Philips Semiconductors

Product specification

SA5777A

Dual air-core gauge driver

1999 Sep 20

6

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

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

maintain the meter accuracy at this maximum deflection. The 0.5 x

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

SL00462