U2481B Datasheet, PDF (7/11 Pages) List of Unclassifed Manufacturers – Automotive Lamp Outage Monitor

TELEFUNKEN Semiconductors
U2481B/ U2482B
Application Hints
Layout Recommendations for Copper
Layer Shunts
Lamp outage monitor systems can be produced most cost-
efficiently if stamped shunt resistors are replaced by
copper layer shunts which are generated with the pc board
layout.
The U2481B and the U2482B are suitable for this
application because of their comparator thresholds,
which are compensated in reference to the temperature
characteristic of copper.
A constant lamp current, I_LAMP = V_Th/R_sh with threshold
voltage V_Th = f(T) and shunt resistor R_sh = f(T), is
achieved if the comparator threshold and the shunt
resistor have identical temperature characteristics. With
the temperature coefficient of copper a_cu = 3.9<10^–3
1/K, a copper shunt changes its nominal value by 52% if
the automotive ambient temperature range of t_amb = –40
to +95°C is taken into consideration.
Examples for sheet resistances of copper shunts
(T_amb = 25°C):
R_A = 0.5 mW/square (35.1 mm layer thickness)
R_A = 0.25 mW/square (70 mm layer thickness)
How to Lay Out Copper Shunts (figure 3)
The width of the copper trace has to be selected in
reference to a low current-effected temperature increase.
The copper trace must be capable of peak currents which
do not blow the fuse. The peak currents are specified by
the car manufacturers.
Example:
A 7.5 A fuse allows a peak current of 26 A (1 s), 15 A
(10 s) or 10 A (60 s).
The copper shunt length has to be calculated between the
two sense connections to the comparator. The connection
of the common reference input of double and triple
comparators has to be considered carefully.
Calculation example for a copper shunt used with a 4 W
bulb.
I_LAMP = 0.325 A
Failure criterion: I = 1/2<I_LAMP
10 A fuse is capable of I = 13.5 A
Copper layer thickness: 70 mm
Comparator threshold voltage (U2481B, U2482B):
V_Th = 3.5 mV
Assumed copper width for temperature increase
DT_cu < 50°C:
W_cu = 2.5 mm (13.5 A, 70 mm)
Two equations for calculation of the shunt resistance:
R_shunt = R_A<L/W_cn
(with R_A = Cu sheet resistance)
R_shunt = V_Th / 1/2 I_LAMP
Thus the length of copper shunt is calculated as:
L = 2<V_Th<W_cn / R_A<I_LAMP
L = 215 mm
For a reasonable pc-layout a meander-shaped shunt
resistor is recommended. The high lamp currents may
cause hot spots at sharp edges of the copper shunts. That
may deteriorate accuracy of the measurement. Therefore
it is recommended to layout the copper shunts with
smoothed curves.
In accordance to figure...the meander may be formed by
4 straight tracks (length L_S each) and 3 connecting 180°
ares (length L are each). If the mean are radius is selected
to r = W_cn the are lenght becomes L_are = p W_cn.
Thus the total lenght is
L = 4 L_S + 3 L_are
= 4 L_S + 3 p W_cn
With L = 215 mm the track length becomes
L_S
+
1
4
(L
*
3
p
W _cn)
L_S = 47.8 mm
Using Fuses as Shunt Resistors
This cost saving method can be used if the following
assumptions are fulfilled:
– Each lamp needs a dedicated fuse
– The fuse socket is mounted within the monitor
module to connect the sense linesare suitable for
this monitor
– task because of their comparator thresholds are
compensated for the fuse temperature coefficient
of a_F = 4.1<10^–3 1/K
Calculation Example for a 55 W Bulb and a
7.5 A fuse:
Measured voltage drop across the fuse: V_F = 52 mV
Measured current: I_LAMP = 4.2 A
Calculated resistance: R_F = 12.4 mW
Selected comparatore threshold:
V_Th = 1/4 V_F = 13 mV (typically);
With a lamp current I_LAMP < V_Th/R_F =< 1.05 A
the comparator detects a blown fuse.
Rev. A1: 21.08.1995
Preliminary Information
7 (11)