MC68HC05V12 Datasheet, PDF (186/246 Pages) Freescale Semiconductor, Inc – HCMOS Microcontreller Unit

Gauge Drivers
Freescale Semiconductor, Inc.
15.5 Technical Note
An auto-zeroing scheme is implemented in the MC68HC705V12 to
reduce errors internal to the chip. Prior to each coil update, during the
auto-zero phase, the amplifier output is disconnected from all FET gates
(see Figure 15-4) while the input is connected to its new input voltage.
On completion of the auto-zero cycle, the amplifier output is connected
to the appropriate FET gate. Since the FET gate and amplifier output are
typically not at the same potential, the FET gate is momentarily pulled
down, until the amplifier control loop re-establishes the correct gate
potential. This abrupt change in gate potential results in voltage spikes,
and thus current spikes in the gauge coil. The magnitude, duration, and
number of spikes are dependent on the coil resistance, gauge supply
voltage (V_GSUP), and the coil current prior to the auto-zero cycle
(Figure 15-4). Only the worst case spike durations, under the specified
test conditions, are shown. Typically, the spike duration and total spike
duration is smaller.
NOTE: Due to the positive and negative spikes, there is negligible d.c. error
introduced.
Technical Data
I₁
I₀
I₁
t₁
t₂
t_MAX
I₀ = Current before spikes
I₁ = Maximum spike magnitude
= (V_GSUP + 0.9 V)/R_Coil
t₁ = Maximum negative spike duration
= 300 µs
t₂ = Maximum positive spike duration
= 200 µs
t_MAX = Maximum duration of all spikes
= 500 µs
Conditions: V_GSUP = 8.00 V
L_Coil = 30 mH
R_Coil = 200 W (typical)
T_A = 27°C
Figure 15-4. Specification for Current Spikes
Gauge Drivers
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MC68HC05V12 — Rev. 2.0