EN5339QI
Thermal Considerations
Thermal considerations are important power supply
design facts that cannot be avoided in the real
world. Whenever there are power losses in a
system, the heat that is generated by the power
dissipation needs to be accounted for. The Enpirion
PowerSoC helps alleviate some of those concerns.
The Enpirion EN5339QI DC-DC converter is
packaged in a 4x6x1.1mm 24-pin QFN package.
The QFN package is constructed with exposed
thermal pads on the bottom of the package. The
exposed thermal pad should be soldered directly on
to a copper ground pad on the printed circuit board
(PCB) to act as a heat sink. The recommended
maximum junction temperature for continuous
operation is 125°C. Continuous operation above
125°C may reduce long-term reliability. The device
has a thermal overload protection circuit designed
to turn off the device at an approximate junction
temperature value of 150°C.
For VIN = 5V, VOUT = 3.3V at 3A, η ≈ 90%
η = POUT / PIN = 90% = 0.90
PIN = POUT / η
PIN ≈ 9.9W / 0.9 ≈ 11W
The power dissipation (PD) is the power loss in the
system and can be calculated by subtracting the
output power from the input power.
PD = PIN – POUT
≈ 11W – 9.9W ≈ 1.1W
With the power dissipation known, the temperature
rise in the device may be estimated based on the
theta JA value (θJA). The θJA parameter estimates
how much the temperature will rise in the device for
every watt of power dissipation. The EN5339QI has
a θJA value of 36 ºC/W without airflow.
Determine the change in die temperature (∆T)
based on PD and θJA.
The EN5339QI is guaranteed to support the full 3A
output current up to 85°C ambient temperature.
The following example and calculations illustrate
the thermal performance of the EN5339QI.
∆T = PD x θJA
∆T ≈ 1.1W x 36°C/W = 39.6°C ≈ 40°C
Example:
The junction temperature (TJ) of the device is
approximately the ambient temperature (TA) plus
the change in temperature. We assume the initial
ambient temperature to be 25°C.
VIN = 5V
VOUT = 3.3V
I
OUT = 3A
TJ = TA + ∆T
First calculate the output power.
POUT = 3.3V x 3A = 9.9W
TJ ≈ 25°C + 40°C ≈ 65°C
The maximum operating junction temperature
(TJMAX) of the device is 125°C, so the device can
operate at a higher ambient temperature. The
maximum ambient temperature (TAMAX) allowed can
be calculated.
Next, determine the input power based on the
efficiency (η) shown in Figure 6.
100
90
TAMAX = TJMAX – PD x θJA
≈ 125°C – 40°C ≈ 85°C
80
70
~90%
60
50
40
30
The ambient temperature can actually rise by
another 60°C, bringing it to 85°C before the device
will reach TJMAX. This indicates that the EN5339QI
can support the full 3A output current range up to
approximately 85°C ambient temperature given the
input and output voltage conditions. Note that the
efficiency will be slightly lower at higher
temperatures and these calculations are estimates.
20
CONDITIONS
VOUT = 3.3V
VIN = 5V
10
0
0
0.5
1
1.5
2
2.5
3
OUTPUT CURRENT (A)
Figure 6: Efficiency vs. Output Current
Enpirion 2012 all rights reserved, E&OE
www.enpirion.com, Page 14
06903
September 12, 2012
Rev: B
ENPIRION [ ENPIRION, INC. ]
