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EN5339QI

3A PowerSoC Low Profile Synchronous Buck DC-DC Converter with Integrated Inductor

Altera Corporation 

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 Altera

Enpirion PowerSoC helps alleviate some of those

concerns.

The Altera 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.

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.

Example:

VIN = 5V

VOUT = 3.3V

IOUT = 3A

First calculate the output power.

POUT = 3.3V x 3A = 9.9W

Next, determine the input power based on the

efficiency (η) shown in Figure 7.

100

90

80

70

60

50

40

30

20

10

0

0

~90%

VOUT = 3.3V

CONDITIONS

VIN = 5V

0.5

1

1.5

2

2.5

3

OUTPUT CURRENT (A)

Figure 7: Efficiency vs. Output Current

EN5339QI

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 (ΔT)

based on PD and θJA.

ΔT = PD x θJA

ΔT ≈ 1.1W x 36°C/W = 39.6°C ≈ 40°C

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.

TJ = TA + ΔT

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.

TAMAX = TJMAX – PD x θJA

≈ 125°C – 40°C ≈ 85°C

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.

06903

June 26, 2015

www.altera.com/enpirion, Page 15

Rev E

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