LT1764EQ View Datasheet(PDF) - Linear Technology
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LT1764EQ Datasheet PDF : 20 Pages
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LT1764 Series
APPLICATIONS INFORMATION
Thermal Considerations
The power handling capability of the device is limited
by the maximum rated junction temperature (125°C).
The power dissipated by the device is made up of two
components:
1. Output current multiplied by the input/output voltage
differential: (IOUT)(VIN – VOUT), and
2. GND pin current multiplied by the input voltage:
(IGND)(VIN).
The GND pin current can be found using the GND Pin
Current curves in the Typical Performance Characteris-
tics. Power dissipation will be equal to the sum of the two
components listed above.
The LT1764 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. For continuous normal conditions, the maximum
junction temperature rating of 125°C must not be
exceeded. It is important to give careful consideration to
all sources of thermal resistance from junction to ambient.
Additional heat sources mounted nearby must also be
considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Surface mount heatsinks and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 1/16" FR-4 board with one ounce
copper.
Table 1. Q Package, 5-Lead DD
COPPER AREA
TOPSIDE* BACKSIDE BOARD AREA
2500mm2 2500mm2 2500mm2
1000mm2 2500mm2 2500mm2
125mm2 2500mm2 2500mm2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
23°C/ W
25°C/ W
33°C/ W
* Device is mounted on topside
Table 2. FE Package, 16-Lead TSSOP
COPPER AREA
TOPSIDE* BACKSIDE
2500mm2 2500mm2
1000mm2 2500mm2
225mm2 2500mm2
100mm2 2500mm2
BOARD AREA
2500mm2
2500mm2
2500mm2
2500mm2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
38°C/ W
43°C/ W
48°C/ W
60°C/ W
* Device is mounted on topside
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 2.5°C/W
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4V to 6V, an output current range of 0mA to
500mA and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX))
where,
IOUT(MAX) = 500mA
VIN(MAX) = 6V
IGND at (IOUT = 500mA, VIN = 6V) = 10mA
So,
P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W
Using a DD package, the thermal resistance will be in the
range of 23°C/W to 33°C/W depending on the copper
area. So the junction temperature rise above ambient will
be approximately equal to:
1.41W(28°C/W) = 39.5°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
TJMAX = 50°C + 39.5°C = 89.5°C
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