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P +
ƪ
ǒ
P
IN
V
DD
Ǔ
2
R
DSON
ƫ
)
ƪ
V
AUX
Q
G
ƒ
ƫ
)
ƪ
V
AUX
I
INTERNAL
ƫ
)
ƪǒ
V
AUX
* V
BIAS
Ǔ
I
OPTO
ƫ
P +
ƪ
ǒ
P
IN
V
DD
Ǔ
2
R
DSON
ƫ
)
ƪ
V
DD
Q
G
ƒ
ƫ
)
ƪ
V
DD
I
INTERNAL
ƫ
)
ƪǒ
V
DD
* V
BIAS
Ǔ
I
OPTO
ƫ
[ ] [ ]
IN
AUX G AUX INTERNAL
DD
2
P
P = x R + V x Q x + V x I
DSON
V
f
é ù
æ ö
ê ú
ç ÷
ê ú
è ø
ë û
P +
ƪ
ǒ
P
IN
V
DD
Ǔ
2
R
DSON
ƫ
)
ƪ
V
DD
Q
G
ƒ
ƫ
)
ƪ
V
DD
I
INTERNAL
ƫ
V
DD
+
ǒ
V
PSE
* 2 V
D
Ǔ
)
ǒ
V
PSE
* 2V
D
Ǔ
2
* 4 P
IN
R
LOOP
Ǹ
2
where V
D
is an input diode drop (0.75 V), R
LOOP
is
TPS23750
TPS23770
SLVS590B – JULY 2005 – REVISED FEBRUARY 2008
Four major contributors to internal heat dissipation are the internal (hotswap) MOSFET I
2
R, the gate drive load,
the internal bias power, and the optocoupler load. These four contributors form a template for the loss
approximations of the common configurations shown in Table 3 . The total loss under low, medium, and high
input voltages should be checked. I
2
R dominant designs fare worse at low input voltage, while an AUX-load loss
driven design may be worse at high input voltage.
Table 3. Power Dissipation
INTERNAL DISSIPATION MODEL
Isolated converter with
AUX override
Isolated converter
without AUX override
Nonisolated converter
with AUX override
Nonisolated converter
without AUX override
• I
INTERNAL
represents the operational current from the Electrical Characteristics table. Approximate that all the
current is due to the controller.
• P
IN
is the converter input power (P
OUT
/efficiency), not the power at the PI.
• f is the converter switching frequency, and I
OPTO
is the optocoupler bias current.
• V
DD
can be calculated as
0 Ω to 20 Ω plus the MOSFET resistance, and P
IN
as above. V
PSE
is 44 V for cases where the MOSFET loss
dominates.
• R
DSON
is the internal pass MOSFET resistor, 0.6 Ω typical and 1 Ω maximum.
• The loss should be checked at different PI voltages to determine the worst case, especially where AUX
override is not used.
A simple thermal model for the junction temperature is:
T
J
= T
A
+ (P × θ
JA
)
where T
J
is the junction temperature, T
A
is the ambient temperature, P is the total power dissipated in the
TPS23750, and θ
JA
is the thermal resistance from the junction to ambient. θ
JA
includes heat paths from the die
through the package directly to air, through the leads to the circuit board, from the PowerPAD to the circuit
board, and from the circuit board to air. The long-term steady-state junction temperature should be kept below
125 ° C.
Consider the case of a buck converter to demonstrate a thermal design:
• The output is 5 V at 1.5 A, with estimated efficiency of 85%.
• The chosen switching MOSFET has a Q
G
of 10 nC, and a switching frequency of 200 kHz.
• Use the worst-case internal MOSFET resistance of 1 Ω .
• Assume an ambient air temperature of 65 ° C.
• Assume a thermal resistance of 45 ° C/W, because the PowerPAD has been connected to a large copper fill,
but is not exactly as shown in SLMA002 .
• Use the worst-case combinations of input voltage and loop resistance per Table 4 .
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