SA5778 Datasheet, PDF(7/16 Page) NXP Semiconductors – Serial triple gauge driver STGD

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Philips Semiconductors

Serial triple gauge driver (STGD)

Product specification

SA5778

Table 1. Truth Table

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ RUN

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ Input

1=High

GOE

Input

1=High

SwControl

1=ON

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ 0

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ 1

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ 0

ÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃÃ 0

Swbatt1,2

Voltage

Off

VBATT

Off

Minor Gauge Driver

Outputs

High Impedance

Enabled

(output forced to zero)

Enabled

High Impedance

Major Gauge

Driver Outputs

System Status

High

Impedance

High

Impedance

Enabled

High

Impedance

Standby mode

Start up mode, sets minor gauge driver to

zero position, and disables major gauge

driver. Load values into STGD via the serial

port.

Normal Operating mode. Periodically

update gauge data as required by the

application.

Power down sequence. Load a series of

values into the STGD to return needles to

zero before power is removed.

Returned to standby mode (same as first

row of table)

THERMAL MANAGEMENT AND POWER

DISSIPATION

The power dissipated by the STGD has three components. The first

term in the equation below represents the power dissipated in the

STGD from current through the coil resistance. This component of

the power dissipation is a function of both the battery voltage and

the coil resistance. Most of the external loads such as the coils are

resistive, so the current drawn by the output buffers is proportional

to the supply voltage, resulting in power dissipation that is

proportional to the square of the supply voltage for these circuits.

The highest power dissipation for a given coil driver will occur when

the coil voltage is being driven to 50% of VBATT. Thus the power

dissipated by each coil driver is (VBATT/2)* (VBATT/2Rc) or

VBATT(VBATT/4Rc). If the coil resistance of the two minor gauge coils

and the two coils of the major gauge all have the same resistance,

then the maximum total power dissipation of the drivers becomes

4*VBATT(VBATT/4Rc) or simply VBATT(VBATT/Rc). Much of the

internal analog circuits appears to the supply pins as a current sink

and is represented by the second term. The current drawn by these

circuits is relatively constant despite changes in supply voltage,

resulting in power dissipation that is proportional to the supply

voltage. Finally some power is dissipated in driving the external PNP

transistor used to control the switched battery supply. The total

power dissipation is a combination of these components and may be

calculated from the formula:

PD=VBATT(VBATT/RC)+VBATT(0.012) +

VOL2(VBATTâVOL2âVBE(PNP))/RB

Where:

The actual value used is dependent on the current needed to

keep the PNP in saturation.

All gauges at 45Â° to a quadrant axis, as this is the highest

internal power dissipation position.

If only the nominal coil resistance is known at a given nominal

ambient temperature such as 25Â°C, the coil operating resistance at

a high temperature ambient may be calculated using the following

formula:

RCA = RCN (1+(0.4%/Â°C)*((TSH+Tamb)â25Â°C))

Where:

RCA = Resistance of Coil at Ambient temperature, including self

heating

RCN = Nominal Resistance of Coil at 25Â°C, without self heating

Tamb = Ambient temperature, Â°C

TSH = Self heating of coil, Â°C

0.4%/Â°C = Resistance increase coefficient for copper

Figure 7 shows power dissipation plotted as a function of coil

resistance and voltage. Since coil resistance is a function of

temperature, the maximum power dissipation plotted will only occur

at the lowest specified operating temperature. The power dissipation

is lowest at the highest ambient temperature because of the

increase in coil resistance with temperature.

This maximum power dissipation will only occur during a fault

condition in which the system voltage rises to 18V, generally

because of a failed voltage regulator controlling the vehicles battery

voltage. Power dissipation will be lower when air core meter

movements with higher nominal coil resistance are used.

PD = Power dissipation in watts

VBATT = Battery supply voltage in volts

RC = Coil resistance in ohms at ambient temperature including

any self heating effects

VOL2 = Output low voltage of the SwCONTROL pin as specified

in the DC Characteristics

VBE(PNP)= The VBE drop of the external PNP transistor

RB = Resistor is series with base of external PNP transistor.

The minimum value of RB = VBATTMAXIOL=16/0.050=320 â¦

1998 Apr 03

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