SA5778 Datasheet, PDF (6/16 Pages) NXP Semiconductors – Serial triple gauge driver STGD

Philips Semiconductors
Serial triple gauge driver (STGD)
Product specification
SA5778
DATA_IN
S_CLK
ST
CS
GOE
RUN
V_BATT
DATA / STATUS SHIFT REGISTERS
STATUS
LATCH
DATA
LATCHES
THERMAL
PROTECTION
SwBATT, BIAS
DIGITAL-to-ANALOG
CONVERTERS and OUTPUT
MULTIPLEXERS
DATA
OUT
SwBATT TRANSISTOR
112° MINOR GAUGES 360° MAJOR GAUGE
SR01120
Figure 5. Gauge Connections to the STGD
18-24V
+
REFERENCE –
V_{BATT 1KΩ}
GOE
RUN
10KΩ
5V
REGULATOR
5V
LOGIC
SwControl
R_B
OUTPUT
BUFFER SUPPLY
DAC REFERENCES
SwBATT1/2
BIAS COILS
V_BATT
EXTERNAL
GAUGE DRIVERS
SR01121
Figure 6. Gauge Enable/Standby Circuit and Over Voltage
Protection Circuit
Figure 6 shows the protection and gauge enable logic for the STGD.
The battery supply voltage V_BATT is monitored, and if the supply
exceeds the specified operating range, the STGD is put in a
shutdown mode in which the output buffers are disabled. The STGD
will also enter the shutdown mode by excessive die temperature,
and will return to normal operation when the die temperature
decreases to within specified limits. Thermal shutdown may occur at
V_BATT supply voltages over 16V at high ambient temperatures near
105°C. Internal logic will continue to function and status may be read
out to determine the source of the shutdown. The STGD may be
placed in a standby mode with a low on both the GOE and RUN
input pin. In this mode, battery current drain is minimized.
The SwBATT1 and SwBATT2 inputs are the supply for the DACs,
and the output buffers driving the coils including the COM output
which stabilizes the voltages applied to the bias coils of the minor
gauges. Both SwBATT1 and SwBATT2 should be connected to the
collector of the control transistor as these inputs are not connected
internally and supply different portions of the circuit. This switched
battery supply is protected from voltages exceeding the specified
operating range and is controlled by the SwCONTROL output. This
supply may optionally be used to supply additional circuits which
operate from unregulated battery supplies but which need protection
from over voltage transients. Typical devices which may benefit from
this protection include the Serial Gauge Driver, SA5775A and Dual
Gauge Driver, SA5777A, which are often used in conjunction with
the STGD in 4 and 5 gauge applications.
This switched battery supply is turned off when the STGD enters the
standby mode in response to the RUN and GOE inputs both being
low, or a V_BATT supply exceeding the specified operating range. The
switched battery supply depends on the RUN signal to prevent
undesired needle movement on the minor gauges when going from
standby to active mode. This movement would otherwise occur if
the voltage to the fixed bias coils of the minor gauges was switched
on before the coil voltages provided by the DACs within the STGD
were defined. The start up jump is prevented as follows. In the sleep
mode the switched battery supply is off, and the gauge drive outputs
of the STGD are in a high impedance state. The gauges are in their
zero position from the previous power-down sequence. When the
RUN input goes high, but the GOE is kept low, the STGD enters the
start up mode in which the minor gauges are driven to zero, the
internal 5V regulator for the logic is turned on, and the switched
battery supply is turned on to supply the bias coil and STGD output
buffers. However, the output buffers for the major gauge remain in
the high impedance output state. The microcontroller may load
values into the STGD via the serial interface while GOE is low.
When the microcontroller applies a high to GOE, the major gauge
output buffers are enabled. When the RUN signal is removed the
STGD continues to operate in the normal mode, however, the
controlling microcontroller should also monitor RUN and, when it
goes low, send a series of values to the STGD to move the needles
to their zero positions before taking GOE low to put the part in the
standby mode.
Table 1 describes the operation and control of the SwBATT supply,
the output buffers, and the operations normally performed by the
microcontroller. Normal operation of a vehicle will follow the
sequence of the truth table from top to bottom. The RUN input is
typically connected to the switched ignition voltage, while GOE is
controlled by the microcontroller.
1998 Apr 03
6