TMC603A Datasheet, PDF (20/44 Pages) TRINAMIC Motion Control GmbH & Co. KG. – Three phase motor driver with BLDC back EMF commutation hallFX™ and current sensing

TMC603A DATA SHEET (V. 1.15 / 2009-Nov-19)
20
For a shunt resistor based measurement, the same formula is true:
For the shunt resistor measurement, care has to be taken not to exceed the voltage range which can
be accepted by the measurement input, i.e. the shunt resistor should be selected in a way that the
voltage drop is at maximum 0.3V at full motor current. This is the maximum voltage which can be
measured. A lower sense resistor gives less power dissipation, but lower currents show with less
resolution.
5.3.1 Current measurement timing
Current measurement is self-timed, in order to only provide valid output voltages. Sampling is active
during the low side ON time. The sampling is delayed by an internal time delay, in order to avoid
sampling of instable values during settling time of the bridge current and amplifiers. Thus, a minimum
ON time is required in order to get a current measurement. The output CURx reflects the current
during the measurement. The last value is held in a track and hold circuit as soon as the low side
transistor switches off.
SAM-
PLEx
0V
BLx
0V
BMx
VVM
0.25V
0V
-0.25V
Internal sample control
External
control
CURx ^VVCC/3
0V
tBLHICURX
Phase
Hold
(undef.)
CURx tracking -BMx
tBLHICURX
Hold
CURx tracking
-BMx
Hold
figure 15: timing of the current measurement
The SAMPLEx pins can be used to refresh the measurement during long on time periods, e.g. when
the motor is in standstill, with the low side being continuously on, e.g. in a hall sensor based block
commutation scheme with the chopper on the high side. In this application, all SAMPLEx pins can be
tied together to one microprocessor output. For advanced applications, where a precise setting of the
current sampling point is desired, e.g. centered to the on-time, SAMPLEx pins can be deactivated at
the desired point of time, enabling the hold stage.
5.3.2 Auto zero cycle
The current measurement amplifiers do an automatic zero cycle during the OFF time of the low side
MOSFETs. The zero offset is stored in internal capacitors. This requires switching off the low side at
least once, before the first measurement is possible, and on a cyclic basis, to avoid drifting away of the
zero reference. This normally is satisfied by the chopper cycle. If commutation becomes stopped, e.g.
due to motor stand still, the respective phase current measurement could drift away. After the first
switching off and on of the low side, the measurement becomes valid again. Therefore, you should
integrate a timer in your commutation, which checks for the low side on time exceeding for example
10ms. If the on time of the respective low side reaches this time limit, you can either use the sample
input SAMPLEx to refresh the current measurement, by switching it high for at least 1µs, or you switch
off the low side for a short time of a few microseconds.
Copyright © 2009 TRINAMIC Motion Control GmbH & Co. KG