SLMA002 datasheet(29/55 Pages) TI1

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(

)

(

)

OUTPUTx

RMS(outputx )

OUTPUTx

(16)

D(cond)

RMS(outputx)

DS(on)

(17)

D(SW) =

(V )

(18)

D(cond)output1

D(SW )output1

D(cond)output2

D(SW )output2

(19)

(

)

TH(pkg)

TH(pad amb)

´ q

+ q

(20)

Power Derating

TPS54283,, TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

where

• D is the duty cycle

• I

OUTPUTx is the DC output current

• ΔI

OUTPUTx is the peak ripple current in the inductor for Outputx

Notice the impact of the operating duty cycle on the result.

Multiplying the result by the RDS(on) of the MOSFET gives the conduction loss.

The switching loss is approximated by:

where

• where C

J is the parallel capacitance of the rectifier diode and snubber (if any)

• f

S is the switching frequency

The total power dissipation is found by summing the power loss for both MOSFETs plus the loss in the internal

regulator.

The temperature rise of the device junction depends on the thermal impedance from junction to the mounting pad

(See the Package Dissipation Ratings table), plus the thermal impedance from the thermal pad to ambient. The

thermal impedance from the thermal pad to ambient depends on the PCB layout (PowerPAD interface to the

PCB, the exposed pad area) and airflow (if any). See the PCB Layout Guidelines, Additional References section.

The operating junction temperature is shown in Equation 20.

The TPS5428x delivers full current at ambient temperatures up to +85

°C if the thermal impedance from the

thermal pad to ambient is sufficiently low enough to maintain the junction temperature below the thermal

shutdown 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 35 illustrates the power derating for elevated

ambient temperature under various airflow conditions. Note that these curves assume that the PowerPAD is

properly soldered to the recommended thermal pad. (See the References section for further information.)

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Product Folder Link(s): TPS54283 TPS54286

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