TPS54383, TPS54386
www.ti.com
SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
2
)
æ
ç
ö
÷
æ
ç
ç
è
ö
÷
÷
ø
DI
(
2
)
OUTPUTx
I
=
D ´
I
(
+
RMS(outputx)
OUTPUTx
ç
ç
è
÷
÷
ø
12
(16)
where
•
•
•
D is the duty cycle
IOUTPUTx is the dc output current
ΔIOUTPUTx 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.
2
P
= I
´R
DS(on)
D(cond)
RMS(outputx)
(17)
(18)
The switching loss is approximated by:
2
(VIN) ´ CJ ´ fS
PD(SW)
=
2
where
•
•
where CJ is the prallel capacitance of the rectifier diode and snubber (if any)
fS 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.
P = P
+ P
+ P
+ P
+ V ´Iq
D
D(cond)output1
D(SW)output1
D(cond)output2
D(SW)output2 IN
(19)
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.
T = T + P ´ q + qTH(pad-amb)
TH(pkg)
(
)
J
A
D
(20)
Power Derating
The TPS5438x delivers full current at ambient temperatures up to +85°C if the thermal impedance from the
thermal pad maintains 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 36 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.)
Copyright © 2007, Texas Instruments Incorporated
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