AD22001N データシートの表示（PDF） - Analog Devices
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AD22001N Datasheet PDF : 8 Pages
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AD22001
Threshold Biasing
The comparators are all activated by a common bias connection
which controls their threshold voltage. The threshold is
compensated for variations in temperature and operating
voltage. The temperature compensation is approximately +3770
ppm/°C. This causes the threshold to track the voltage across a
copper shunt operated at constant current and varying
temperature. The comparators are intended for use with small
value shunts made using normal copper tracks on PC cards.
+VBATT
CHIP ENABLE
1
+VS
2
AD22001
GND
20
FUSE CHECK
19
POWER
FUSE
OUT
3
4
LAMP RS
5
FUSE
CHECK
6
OUT 1 & 2
7
LAMP
8
RS
9
LAMP
10
18
OUT 5
LAMP
17
RS
16
LAMP FUSE
15
RS
14
13
OUT
3&4
FUSE
12
OUT
(3 × 4)
11
RS
FUSE
NOTE
RS ARE COPPER-TRACK SHUNT RESISTORS ON THE
PRINTED CIRCUIT BOARD.
Figure 4. Application Suggestion
The power supply voltage compensation alters the comparator
threshold with a 50% sensitivity around the nominal 13.5 V
powering voltage. That is, a 10% change in battery voltage
results in a 5% change in threshold voltage. This compensation
approximately tracks the current in a hot lamp which typically
varies nonlinearly with applied voltage, with approximately 50%
sensitivity.
ON-OFF and Power Control
The AD22001 is intended to be powered directly from the
battery voltage. In normal operation an internal switch connects
internal circuits and the Power Out pin to the battery voltage,
under control of the Chip Enable. When Chip Enable is high
(above 9 V) the switch is on, the AD22001 is active and the
Power Out pin will be close to battery potential. When Chip
Enable is low, or open circuit, the switch will open and the
AD22001 will be inactive and Power Out will go low.
In order to protect the external circuits connected to Power Out
from overvoltage, the power switch is “soft” and limits the
output voltage to typically 16 V. If the applied voltage exceeds
this value, both Power Out and the internal voltage (from which
the threshold is derived) will limit. The limited Power Out
voltage will be available for moderate overvoltage conditions
such as double battery. If the applied voltage goes above
typically 33 V, the internal switch will be turned off, the lamp
monitor circuits will be inactivated and Power Out will go low
for the duration of the overvoltage.
Copper Resistors
Low value resistors can be produced as part of the printed
circuit pattern on copper clad boards. These resistors will have a
large positive TC, but this effect is compensated by the
temperature sensitive threshold in the AD22001.
At a design temperature of 24°C the sheet resistance, ρS, of 35 µm
copper cladding is about 0.5 milliOhms per square (mΩ/sq),
and 70 µm copper is about 0.25 mΩ/sq. That is, a resistor made
from 35 µm copper with an aspect ratio (length/width) of 1 will
have a resistance of 0.5 mΩ, while with an aspect ratio of 4 its
resistance would be 2 mΩ, etc. The resistor can be designed for
a given value R by noting R = ρSL/W, so that L = RW/ρS.
The sheet resistance ρS is based on the thickness of copper
cladding (with 1 ounce per square foot (oz./ft2) copper at
0.0014" approx. equal to 35 µm, and 2 oz./ft2 copper at 0.0028"
approx. equal to 70 µm) and the temperature. Choosing the
resistance value to correspond to the desired current trip level
and AD22001 threshold at 22°C, the 24°C “round number” is
convenient allowing a 2°C gradient from the resistor to the IC.
Choose W, the track width, as the larger of two values to be
determined. First the conductor should be sized so that the
nominal operating current results in only a small temperature
rise. This can be determined from the table from MIL STD-
275C, 9 Jan. 1970 (Figure 8).
Second, the width must be great enough to survive the peak
currents which are allowed by the fuse which supplies the track.
This can be determined from recommendations by the laminate
manufacturer, or the MIL table.
Using the larger of the two values determined for W, the length
of the resistor can be calculated. The resistor is defined by
“Kelvin” connections to the ends which sense the voltage across
this section of the track, and apply it to the differential input of
the AD22001 comparator.
As an example, consider the case of a parking lamp which
nominally draws 325 mA, and is on a circuit fused for 10A.
Suppose the board uses 70 µm (2 oz.) copper cladding, then a
conductor width of a few mils will suffice for the nominal
current. However, the 10A fuse may allow as much as 13.5A
current before it opens. A conductor 0.1" in width will carry
13.5A with less than 50°C rise in temperature, according to the
table.
Selecting 1/2 the nominal current as the AD22001 trip level
gives:
R = 1.75 mV/(325 mA/2)
or R = 10.769 mΩ.
Substituting these values in the expression for L,
L = 10.769 mΩ ϫ 0.1" /0.25 mΩ per sq
or L = 4.3".
A track of at least 0.1" in width should be used to convey this
current anywhere on the board. A section 4.3" long should be
sampled by Kelvin connections to the comparator differential
inputs.
This case, where the peak current available is large while the
sensed current is small, results in extremes of resistor length. In
such cases it may be convenient to bend or fold the shunt
REV. A
–5–
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