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Power Dissipation and Junction Temperature
P
D
(
max
)
+
T
J
max *T
A
R
QJA
(1)
P
D
+
ǒ
V
IN
*V
OUT
Ǔ
I
OUT
(2)
Programming the TPS79301 Adjustable LDO Regulator
V
OUT
+ V
REF
ǒ
1 )
R
1
R
2
Ǔ
(3)
R
1
+
ǒ
V
OUT
V
REF
* 1
Ǔ
R
2
(4)
C
1
+
(3 x 10
*7
) x (R
1
) R
2
)
(R
1
x R
2
)
(5)
TPS793xx
SLVS348K – JULY 2001 – REVISED OCTOBER 2007
Specified regulator operation is assured to a junction temperature of +125 ° C; the maximum junction temperature
should be restricted to +125 ° C under normal operating conditions. This restriction limits the power dissipation the
regulator can handle in any given application. To ensure the junction temperature is within acceptable limits,
calculate the maximum allowable dissipation, P
D(max)
, and the actual dissipation, P
D
, which must be less than or
equal to P
D(max)
.
The maximum power dissipation limit is determined using Equation 1 :
Where:
• T
J
max is the maximum allowable junction temperature.
• R
θ JA
is the thermal resistance junction-to-ambient for the package (see the Dissipation Ratings Table ).
• T
A
is the ambient temperature.
The regulator dissipation is calculated using Equation 2 :
Power dissipation resulting from quiescent current is negligible. Excessive power dissipation triggers the thermal
protection circuit.
The output voltage of the TPS79301 adjustable regulator is programmed using an external resistor divider as
shown in Figure 23 . The output voltage is calculated using Equation 3 :
Where:
• V
REF
= 1.2246V typ (the internal reference voltage)
Resistors R
1
and R
2
should be chosen for approximately 50 μ A divider current. Lower value resistors can be used
for improved noise performance, but the solution consumes more power. Higher resistor values should be
avoided as leakage current into/out of FB across R
1
/R
2
creates an offset voltage that artificially
increases/decreases the feedback voltage and thus erroneously decreases/increases V
OUT
. The recommended
design procedure is to choose R
2
= 30.1k Ω to set the divider current at 50 μ A, C
1
= 15pF for stability, and then
calculate R
1
using Equation 4 :
In order to improve the stability of the adjustable version, it is suggested that a small compensation capacitor be
placed between OUT and FB. For voltages less than 1.8V, the value of this capacitor should be 100pF. For
voltages greater than 1.8V, the approximate value of this capacitor can be calculated as shown in Equation 5 :
The suggested value of this capacitor for several resistor ratios is shown in the table below. If this capacitor is
not used (such as in a unity-gain configuration) or if an output voltage less than 1.8V is chosen, then the
minimum recommended output capacitor is 4.7 μ F instead of 2.2 μ F.
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