EN23F0QI Datasheet, PDF (21/26 Pages) Enpirion, Inc. – 15A Voltage Mode Synchronous Buck PWM

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 EN23F0QI 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 EN23F0QI 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 EN23F0QI.
Example:
V_IN = 12V
V_OUT = 1.2V
I_OUT = 15A
First calculate the output power.
P_OUT = 1.2V x 15A = 18W
Next, determine the input power based on the
efficiency (η) shown in Figure 11.
Efficiency vs. Output Current
100
90
80
70
60
50
40
30
VOUT = 3.3V
CONDITIONS
20
VOUT = 1.8V
V_IN = 12.0V
AVIN = 3.3V
10
VOUT = 1.2V
Dual Supply
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
OUTPUT CURRENT (A)
Figure 11: Efficiency vs. Output Current
EN23F0QI
For V_IN = 12V, V_OUT = 1.2V at 15A, η ≈ 80%
η = P_OUT / P_IN = 80% = 0.8
P_IN = P_OUT / η
P_IN ≈ 18W / 0.8 ≈ 22.5W
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
≈ 22.5W – 18W ≈ 4.5W
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 EN23F0QI has
a θ_JA value of 13 ºC/W without airflow.
Determine the change in temperature (ΔT) based
on P_D and θ_JA.
ΔT = P_D x θ_JA
ΔT ≈ 4.5W x 13°C/W = 58.5°C ≈ 59°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 + ΔT
T_J ≈ 25°C + 59°C ≈ 84°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 – 59°C ≈ 66°C
The maximum ambient temperature the device can
reach is 66°C given the input and output conditions.
Note that the efficiency will be slightly lower at
higher temperatures and this calculation is an
estimate.
©Enpirion 2012 all rights reserved, E&OE
Enpirion Confidential
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