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

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

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

ρ

The sheet resistance

ρ

cladding (with 1 ounce per square foot (oz./ft

0.0014" approx. equal to 35

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