Datasheet

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( )
( )
2
2
OUTPUTx
RMS(outputx) OUTPUTx
I
I D I
12
æ ö
æ ö
D
ç ÷
ç ÷
= ´ +
ç ÷
ç ÷
ç ÷
è ø
è ø
(16)
2
D(cond) RMS(outputx) DS(on)
P I R= ´
(17)
2
P
D(SW)
=
(V ) C f
IN J S
2
´ ´
(18)
D D(cond)output1 D(SW )output1 D(cond)output2 D(SW )output2 IN
P P P P P V Iq= + + + + ´
(19)
( )
J A D TH(pkg) TH(pad amb)
T T P
-
= + ´ 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
• Δ I
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 R
DS(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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