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AD22001N Datasheet(PDF) 5 Page - Analog Devices |
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AD22001N Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page AD22001 REV. A –5– 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. GND FUSE CHECK OUT 5 POWER OUT LAMP LAMP FUSE FUSE OUT (3 × 4) OUT 3 & 4 FUSE FUSE CHIP ENABLE LAMP LAMP LAMP OUT 1 & 2 FUSE CHECK AD22001 1 2 3 4 5 6 7 8 9 10 NOTE R S ARE COPPER-TRACK SHUNT RESISTORS ON THE PRINTED CIRCUIT BOARD. 11 20 19 18 17 16 15 14 12 13 +V S R S R S +V BATT R S R S R S 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, ρ S, of 35 µm copper cladding is about 0.5 milliOhms per square (m Ω/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 m Ω, while with an aspect ratio of 4 its resistance would be 2 m Ω, etc. The resistor can be designed for a given value R by noting R = ρ SL/W, so that L = RW/ ρ S. The sheet resistance ρ S is based on the thickness of copper cladding (with 1 ounce per square foot (oz./ft 2) 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 m Ω. Substituting these values in the expression for L, L = 10.769 m Ω 0.1" /0.25 m Ω 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 |
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