Datasheet

Board thickness 62 mils
Boardsize
Boardmaterial FR4
Coppertrace/heatsink 1oz
Exposedpadmounting 63/67tin/leadsolder
HeatsinkArea
1 oz Copper
3.2in 3.2in´
P =
D(max)
T T-
Jmax A
R
qJA(system)
P =
D(total)
V V-
IN1 OUT1
( (
V V-
IN2 OUT2
( (
´ I +V ´ +
OUT1 IN1
I
2
Q
I
2
Q
´ ´I +V
OUT2 IN2
P =
D(total)
V V-
IN1 OUT1
( (
V V-
IN2 OUT2
( (
´ I +
OUT1
´ I
OUT2
P =
D(max)
T T-
Jmax A
R
qJA(system)
=
+125 C 55 C-° °
+50°C/W
=1.4W
P =
D(total)
V V-
IN1 OUT1
( (
V V-
IN2 OUT2
( (
´ I +
OUT1
´ I
OUT2
=(5.0 3.3) 0.5+(2.8 1.8) 0.8=1.25W- -
´ ´
TPS70445, TPS70448
TPS70451, TPS70458
TPS70402
www.ti.com SLVS307F –SEPTEMBER 2000–REVISED APRIL 2010
Figure 36 is an example of a thermally-enhanced PWB layout for use with the new PWP package. This board
configuration was used in the thermal experiments that generated the power ratings shown in Figure 34 and
Figure 35. As discussed earlier, copper has been added on the PWB to conduct heat away from the device. R
qJA
for this assembly is illustrated in Figure 34 as a function of heat-sink area. A family of curves is included to
illustrate the effect of airflow introduced into the system.
Figure 36. PWB Layout (Including Copper Heatsink Area) for Thermally-Enhanced PWP Package
From Figure 34, R
qJA
for a PWB assembly can be determined and used to calculate the maximum
power-dissipation limit for the component/PWB assembly, with the equation:
where:
• T
Jmax
is the maximum specified junction temperature (+150°C absolute maximum limit, +125°C recommended
operating limit) and T
A
is the ambient temperature. (1)
P
D(max)
should then be applied to the internal power dissipated by the TPS704xx regulator. The equation for
calculating total internal power dissipation of the TPS704xx is:
(2)
Since the quiescent current of the TPS704xx is very low, the second term is negligible, further simplifying the
equation to:
(3)
For the case where T
A
= +55°C, airflow = 200 ft/min, copper heat-sink area = 4 cm
2
, the maximum
power-dissipation limit can be calculated. First, from Figure 34, we find the system R
qJA
is +50°C/W; therefore,
the maximum power-dissipation limit is:
(4)
If the system implements a TPS704xx regulator, where V
IN1
= 5.0V, V
IN2
= 2.8 V, I
OUT1
= 500 mA, and I
OUT2
=
800 mA, the internal power dissipation is:
(5)
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