AN426

Philips Semiconductors www.datasheet4u.com Application note Controlling air core meters with the 87C751 and SA5775 AN426 INTRODUCTION Often, certain classes of microcontroller applications surface where large amounts of on-chip resources such as a large program memory space and numerous I/O pins are not required. These applications are typically cost sensitive and desirable attributes of the MCU include low cost and modest on-chip resources such as program and data memory, I/O, and timer-counters. Substantial benefits of reduced design cycle time can be realized by using an industry-standard architecture having software compatibility with existing popular microcontrollers. SA5775 Serial Gauge Driver The SA5775 is a monolithic driver for controlling air core meters typically used in automotive instrument clusters and is shown in Figure 3. The SA5775 receives a 10-bit serial word and converts that word to four voltage outputs that appear at the SINE+, SINE–, COSINE+, and COSINE– outputs. The differential voltage at the SINE outputs are applied to one coil of the meter and the COSINE outputs are applied to the other coil of the meter. The currents through these coils produce a resultant magnetic force which is the vector sum of the magnetic forces produced by each of the two coils. Since the currents through the coils are bidirectional this magnetic vector can rotate through a full 360 degrees. The magnetized disc within the air core meter will follow the rotating vector an...