TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
19
5.3 Current measurement amplifiers
The TMC603 amplifies the voltage drop in the three lower MOSFET transistors in order to allow
current measurement without the requirement for current sense (shunt) resistors. This saves cost and
board space, as well as the additional power dissipation in the shunt resistors. However, additional
shunt resistors can be added, e.g. source resistors for each lower MOSFET or a common shunt
resistor in the bridge foot point, in order to increase voltage drop and to have a more exact
measurement.
The TMC603 CURx outputs deliver a signal centered to 1/3 of the 5V VCC supply. This allows
measurement of both, negative and positive signals, while staying compatible to a 3.3V
microcontroller. The current amplifier is an inverting type.
+VCC
R
amplify
4.5x or 18x
SENSE_HI
BMx
D
A
R
R
1/3 VCC
SWC
track & hold stage
CURx
A
autozero
add 1/3 VCC
offset
automatic
sample
point delay
BLx
D
D
SAMPLEx
figure 14: schematic of current measurement amplifiers
Pin
Comments
CURx
Output of current measurement amplifier. The output signal is centered to 1/3 VCC.
SENSE_HI
Switches current amplifiers to high sensitivity (high level). Control by processor to get
best sensitivity and resolution for measurement.
SAMPLEx
Optional external control for current measurement sample/hold stage. Set to positive
level, if unused
The voltage drop over the MOSFET is calculated as follows:
whereas x is the ADC output value, x0 is the ADC output value at zero current (e.g. 85 for an 8 bit ADC
with 5V reference voltage), ADCMAX is the range of the ADC (e.g. 256 for an 8 bit ADC), VADCREF is the
reference voltage of the ADC and ACUR is the currently selected amplification (absolute value) of the
TMC603.
With this, the motor current can be calculated using the on resistance RDSON (at 10V) of the MOSFET:
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.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
