Power Supply, Pins 19 and 20
The Ics have two supply pins. The IC itself is supplied via
Pin 19 (stabilized voltage, POR-circuitry). Pin 20
supplies only the current sources of the comparators. Both
pins have an an internal 21 V Zener Diode for protection.
For reasons of interference protection and surge
immunity the supply voltage pins have to be provided
with RC-circuitries as shown in figure... The resistors
limit the current in the case of overvoltage, whereas the
capacitors smooth the supply voltage.
R = 150 W; K = 0.1 mF
R = 1000 W; K = 0.1 mF
An external diode protects the IC against battery reversal.
When the supply voltage is switched on, a power-on-reset
pulse is generated internally which resets the brake lamp
monitor latch and the counter stages.
Oscillator, Pin 15
The RC-oscillator is the time base for clock 1 and clock 2
(see block diagram) which are desired from a divider
stage and fed to the two upward and downward counters.
The oscillator frequency fosc is mainly determined by the
external R/C components and an integrated resistor. The
capacitor K is charged by the external resistor and
discharged by the integrated one. Because of the temper-
ature characteristic and the tolerances of the integrated
2 kW-resistor, the external one has to have considerable
higher value in order to achieve a stable frequency.
Calculation of the frequency:
(0.74 Rosc ) 2260 W)
With the recommended values
Rosc = 82 kW and Cosc = 10 nF
fosc ≈ 1.58 kHz
Comparators K1 and K2: threshold voltage of typically
Vth = 13 mV; designed to monitor high- and low-beam
bulbs, where the fuses can be used as shunt resistors.
Comparators K3 to K7: threshold voltage of typically
Vth = 3.5 mV; designed for all other monitor purposes.
Due to the integrated compensation circuitry, these
comparators are suitable for pc layer copper shunts.
A bulb is detected as “good” as long as the voltage drop
via the shunt resistor exceeds the threshold voltage. The
pilot lamp is off. A failed bulb is detected if the voltage
drop via the shunt resistor is smaller than the threshold
voltage. The pilot lamp is on. An external transistor is
necessary to drive the pilot lamp.
In order to disable monitoring when the lamps are
switched off, therefore the comparator outputs are
disabled if the reference voltage Vref < VMin with
VMin = 0.33 VB.
As protection against transients on the supply, all
comparator inputs require external resistors. Their
tolerances directly influence the comparator threshold
accuracy. Resistors with a " 1% tolerance are
Single comparators: The inputs (K1, K2, K4, K6) sink
currents of typically 10 mA each. During switch-over, the
currents of reference and input pins are identical.
Therefore identical external protection resistors
(R = 1 kW) are required.
Double comparator: The reference input (K5) sinks a
current of typically 20 mA; its inputs sink currents of
typically 10 mA each during switch-over. Therefore the
external protection resistors must provide R = 1 kW for
REFK5 and R = 2 kW each for IN1K5 and IN2K5.
Triple comparators: The reference inputs (K3 and K7)
sink currents of typically 30 mA. All inputs (IN1K3, IN2K3
IN3K3, and IN1K7, IN2K7, IN3K7) sink the identical typical
current of 10 mA during switch-over. Therefore the
external protection resistor must provide R = 1 kW for
REFK3 and REFK7 and R = 3 kW at each input.
The internal EXOR conjunction of comparators K1 and
K2 allows fuse monitoring for both lamps. Even a
simultaneous blowout of both fuses is detected.
The pins INFK3, INFK5 and INFK6 with their EXOR gates
can be used for additional fuse monitoring.
Detection threshold for a blown fuse is Vref < VMin with
VMin = 0.33 VB (VB = reference voltage of comparators).
Pin INF is used for brake fuse monitoring. With a voltage
drop VF > 4.8 V across the fuse, the comparator detects
a blown fuse. A voltage drop VF < 2.8 V represents a
“good” fuse. U2481B’s comparator K7 is connected to a
latch, thus a brake lamp outage is memorized.
Rev. A1: 21.08.1995