TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
20
External
control
Internal sample control
SAM-
0V
PLEx
0V
BLx
VVM
0.25V
0V
BMx
-0.25V
VVCC/3
0V
CURx
Phase
tBLHICURX
tBLHICURX
Hold
(undef.)
CURx tracking
-BMx
CURx tracking -BMx
Hold
Hold
figure 15: timing of the current measurement
The SAMPLEx pins will normally be unused and can be tied to VCC. 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.
5.3.3 Measurement depending on chopper cycle
If the low side on-time on one phase tBLHICURX is too low, a current measurement is not possible. The
TMC603 automatically does not sample the current if the minimum low side-on time 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 from the difference of the remaining two current measurements.
5.3.4 Compensating for offset voltages
In order to measure low current values precisely, the “zero” value (x0) of 1/3 VCC can 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
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, the use of at least one additional
shunt resistor in the common ground line of all three half bridges provides a stable current
measurement base, which allows in-system adjustment of the relative MOSFET resistances. Further,
the TMC603 measurement amplification is slightly different for positive and negative currents.
Copyright © 2008 TRINAMIC Motion Control GmbH & Co. KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
