0

0.4

0.6

0.8

1.0

1.8

0.2

0

20

40

60

140

80

100

120

T

0

150

250

500

LFM

1.2

1.6

1.4

LFM = 0

LFM = 150

LFM = 250

LFM = 500

TPS54290, TPS54291, TPS54292

SLUS973 – OCTOBER 2009

www.ti.com

POWER DERATING

The TPS5429x delivers full current at wide duty cycles at ambient temperatures up to 85°C if the thermal

impedance from the thermal pad is sufficient to maintain the junction temperature below the thermal shut down

level. At higher ambient temperatures, the device power dissipation must be reduced to maintain the junction

temperature at or below the thermal shutdown level. Figure 18 illustrates the power derating for elevated ambient

temperature under various air flow conditions. Note that these curves assume the PowerPAD is soldered to the

recommended thermal pad. See References for further information.

Figure 18. Power Derating Curves

PowerPAD PACKAGE

The PowerPAD package provides low thermal impedance for heat removal from the device. The PowerPAD

derives its name and low thermal impedance from the large bonding pad on the bottom of the device. The circuit

board must have an area of solder-tinned-copper underneath the package. The dimensions of this area depend

on the size of the PowerPAD package. Thermal vias connect this area to internal or external copper planes and

should have a drill diameter sufficiently small so that the via hole is effectively plugged when the barrel of the via

is plated with copper. This plug is needed to prevent wicking the solder away from the interface between the

package body and the solder-tinned area under the device during solder reflow. Drill diameters of 0.33 mm (13

mils) works well when 1-oz copper is plated at the surface of the board while simultaneously plating the barrel of

the via. If the thermal vias are not plugged when the copper plating is performed, then a solder mask material

should be used to cap the vias with a diameter equal to the via diameter of 0.1 mm minimum. This capping

prevents the solder from being wicked through the thermal vias and potentially creating a solder void under the

package. (See Additional References)

18

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