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LT1117CM-5 Datasheet(PDF) 7 Page - Linear Technology |
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LT1117CM-5 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1117/LT1117-2.85 LT1117-3.3/LT1117-5 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 THERMAL RESISTANCE TOPSIDE* BACKSIDE 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 APPLICATI HI TS 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: PD = ( VIN – VOUT )( IOUT ) Maximum junction temperature will be equal to: TJ =TA(MAX) + PD(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 100 Ω-200Ω. The size of the required adjust pin capacitor is a function of the input ripple frequency. At 120Hz, with R1=100 Ω, the adjust pin capacitor should be > 13 µF. 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 (VOUT/VREF). 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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