TMC603A Datasheet, PDF(21/44 Page) TRINAMIC Motion Control GmbH & Co. KG. – Three phase motor driver with BLDC back EMF commutation hallFX™ and current sensing

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TMC603A Datasheet, PDF (21/44 Pages) TRINAMIC Motion Control GmbH & Co. KG. – Three phase motor driver with BLDC back EMF commutation hallFX™ and current sensing

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TMC603A DATA SHEET (V. 1.15 / 2009-Nov-19)

5.3.3 Measurement depending on chopper cycle

If the low side on-time on one phase tBLHICURX is too short, a current measurement is not possible. The

TMC603 automatically does not sample the current if the minimum low side-on time requirement is not

met. This condition can arise in normal operation, e.g. due to the commutation angle defined by a sine

commutation chopper scheme. The respective CURx output then does not reflect the phase current.

Thus, the CURx output of a phase should be ignored, if the on-time falls below the minimum low side

on-time for current measurement (please refer to maximum limit). The correct current value can easily

be calculated using the difference of the remaining two current measurements. This results from the

fact that the sum of all three currents equals zero (IU+IV+IW = 0). This way, all motor currents are

always known from the measurement of two phase currents. It is important to know all three phase

currents for a sine commutated motor. For block commutation, there is always one low side active and

the full current can be seen at this low side.

5.3.4 Compensating for offset voltages

In order to measure low current values precisely, the âzeroâ value (x0) of 1/3 VCC should be measured

via the ADC, rather than being hard coded into the measurement software. This is possible by doing a

first current measurement during motor stand-still, with no current flowing in the motor coils, e.g.

during a test phase of the unit. The resulting value can be stored and used as zero reference.

However, the influence of offset voltages can be minimized, by using the high sensitivity setting of the

amplifiers for low currents, and switching to low sensitivity for higher currents.

5.3.5 Getting a precise current value using MOSFET on-resistance

The on-resistance of a MOSFET has a temperature co-efficient, which should not be ignored. Thus,

the temperature of the MOSFETs must be measured, e.g. using an NTC resistor, in order to

compensate for the variation. Also, the initial RDSON depends upon fabrication tolerance of the

MOSFETs. If exact measurement is desired, an adjustment should be done during initial testing of

each product. For applications, where an adjustment is not possible, external sense resistors can be

used instead. A single resistor in the GND line often is sufficient for block commutation. For sine

commutation, three sense resistors should be used.

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