SA5777A Datasheet, PDF (6/12 Pages) NXP Semiconductors – Dual air-core gauge driver

Philips Semiconductors
Dual air-core gauge driver
Product specification
SA5777A
FUNCTIONAL DESCRIPTION
The SA5777A dual air-core gauge driver logic Block Diagram shows
the two 10-bit input shift registers and two 9-bit parallel latches, and
two 7+-bit DACs. The MSB is a dummy bit required for compatibility
with the SA5775A. The DACs generate output voltages that are
offset within the supply rails to give the output buffers enough
headroom to operate. With a 14 V supply, the typical output swing is
from 1 V to 11.5 V. The MUX generates the two required quadrants
by switching the 56° data from the DAC to the appropriate output
buffer. The output buffers provide the necessary current to drive the
air-core gauge. The output buffers are always connected to the
coils and can sink and source sufficient current so that inductive
kickback is eliminated during normal operation.
The primary function of the SA5777A IC is to generate the transfer
function that maps an input code into the correct voltages for linearly
controlling the coils of an air-core gauge display (Figure 3). The
SA5777A has been implemented using the tangent drive algorithm.
Therefore, one coil on each meter will be driven with an output
approximating the tangent function, the other coils will be biased at
1/2 V_BB. The internal DAC is designed to operate over a 7+ bit (56°)
data range. An extended range can be achieved by changing the
relationship between the bias coil and the driver coil. As the current
through the bias coil is reduced, the full scale deflection is
increased. Theoretically, this deflection could approach 180°, but
practical limitations of accuracy, resolution, and torque restrict the
full scale range to approximately 112° (Figure 4). This full scale
range corresponds to a bias coil voltage of 0.5 x V_BB and a full scale
tangent voltage of 0.744 x V_BB. The DAC has been tailored to
maintain the meter accuracy at this maximum deflection. The 0.5 x
V_BB bias coil voltage is obtained by connecting the bias coils of the
two meters in series across V_BB. This gives bias stability over
temperature. The internal bias generator is used to offset any
inaccuracies due to meter mismatches. This circuit receives
commands via an internal serial data interface port which is SPI
compatible. These parts can be serially cascaded with other
SA5777A ICs and/or SA5775A ICs to interface signals in multi-chip
systems. The SA5777A has a typical resolution of 0.35° over a full
scale deflection of approximately 112° and is guaranteed to be
monotonic. The input data is directly proportional to the displayed
angle in degrees (Figure NO TAG). Input code 0 gives an output
angle of 0°, code 319 (decimal) will generate a full scale output of
112.15°. Codes higher than decimal 319 will not be loaded into the
DAC latches and will leave the coil output buffers unchanged.
However, codes greater than 319 can be shifted through the
SA5777A intact if other parts are cascaded. The SA5777A is
capable of sourcing and sinking up to 100mA per differential driver
to control either one or two air-core gauge displays directly.
On-chip overcurrent and thermal shut-down logic prevents the chip
from overheating due to high current fault conditions. When a
shut-down condition is detected, the protection circuit disables the
coil output buffers (i.e., C1+, C1–, C2+, C2–, COM). The coil output
buffers remain in this condition until the first falling edge of CS that
occurs after the die temperature has decreased to about 140°C or
the overcurrent condition has been removed. During shut-down, the
digital portion of this IC continues to operate normally.
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
–2.00
–4.00
–6.00
–8.00
–10.00
–12.00
–14.00
31
63
95
127
159
191
223
255
287
319
INPUT CODE
Figure 3. Typical output voltage vs input code (V_BB = 14 V)
SL00462
1999 Sep 20
6