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參數(shù)資料
| 型號： | SA5778 |
| 廠商： | NXP Semiconductors N.V. |
| 英文描述： | Serial triple gauge driver STGD |
| 中文描述： | 串行三規(guī)司機STGD |
| 文件頁數(shù)： | 7/16頁 |
| 文件大?。?/td> | 122K |
| 代理商： | SA5778 |
Philips Semiconductors
Product specification
SA5778
Serial triple gauge driver (STGD)
1998 Apr 03
7
Table 1.
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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.
áá
1=High
á
RUN
á
á
á
GOE
Input
á
á
ááá
ááá
SwControl
1=ON
á
á
á
Swbatt1,2
Voltage
á
á
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á
Minor Gauge Driver
Outputs
á
á
High
ááá
ááá
ááá
ááá
Enabled
ááá
Impedance
á
Major Gauge
Driver Outputs
á
á
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áááááááááá
Standby mode
á
á
0
áá
áá
áá
áá
0
áá
á
á
á
1
á
á
á
á
ááá
ááá
1
ááá
ááá
ááá
á
á
áá
áá
V
BATT
áá
áá
á
ááááá
ááááá
Enabled
áá
ááááá
á
Enabled
(output forced to zero)
á
High
port.
á
á
Start up mode, sets minor gauge driver to
zero position, and disables major gauge
driver. Load values into STGD via the serial
á
1
á
1
á
1
á
V
BATT
á
Enabled
á
values into the STGD to return needles to
Enabled
á
update gauge data as required by the
á
á
á
á
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á
á
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row of table)
á
á
0
á
0
á
0
á
Off
á
High Impedance
á
á
Returned to standby mode (same as first
The highest power dissipation for a given coil driver will occur when
the coil voltage is being driven to 50% of V
BATT
. Thus the power
dissipated by each coil driver is (V
BATT
/2)* (V
BATT
/2Rc) or
V
BATT
(V
BATT
/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*V
BATT
(V
BATT
/4Rc) or simply V
BATT
(V
BATT
/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:
P
D
=V
BATT
(V
BATT
/R
C
)+V
BATT
(0.012) +
V
OL2
(V
BATT
–V
OL2
–V
BE(PNP)
)/R
B
Where:
P
D
= Power dissipation in watts
V
BATT
= Battery supply voltage in volts
R
C
= Coil resistance in ohms at ambient temperature including
any self heating effects
V
OL2
= Output low voltage of the SwCONTROL pin as specified
in the DC Characteristics
V
BE(PNP)
= The V
BE
drop of the external PNP transistor
R
B
= Resistor is series with base of external PNP transistor.
The minimum value of R
B
= V
BATTMAX
I
OL
=16/0.050=320
The actual value used is dependent on the current needed to
keep the PNP in saturation.
All gauges at 45
°
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 25
°
C, the coil operating resistance at
a high temperature ambient may be calculated using the following
formula:
R
CA
= R
CN
(1+(0.4%/
°
C)*((T
SH
+T
amb
)–25
°
C))
Where:
R
CA
= Resistance of Coil at Ambient temperature, including self
heating
R
CN
= Nominal Resistance of Coil at 25
°
C, without self heating
T
amb
= Ambient temperature,
°
C
T
SH
= Self heating of coil,
°
C
0.4%/
°
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.
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