GM1117S
1A LOW DROPOUT
PRECISION VOLTAGE REGULATOR
Heatsink Requirements
When an integrated circuit operates with an appreciable current, its junction temperature is elevated.
It is important to consider its thermal limits in order to achieve acceptable performance and reliability.
This limit is determined by summing the individual parts consisting of a series of temperature rises
from the semiconductor junction to the operating environment.
The heat generated at the device junction flows through the die to the die attach pad, through the
lead frame to the surrounding case material, to the printed circuit board, and eventually to the
ambient environment. Below is a list of variables that may affect the thermal resistance and in turn
the need to a heatsink.
ΘJC, Junction to Case Thermal Resistance ΘCA, Case to Ambient Thermal Resistance
1.
2.
3.
4.
5.
Lead Frame Size & Material
Number of conducting Pins
Die Size
1.
2.
3.
4.
5.
6.
7.
8.
Mounting Pad Size, Material & Location
Placement of Mounting Pad
PCB Size & Material
Die Attach Material
Traces Length & Width
Adjacent Heat Source
Volume of Air
Molding Compound Size & Material
Ambient Temperature
Shape of Mounting Pad
The GM1117S regulators have internal thermal shutdown to protect the device from over heating.
Under all possible operating conditions, the junction temperature of the GM1117S must be within in
the range of 0°C to 125°C. A heatsink may be required depending on the maximum power
dissipation and maximum ambient temperature of the application. To determine if a heatsink is
needed, the power dissipated y the regulator, PD, must be calculated according to the circuit below:
GM1117S
IIN
IIN = IL + IG
PD = (VIN – VOUT) X IL + VIN X IG
VIN VOUT
Gnd
VIN
VOUT
IL
IG
The next parameter which must be calculated is the maximum allowable temperature rise, TR(max):
TR(max) = TJ(max) – TA(max)
Where TJ(max) is the maximum allowable junction temperature (125°C) and TA(max) is the maximum
ambient temperature which will be encountered in the application.
Using the calculated values for TR(max) and PD the maximum allowable value for the junction-to-ambient
thermal resistance (θJA) can be calculated:
ΘJA(max) = TR(max) X PD
If the maximum allowable value for ΘJA is found to be > 136°C/W for SOT223 packages or > 79°C/W
for TO220 package or > 92°C/W for TO252 package, no heat sink is needed since the package
alone will dissipate enough heat to satisfy these requirements. If the calculated value for ΘJA falls
below these limits, a heat sink is required.
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GAMMA [ GAMMA MICROELECTRONICS INC. ]
