AND8349-D Datasheet, PDF (2/6 Pages) ON Semiconductor – Automotive Applications The Use of Discrete

AND8349/D
A CCR is a nearly ideal current source providing constant
current regardless of applied voltage above its operating
minimum. In simple terms, a CCR can be considered a
nonlinear voltage controlled resistor.
The Power Dissipation (P_d) in an LED is P_d = I V. Since
the CCR acts as a voltage controlled resistor while the
resistor biasing fixes the resistor value, the LED Power
dissipation is shown to be nearly constant over a variable
battery range. (Figure 3)
For automotive CHMSLs, a constant current source for
LEDs reduces stress conditions caused by overdriving with
current as compared to resistor biasing. A Reverse
protection diode (MBRS140T3 in Figure 4) prevents a
reverse voltage condition which can permanently damage an
LED and protects the CCR from conducting in the reverse
bias mode (Figure 1).
A basic CHMSL configuration with 3 Red LEDs in series
is shown in Figure 4. A CCR provides a uniform intensity
over full line voltage swings and greatly reduces LED power
dissipation as compared to common resistor biasing. At 9 V
battery input, a CCR provides a higher current than a typical
biasing resistor value would provide (Figure 3). At 16 V a
stable, constant current is supplied by the CCR.
Figure 2 shows a comparison of CCR vs. Resistor Bias
current over battery voltage variation from 9 V to 16 V. The
LED current, and therefore intensity, is constant with the
CCR device compared to the resistor bias.
40
T_A = 25°C
35 _{Circuit Current with}
CCR Device
30
25
Circuit Current
with 250 W
20
15
10
9
10
11
Representative Test Data
for Figure 6 Circuit, Current
of LEDs, FR−4 @ 300 mm²,
1 oz Copper Area
12
13
14
15
16
V_in (V)
Figure 2. Series Circuit Current
240
220 ^{TA = 25°C}
200 ^{LED Power with}
CCR Device
180
160
140
120 ^{LED Power}
with 250 W
100
80
60
9
10
11
Representative Test Data
for Figure 6 Circuit, Pd of
LEDs, FR−4 @ 300 mm²,
1 oz Copper Area
12 13 14
15 16
V_in (V)
Figure 3. LED Power
MBRS140T3
CCR
CCR
CCR
CCR
CCR
Figure 4.
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