LT1117_02 Datasheet, PDF (7/12 Pages) Linear Technology – 800mA Low Dropout Positive Regulators Adjustable and Fixed 2.85V, 3.3V, 5V

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APPLICATIO HI TS
Thermal Considerations
LT1117 series regulators have internal thermal limiting
circuitry designed to protect the device during overload
conditions. For continuous normal load conditions how-
ever, 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. For the
SOT-223 package, which is designed to be surface
mounted, additional heat sources mounted near the de-
vice must also be considered. Heat sinking is accom-
plished using the heat spreading capability of the PC board
and its copper traces. The thermal resistance of the
LT1117 is 15°C/W from the junction to the tab. Thermal
resistances from tab to ambient can be as low as 30°C/W.
The total thermal resistance from junction to ambient can
be as low as 45°C/W. This requires a reasonable sized PC
board with at least one layer of copper to spread the heat
across the board and couple it into the surrounding air.
Experiments have shown that the heat spreading copper
layer does not need to be electrically connected to the tab
of the device. The PC material can be very effective at
transmitting heat between the pad area, attached to the tab
of the device, and a ground plane layer either inside or on
the opposite side of the board. Although the actual thermal
resistance of the PC material is high, the Length/Area ratio
of the thermal resistor between layers is small. The data in
Table 1 was taken using 1/16" FR-4 board with 1oz. copper
foil. It can be used as a rough guideline in estimating
thermal resistance.
Table 1.
COPPER AREA
TOPSIDE* BACKSIDE
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 Sq. mm 2500 Sq. mm 2500 Sq. mm
45°C/W
1000 Sq. mm 2500 Sq. mm 2500 Sq. mm
45°C/W
225 Sq. mm 2500 Sq. mm 2500 Sq. mm
53°C/W
100 Sq. mm 2500 Sq. mm 2500 Sq. mm
59°C/W
1000 Sq. mm 1000 Sq. mm 1000 Sq. mm
52°C/W
1000 Sq. mm 0
1000 Sq. mm
55°C/W
* Tab of device attached to topside copper
LT1117/LT1117-2.85
LT1117-3.3/LT1117-5
The thermal resistance for each application will be affected
by thermal interactions with other components on the
board. Some experimentation will be necessary to deter-
mine the actual value.
The power dissipation of the LT1117 is equal to:
P_D = ( V_IN – V_OUT )( I_OUT )
Maximum junction temperature will be equal to:
T_J = T_A(MAX) + P_D(Thermal Resistance (junction-to-
ambient))
Maximum junction temperature must not exceed 125°C.
Ripple Rejection
The curves for Ripple Rejection were generated using an
adjustable device with the adjust pin bypassed. These
curves will hold true for all values of output voltage. For
proper bypassing, and ripple rejection approaching the
values shown, the impedance of the adjust pin capacitor,
at the ripple frequency, should be < R1. R1 is normally in
the range of 100Ω to 200Ω. The size of the required adjust
pin capacitor is a function of the input ripple frequency. At
120Hz, with R1 = 100Ω, the adjust pin capacitor should be
> 13µF. At 10kHz only 0.16µF is needed.
For fixed voltage devices, and adjustable devices without
an adjust pin capacitor, the output ripple will increase as
the ratio of the output voltage to the reference voltage
(V_OUT/ V_REF). For example, with the output voltage equal to
5V, the output ripple will be increased by the ratio of
5V/1.25V. It will increase by a factor of four. Ripple
rejection will be degraded by 12dB from the value shown
on the curve.
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