EN2342QI Datasheet, PDF (18/23 Pages) Altera Corporation – 4A PowerSoC Voltage Mode Synchronous PWM Buck with Integrated Inductor

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 EN2342QI DC-DC converter is
packaged in an 8x11x3mm 68-pin QFN package.
The QFN package is constructed with copper lead
frames that have exposed thermal pads. The
exposed thermal pad on the package 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 EN2342QI is guaranteed to support the full 4A
output current up to 85°C ambient temperature.
The following example and calculations illustrate
the thermal performance of the EN2342QI.
Example:
V_IN = 12V
V_OUT = 3.3V
I_OUT = 4A
First calculate the output power.
P_OUT = 3.3V x 4A = 13.2W
Next, determine the input power based on the
efficiency (η) shown in Figure 9.
100
90
80
70
60
50
40
30
20
10
0
0
Efficiency vs. Output Current
91%
VOUT = 3.3V
CONDITIONS
V_IN = 12.0V
0.5 1 1.5 2 2.5 3 3.5 4
OUTPUT CURRENT (A)
Figure 9: Efficiency vs. Output Current
EN2342QI
For V_IN = 12V, V_OUT = 3.3V at 4A, η ≈ 91%
η = P_OUT / P_IN = 91% = 0.91
P_IN = P_OUT / η
P_IN ≈ 13.2W / 0.9 ≈ 14.51W
The power dissipation (P_D) is the power loss in the
system and can be calculated by subtracting the
output power from the input power.
P_D = P_IN – P_OUT
≈ 14.51W – 13.2W ≈ 1.31W
With the power dissipation known, the temperature
rise in the device may be estimated based on the
theta JA value (θ_JA). The θ_JA parameter estimates
how much the temperature will rise in the device for
every watt of power dissipation. The EN2342QI has
a θ_JA value of 18 ºC/W without airflow.
Determine the change in temperature (∆T) based
on P_D and θ_JA.
∆T = P_D x θ_JA
∆T ≈ 1.31W x 18°C/W = 23.5°C ≈ 24°C
The junction temperature (T_J) of the device is
approximately the ambient temperature (T_A) plus
the change in temperature. We assume the initial
ambient temperature to be 25°C.
T_J = T_A + ∆T
T_J ≈ 25°C + 24°C ≈ 49°C
The maximum operating junction temperature
(T_JMAX) of the device is 125°C, so the device can
operate at a higher ambient temperature. The
maximum ambient temperature (T_AMAX) allowed can
be calculated.
T_AMAX = T_JMAX – P_D x θ_JA
≈ 125°C – 24°C ≈ 101°C
The maximum ambient temperature the device can
reach is 101°C given the input and output
conditions. Note that the efficiency will be slightly
lower at higher temperatures and this calculation is
an estimate.
www.altera.com/enpirion, Page 18
09520
February 21, 2014
Rev A