Datasheet

P
T
+
ǒ
T
J
* T
A
Ǔ
q
JA
(Watts)
UDG-02139
I
ANTI-CROSS
CONDUCTION
SYNCHRONOUS
RECTIFIER ON
BODY DIODE
CONDUCTION
BODY DIODE
CONDUCTION
HIGH SIDE ON
I
D1
I
D2
I
O
SW
0
}
d 1-d
P
SW(fsw)
+
ǒ
V
IN
I
OUT
t
SW
Ǔ
f
SW
(Watts)
I
RMS
+ I
OUT
d
Ǹ
ǒ
A
RMS
Ǔ
P
COND
+
ǒ
I
RMS
Ǔ
2
R
DS(on)
ǒ
1 ) TC
R
ƪ
T
J
* 25
O
C
ƫ
Ǔ
(Watts)
TPS40054
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SLUS593H DECEMBER 2003REVISED JULY 2012
HIGH-SIDE MOSFET POWER DISSIPATION
The power dissipated in the external high-side MOSFET is comprised of conduction and switching losses. The
conduction losses are a function of the I
RMS
current through the MOSFET and the R
DS(on)
of the MOSFET. The
high-side MOSFET conduction losses are defined by Equation 33.
where
TC
R
is the temperature coefficient of the MOSFET R
DS(on)
(33)
The TC
R
varies depending on MOSFET technology and manufacturer, but typically ranges between 3500
ppm/°C and 7000 ppm/°C.
The I
RMS
current for the high-side MOSFET is described in Equation 34.
(34)
The switching losses for the high-side MOSFET are descibed in Equation 35.
where
I
O
is the DC output current
t
SW
is the switching rise time, typically < 20 ns
f
SW
is the switching frequency (35)
Typical switching waveforms are shown in Figure 14.
Figure 14. Inductor Current and SW Node Waveforms
The maximum allowable power dissipation in the MOSFET is determined by Equation 36.
where
P
T
= P
COND
+ P
SW(fsw) (W)
θ
JA
is the package thermal impedance (36)
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