AND8391-D Datasheet, PDF (5/8 Pages) ON Semiconductor – Thermal Considerations for the ON Semiconductor

AND8391/D
The negative temperature coefficient trend of a SOD−123
CCR has a benefit as it avoids thermal runaway. There are
two areas of interest on the curves of Figure 9. The first is for
a given T_A. Each curve shows a decrease in I_reg(SS) as Vak
increases and therefore P_D increases. There also is the
ambient temperature affect on I_reg for a fixed Vak condition.
Both the SOD−123 (Figure 9) and SOT−223 (Figure 10)
show a decrease in I_reg(SS) as T_A increases.
37
36
35 _{TA = −40°C}
34
33
[−0.073 mA/°C
32
Typ @ Vak = 7.5 V
31
30
29
28
T_A = 25°C
[−0.059 mA/°C
27
Typ @ Vak = 7.5 V
26
25 _{TA = 85°C}
24
3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Vak (V)
Figure 9. Typical SOD−123 30 mA, 300 mm²,
1 oz Cu, In Still Air
The following design examples will show how to
determine which package device and the Cu needed for a
simple circuit.
Circuit Design
Example 1:
For a series circuit (Figure 11), the power dissipation of
the CCR is determined by:
(V_source – (V_LEDS + V_RPD)) x I_reg. Using the worst case
scenario; i.e, highest V_source, Lowest LED V_F, and highest
target I_reg. Using a 16 V source (auto voltage regulator high
output) driving two white LEDs with a Vf of 4.2 V, a reverse
protection diode (RPD) with a V_F of 0.2 V and 30 mA I_reg
would give: (16 V − (2 x 4.2 V + 0.2 V)) x 0.030 A = 7.4 V
x 0.03 A = 222 mW.
For an ambient temperature of 85°C, from the P_D curves
of Figures 1 and 3 a SOD−123 with 500 mm² 1 oz Cu would
See ON Semiconductor application note AND8223/D for
additional information.
SOD−123 devices exhibit a greater negative temperature
coefficient as shown in Figure 9 than corresponding
SOT−223 devices as shown in Figure 10, due to the
difference in the package R_qJA. The SOD−123 package
reaches thermal saturation with less power applied than the
SOT−223 package.
36
35
34
33
T_A = −40°C
32
[−0.088 mA/°C
Typ @ Vak = 7.5 V
31
30
29
28
27
T_A = 25°C
[−0.058 mA/°C
Typ @ Vak = 7.5 V
26
25
[−0.061 mA/°C
24
23
T_A = 85°C
22
Typ @ Vak = 7.5 V
21
T_A = 125°C
20
3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Vak (V)
Figure 10. Typical SOT−223 30 mA, 300 mm²,
2 oz Cu, In Still Air
suffice. A SOT−223 with 100 mm² 1 oz Cu would also
work.
Example 2:
Three Red LEDs with each having a V_F of 2.0 Vdc @ 30
mA. Automotive battery voltage of 16 Vdc. Ambient
temperature max of 85°C. Available heat sink area for
device is 300 mm² of 1 oz Cu.
P_D of device = (16 Vdc – (3 x 2.0 Vdc) + 0.2 Vdc) x 30 mA
= 294 mW
SOD−123 P_D max @ 85°C, 300 mm² of 1 oz Cu = 182 mW
SOT−223 P_D max @ 85°C, 300 mm² of 1 oz Cu = 598 mW
The SOT−223 gives a margin of safety in the application.
Or, knowing that 294 mW of power needs to be dissipated,
we can select a SOT−223 device using 100 mm² of 1 oz Cu.
+DC
−DC
Reverse Battery Protection Diode (RPD)
1 ^D1
MBRS140T3
Cathode
Anode
Q1 CCR
NSI45030T1G
Automotive LED’s (3 mm² − 4 Lead)
D2
D3
21
D4
21
2
LED
LED
LED
1
Figure 11.
http://onsemi.com
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