AND8391-D Datasheet, PDF (6/8 Pages) ON Semiconductor – Thermal Considerations for the ON Semiconductor

AND8391/D
The following graphs show the relationship between
I_reg(SS) and T_A for both the SOD−123 and SOT−223 for a
stated Cu area and thickness in still air. They also give the
slope of the line which can be used to estimate T_J at a specific
T_A.
The formula for estimating T_J is: T_J = (P_D x R_qJA) + T_A
(R_qJA value from datasheet)
For the SOD−123 @ 25°C, T_J = (225 mW x 360°C/W) +
25°C = 106°C (as shown on the graph).
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35 ^PD ^{[ 260 mW}
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I_reg(SS) vs. T_A
R_qJA [ 360°C/W
Vak = 7.5 V
33
32 _{Est. TJ [ 54°C}
31
30
29
28
27
26
25
Between −40°C & 25°C
−0.073 mA/°C
P_D [ 225 mW
Est. T_J [ 106°C
Between 25°C & 85°C
−0.059 mA/°C
P_D [ 198 mW
24
23
−40
−30
−20
−10
0
10
Est. T_J [ 156°C
20
30
40
50
60
70
80
T_A (°C)
Figure 12. Typical SOD−123 30 mA, 300 mm², 1 oz Cu, In Still Air
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35
P_D [ 268 mW
34
33 ^{Est. T}J ^{[ −14°C}
R_qJA [ 96°C/W
Vak = 7.5 V
32
Between −40°C & 25°C
31
−0.088 mA/°C
30
P_D [ 225 mW
29
28
27
26
25
24
23
−40
−30 −20
Est. T_J [ 47°C
P_D [ 192 mW
Between 25°C & 85°C
−0.072 mA/°C
Est. T_J [ 103°C
Between 85°C & 125°C
−0.061 mA/°C
P_D [ 174 mW
Est. T_J [ 142°C
−10 0 10 20 30 40
60 70 80 90 100 110 120 130
T_A (°C)
Figure 13. Typical SOT−223 30 mA, 300 mm², 2 oz Cu, In Still Air
PWM Current Control
The power dissipation of the CCR can be reduced when
used in a pulse width modulation (pwm) controlled circuit
Figure 14. The dc average current will be I_reg(SS) x duty
cycle %. For a typical 30 mA CCR at 20% duty cycle, T_A of
25°C, the average current through the LEDs will be 6.0 mA.
CCR
Anode
Lead Input
Control
Input
CCR
Cathode
Output
Figure 14.
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