TPIC84000-Q1 Datasheet, PDF (8/29 Pages) Texas Instruments – LOW-FREQUENCY ANTENNA DRIVER FOR PASSIVE START AND PASSIVE ENTRY

TPIC84134-Q1
SLDS176 – AUGUST 2010
www.ti.com
Current Measurement
From a system's point of view, diagnostics of the antenna operation is done by measuring the load current
across the antenna and providing the measured value to the microcontroller via SPI. The microcontroller
retrieves the current value and evaluates if there is a failure or not. Within the TPIC84134 the current
measurement is done at each Low Side-transistor (LS), though the measurement of the various currents is done
sequentially (i.e. first LS1 measurement followed by the LS2 measurement, etc.). The actual measured analog
values are converted into five bit resolution digital values and are then stored in the control and status register.
As the load current can take between 2 to 10 wave-forms to reach its maximum value; current measurement can
depend on the actual application and the specific Q-factor of the antenna circuit. Therefore it is necessary to
program the exact time when the current measurement must be performed, and it is also necessary to measure
at the specific time within the wave to measure the maximum value.
A programmed parameter indicates which outputs are measured, where the low side of the programmed output
is measured sequentially. Here the edge (rising or falling) is also programmed, where during the odd (1st, 3rd,
5th, etc) rising or falling edge, the current on the low side of the first programmed output is measured; during the
even (2nd, 4th, 6th, etc) rising or falling edge, the current in the low side of the second programmed output is
measured. The measurement is performed continuously and the current value updated (over written) every time
within the Control and Status register.
Figure 4 shows an example of current measurement in a full-bridge configuration. In order to diagnose the
operation of the device in full-bridge, it is necessary to measure the current of LS1 and LS2. The two measured
values are stored in the Control and Status register.
Odd rising edge
Even rising edge
I _Antenna
I_ LS 2
I_ LS 1
LS2 measurement at second rising edge
(Timer2 programmed to 68: 31.902 µs)
LS1 measurement at first rising edge
(Timer1 programmed to 76: 35.627 µs)
0 7.5 14.9 22.4 29.8 37.3
0 7.5 14.9 22.4 29.8 37.3
Figure 4. Current Measurement Timing For Full Bridge Setup
Time
(µs)
Control Logic
The control logic block contains the SPI interface along with all the other circuitry necessary to convert the SPI
commands into the desired outputs.
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