US1260
In this application, the US1260 is used on the VGA card
to convert 3.3V supply to 2.7V to power the Intel 740
chip rather than the conventional LDO which due to its
1.5V minimum dropout spec must use the 5V supply to
achieve the same result. The difference is a substantial
decrease in the power dissipation as shown below.
The maximum power dissipation of 740 chip is 5.8W,
which at 2.7V results in Io=5.8/2.7=2.15A
Example # 2 :
Assuming the following specifications :
VIN = 3.3V
VOUT 2 = 1.5 V
VOUT 1 = 2.5 V
IOUT 2 MAX = 1.5A
IOUT 1 MAX = 0.2 A
TA = 35°C
a) Using standard LDO, the power dissipated in the de-
vice is;
Pd=(Vin - Vo)*Io=(5-2.7)*2.15=4.94W
Using surface mount TO263 package with 25° C/Wjunc-
tion to air thermal resistance results in:
Tj=Pd*qja + Ta=(4.94)(25) + 25=148 ° C
This is very close to the thermal shutdown of the IC.
b) Using 1260, the power dissipated in the device is dras-
tically reduced by using 3.3V supply instead of 5V
Pd=(Vin - Vo)*Io=(3.3-2.7)*2.15=1.3W
Using surface mount TO263 package with 25° C/Wjunc-
tion to air thermal resistance results in:
The steps for selecting a proper heat sink to keep the
junction temperature below 135°C is given as :
1) Calculate the maximum power dissipation using :
PD = IOUT1 ´ VIN - VOUT1 + IOUT2 ´ VIN - VOUT2
PD = 0.2 ´ 3.3 - 2.5 +1.5 ´ 3.3 - 1.5 = 2.86 W
(
)
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)
2) Assuming a TO-263 surface mount package, the junc-
tion to ambient thermal resistance of the package is:
Tj=Pd*qja + Ta=(1.3)(25) + 25=57 ° C
A reduction of 91° C in junction temperature.
qJA = 30°C / W for 1" square pad area
3) The maximum junction temperature of the device is
calculated using the equation below :
TJ = TA + PD ´ qJA
TJ = 35 + 2.86 ´ 30 =121 °C
Since this is lower than our selected 135°C maximum
junction temperature (150°C is the thermal shutdown of
the device), TO-263 package is a suitable package for
our application.
Layout Consideration
The US1260 like all other high speed linear regulators
need to be properly laid out to insure stable operation.
The most important component is the output capaci-
tor, which needs to be placed close to the output
pin and connected to this pin using a plane con-
nection with a low inductance path.
US1260 in Ultra LDO, Single Output Application
The US1260 can also be used in single supply applica-
tions where the difference between input and output is
much lower than the standard 1.5V dropout that is ob-
tainable with standard LDO devices. The schematic in
figure 6 shows the application of the US1260 in a single
supply with the second LDO being disabled.
Rev. 1.9
3/22/99
3-6
UNISEM [ UNISEM ]
