Texas Instruments TPS7A3301EVM-061, User Manual, Page: 4 / 14

C002
J A D JA
T T P
= + ´ q
( )
J A
JA
D
T T
P
-
q =
JA JC CS SA
q = q + q + q
Thermal Guidelines and Layout Recommendations
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5.2 Output Load Transient
Figure 3 shows the load transient response (Vout - C1, yellow) for a full-load step transient from 100 mA
to 1 A (C3, blue). This test was run with the EVM set up for – 5-V – VOUT and – VIN was set at – 8 V.
Figure 3. Load Step and Transient Response
6 Thermal Guidelines and Layout Recommendations
Thermal management is a key component of the design of any power converter and is especially
important when the power dissipation in the LDO is high. Use the following formula to approximate the
maximum power dissipation for the particular ambient temperature:
(3)
where T
J
is the junction temperature, T
A
is the ambient temperature, P
D
is the power dissipation in the
device (watts), and θ
JA
is the thermal resistance from junction to ambient. All temperatures are in degrees
Celsius. The maximum silicon junction temperature, T
J
, must not be allowed to exceed 150 ° C. The layout
design must use copper trace and plane areas smartly, as thermal sinks, in order not to allow T
J
to exceed
the absolute maximum rating under all temperature conditions and voltage conditions across the part. The
designer must consider carefully the thermal design of the PCB for optimal performance over temperature.
The actual allowable power dissipation on a PCB is a strong function of its layout.
Heat flows from the device to the ambient air through many paths, each of which represents resistance to
the heat flow; this resistance is called thermal resistance.
The total thermal resistance of a system is defined by Equation 4 :
(4)
where θ
JA
is the thermal resistance (in ° C/W), T
J
is the allowable junction temperature of the device (in
° C), T
A
is the maximum temperature of the ambient cooling air (in ° C), and P
D
is the amount of power
(heat) generated by the device (in W).
Whenever a heatsink is installed, the total thermal resistance ( θ
JA
) is the sum of all the individual
resistances from the device, going through its case and heatsink to the ambient cooling air.
(5)
Realistically, the user can only control two resistances, θ
CS
and θ
SA
. Therefore, for a device with a known
θ
JC
, θ
CS
and θ
SA
become the main design variables in selecting a heat sink.
4 TPS7A3301EVM-061 SLVU602 – November 2011
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