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LT1117CM-3.3PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT1117CM-3.3PBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LT1117/LT1117-2.85 LT1117-3.3/LT1117-5 8 1117fd APPLICATIONS INFORMATION Output Voltage The LT1117 develops a 1.25V reference voltage between the output and the adjust terminal (see Figure 2). By placing a resistor between these two terminals, a constant current is caused to flow through R1 and down through R2 to set the overall output voltage. Normally this current is chosen to be the specified minimum load current of 10mA. Because IADJ is very small and constant when compared to the current through R1, it represents a small error and can usually be ignored. For fixed voltage devices R1 and R2 are included in the device. 1117 F02 R2 R1 VOUT VIN VREF IADJ 50μA LT1117 IN ADJ OUT VOUT = VREF 1 + + IADJ R2 R2 — R1 + Figure 2. Basic Adjustable Regulator Load Regulation Because the LT1117 is a 3-terminal device, it is not possible to provide true remote load sensing. Load regulation will be limited by the resistance of the wire connecting the regulator to the load. The data sheet specification for load regulation is measured at the output pin of the device. Negative side sensing is a true Kelvin connection, with the bottom of the output divider returned to the negative side of the load. Although it may not be immediately obvious, best load regulation is obtained when the top of the resistor divider (R1) is returned directly to the output pin of the device, not to the load. This is illustrated in Figure 3. Connected as shown, RP is not multiplied by the divider ratio. If R1 were connected to the load, the effective resistance between the regulator and the load would be: R RR R R PP × + = 21 1 , Parasitic Line Resistance 1117 F03 R2 R1 VIN RP PARASITIC LINE RESISTANCE RL CONNECT R1 TO CASE CONNECT R2 TO LOAD LT1117 IN ADJ OUT Figure 3. Connections for Best Load Regulation For fixed voltage devices the top of R1 is internally Kelvin connected, and the ground pin can be used for negative side sensing. Thermal Considerations LT1117 series regulators have internal thermal limiting circuitry designed to protect the device during overload conditions.Forcontinuousnormalloadconditionshowever, 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 device must also be considered. Heat sinking is accomplished 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. |
Similar Part No. - LT1117CM-3.3PBF |
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Similar Description - LT1117CM-3.3PBF |
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