ADP3338
As shown in Figures 4a–c, the amount of copper the ADP3338
is mounted to affects the thermal performance. When mounted
to 2 oz. copper with just the minimal pads, Figure 4a, the θJA is
126.6°C/W. By adding a small copper pad under the ADP3338,
Figure 4b, reduces the θJA to 102.9°C/W. Increasing the copper
pad to 1 square inch, Figure 4c, reduces the θJA even further
to 52.8°C/W.
THERMAL OVERLOAD PROTECTION
The ADP3338 is protected against damage due to excessive power
dissipation by its thermal overload protection circuit. Thermal
protection limits the die temperature to a maximum of 160°C.
Under extreme conditions (i.e., high ambient temperature and
power dissipation) where the die temperature starts to rise above
160°C, the output current will be reduced until the die tempera-
ture has dropped to a safe level.
Current and thermal limit protections are intended to protect
the device against accidental overload conditions. For normal
operation, the device’s power dissipation should be externally
limited so that the junction temperature will not exceed 150°C.
CALCULATING POWER DISSIPATION
Device power dissipation is calculated as follows:
a.
b.
c.
PD = V −VOUT × I
+ V × I
(
)
(
)
IN
LOAD
IN
GND
Figure 4. PCB Layouts
Where ILOAD and IGND are load current and ground current, VIN
and VOUT are the input and output voltages respectively.
Use the following general guidelines when designing printed
circuit boards:
Assuming worst-case operating conditions are ILOAD = 1.0 A,
GND = 10 mA, VIN = 3.3 V and VOUT = 2.5 V, the device power
dissipation is:
1. Keep the output capacitor as close to the output and ground
pins as possible.
I
2. Keep the input capacitor as close to the input and ground
pins as possible.
PD = 3.3V – 2.5V 1000 mA + 3.3V 10 mA = 833 mW
(
)
(
)
So, for a junction temperature of 125°C and a maximum ambi-
ent temperature of 85°C, the required thermal resistance from
junction to ambient is:
3. PC board traces with larger cross sectional areas will remove
more heat from the ADP3338. For optimum heat transfer,
specify thick copper and use wide traces.
4. The thermal resistance can be decreased by adding a copper
pad under the ADP3338 as shown in Figure 4b.
125°C – 85°C
θJA
=
= 48°C/W
0.833W
5. If possible, utilize the adjacent area to add more copper
around the ADP3338. Connecting the copper area to the
output of the ADP3338, as shown in Figure 4c, is best but
will improve thermal performance even if it is connected to
other signals.
PRINTED CIRCUIT BOARD LAYOUT
CONSIDERATIONS
The SOT-223’s thermal resistance, θJA, is determined by the
sum of the junction-to-case and the case-to-ambient thermal
resistances. The junction-to-case thermal resistance, θJC, is
determined by the package design and specified at 26.8°C/W.
However, the case-to-ambient thermal resistance is determined
by the printed circuit board design.
6. Use additional copper layers or planes to reduce the thermal
resistance. Again, connecting the other layers to the output
of the ADP3338 is best, but not necessary. When connecting
the output pad to other layers use multiple vias.
REV. 0
–7–
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