DocID023491 Rev 3

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AN4144

Thermal drift compensation guidelines

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Thermal drift compensation guidelines

The compensation of the thermal drift of the phase resistance is not automatically performed

by STMicroelectronics devices, but a compensation factor (KTHERM) is made available

through a dedicated register.

The thermal compensation factor (KTHERM) is a scalar value between 1 (no compensation)

and 1.5, which is applied to the PWM value obtained by the other compensation systems

(BEMF compensation and supply voltage compensation).

As shown in Equation 11 of Section 1.4 on page 12, the thermal drift of the resistance is

For this reason the thermal drift compensation becomes more important when the driven

motor has a high phase resistance value.

A suggested implementation of the thermal drift compensation is based upon the phase

current measurement during the non-operative period of the motor (if present).

The proposed method requires an initial calibration of the system (performed when the

motor is cold) and described by the following sequence:

1.

The motor is stopped in a specific electrical position.

2.

3.

The output voltage is increased at a low rate through the KVAL_HOLD parameter.

4.

When the calibration current is reached, the respective KVAL_HOLD value must be

stored (KCAL) in the MCU memory.

Since the KCAL value is related to the design parameters only, this calibration can also be

performed during the design of the application. However, because of the variation of the

application characteristics (bus voltage, motor phase resistance, etc.) and the device

circuitry (current sensing), it would be useful to perform the calibration sequence on each

system instead of defining a single KCAL value during the application design.

When a significant change of the motor temperature is expected, the following

compensation sequence can be performed:

1.

The motor is stopped in the same electrical position used during the calibration.

2.

3.

The KVAL_HOLD value is set to KCAL.

4.

The compensation coefficient (K_THERM) is increased or decreased in order to reach

the calibration current using the overcurrent/stall information provided by the device.