Application Hints

THEORY OF OPERATION UNDER-CURRENT LIMIT

DETECTOR

TLH8707 – 6

Lamp Fault Detector

FIGURE 3 Equivalent Automotive Lamp Circuit

The diagram of

Figure 3 represents the typical lamp circuit

found in most automobiles Switch S1 represents a dash-

board switch discrete power device relay andor flasher

actual circuit component (such as a 01X 1W carbon resis-

tor) or it can represent the resistance of some or all of the

wiring harness The load represented here as a single bulb

can just as easily be two or more bulbs in parallel such as

front and rear parking lights or left and right highbeams etc

One of the easiest methods to electronically monitor proper

zero (or to half their former values in the case of two bulbs

wired in parallel) A comparator circuit can then monitor this

sense voltage and alert the system or system user (eg

power an LED) if this sense voltage drops below a predeter-

mined level (defined as the threshold voltage)

Typical sense voltages range from tens to hundreds of milli-

volts Not only does this sense voltage vary nonlinearly with

the battery voltage it may vary significantly with ambient

temperature depending on the temperature coefficient (TC)

of the sense resistor or wiring harness Since these nonlin-

ear characteristics can vary from system to system and

sometimes even within a single system provisions must be

made to accommodate them There are two general meth-

odologies to accomplish this

The first method uses only one bulb per monitoring circuit A

sense resistor is selected to give 50 – 100 mV of sense volt-

age in an operational circuit and a comparator threshold

detecting voltage of approximately 10 mV is set Even if

component tolerances battery line variations and tempera-

ture coefficients cause the sense voltage to vary 31 or

more circuit operation will not be affected

The second method must be used if two or more bulbs are

wired in parallel and it is necessary to detect if any single

lamp fails This is often desirable as it reduces the number

of comparators and displays and system cost by at least a

factor of two In this case the sense voltage will drop by

only half (or less) of it’s original value For example a nomi-

nal 100 mV drop across the sense resistor will drop to

50 mV if one of two bulbs fail Therefore a threshold detec-

tion voltage between 50 and 100 mV is required (since a

10 mV threshold would alert the system only if both bulbs

failed) Yet a fixed threshold of 75 mV may not work if the

nominal 100 mV sense voltage can vary 31 due to the fac-

tors mentioned earlier What is required is a comparator with

a threshold-detecting voltage that tracks the nominal sense

voltage as battery line and ambient temperature change

Thus while the sense voltage may nominally be anywhere

from 50 to 150 mV the threshold voltage will always be

roughly 75% of it or 37 mV to 112 mV and will detect the

failure of either of two bulbs

The LM1946 integrated circuit contains five comparators es-

pecially designed for lamp monitoring requirements Since

all lamps in a system share the same battery voltage and

ambient temperature accommodations for these variations

need to be made only once at the IC and each threshold of

the five comparators then tracks these variations

SETTING THE COMPARATOR THRESHOLD VOLTAGE

The threshold voltage at which the comparator output

changes state is user-set in order to accommodate the

many possible system designs The input bias currents are

purposely high to accomplish this and are each equal to the

user-set current into the Iset pin (more on this later) Typi-

cally around 20 mA the effect of this across the sense resis-

gible and can be ignored However when resistors R1 and

R2

(Figure 4) are added to the circuit a shift in the threshold

voltage is effected This occurs since each input has been

affected by different IR drops The LM1946 behaves like

any other comparator in that the output switches when the

input voltage at the IC pins is zero millivolts (ignoring offset

voltage for the moment) If the output therefore has just

switched states due to just the right threshold voltage

across the sense resistor then the sum of voltages around

the resistor loop should equal zero

TLH8707 – 9

FIGURE 4 Input Bias Current

Assuming Voffset m Vthrshld

5