias

G Vbemf

⋅

UM0971 Datasheet(PDF) 16 Page - STMicroelectronics