Philips Semiconductors

Product specification

SA5777A

Dual air-core gauge driver

1999 Sep 20

8

TFO

TRO

TCSH

TSCLKH

TSCLKL

THI

TSU

FSCLK

GAUGE 2

GAUGE 1

D0

D1

D2

D3

D4

D5

D6

D7

D8

D9

D10 D11 D12 D13 D14 D15 D16 D17 D18 D19

LSB

MSB LSB

MSB

20 SCLK CYCLES

CS

SCLK

DATAIN

DATAOUT

D19

D18

D1

D0

D19*

D18*

D1*

D0*

80%

20%

TCSL

1

2

19

20

*DO IS THE PREVIOUSLY LOADED DATA WORD

SL00465

Figure 6. Serial interface timing

Serial Interface

The SA5777A is controlled through a serial interface with the following control functions (reference Figure 6):

CS

Active high chip select. Enables the SA5777A to receive serial input data. The falling edge of CS loads a new 20-bit data word

into the internal DAC registers which updates the tangent coil output buffers (C1+, C1–, C2+, C2–).

first, gauge 1 first.

pin delayed by 20 clock cycles. This pin can be used to cascade several SA5777As with one CS line to load all of the

SA5777As concurrently.

Power Moding

The SA5777A has a power-on reset capability. On the rising edge

output buffers (C1+, C1–, C2+, C2–, COM) are disabled.

Coil Output Buffer Control

The coil buffers (C1+, C1–, C2+, C2–, COM) are disabled:

2. When OE is taken low or held low. The data registers for the

outputs can still be updated while OE is low. When OE is taken

high, the current output data value is displayed. A falling edge

on CS will be required to activate the outputs if a fault condition

has occurred prior to the OE going high.

3. Due to an overcurrent condition on either of the coil output

buffers. The coil output buffers will be enabled after the next CS

high-to-low transition; assuming OE is high. If the overcurrent

condition has not been removed, the outputs will immediately

return to their disabled condition. The ST pin will indicate status

of the coil outputs.

4. Due to excessive power dissipation (i.e., thermal shut-down).

The die temperature must go below 140

°C before a falling edge

on the CS pin will clear this fault condition and allow the coil

outputs to go active.

Application Notes

The air-core gauge is constructed of two coils would on a cavity at

90

° to each other. Inside the cavity there is a disk which is

magnetized on its diameter. The currents through the coils generate

a resultant magnetic vector that causes the magnetic disc to move

until the magnetic fields are aligned. If the ratio of the currents in the

two coils follows the tangent function, then the transfer characteristic

relating the input data to output angle is linear.

Maximum current is when output is at zero and full scale degrees,

Copper wire has a typical temperature coefficient of 0.4%/C