Datasheet LT1117, LT1117-2.85, LT1117-3.3, LT1117-5 (Analog Devices) - 8

LT1117/LT1117-2.85 LT1117-3.3/LT1117-5
APPLICATIONS INFORMATION Output Voltage
RP PARASITIC The LT1117 develops a 1.25V reference voltage between the LT1117 LINE RESISTANCE V IN OUT output and the adjust terminal (see Figure 2). By placing a IN ADJ resistor between these two terminals, a constant current is caused to flow through R1 and down through R2 to R1 RL CONNECT set the overall output voltage. Normally this current is R1 TO CASE chosen to be the specified minimum load current of 10mA. R2 Because IADJ is very small and constant when compared to the current through R1, it represents a small error and 1117 F03 can usually be ignored. For fixed voltage devices R1 and CONNECT R2 TO LOAD R2 are included in the device.
Figure 3. Connections for Best Load Regulation
LT1117 V IN OUT V IN OUT + For fixed voltage devices the top of R1 is internally Kelvin ADJ VREF R1 connected, and the ground pin can be used for negative IADJ 50μA side sensing. R2 R2 VOUT = VREF 1 + + I — ADJ R2 R1
Thermal Considerations
1117 F02 LT1117 series regulators have internal thermal limiting
Figure 2. Basic Adjustable Regulator
circuitry designed to protect the device during overload conditions. For continuous normal load conditions however,
Load Regulation
the maximum junction temperature rating of 125°C must not be exceeded. Because the LT1117 is a 3-terminal device, it is not possible to provide true remote load sensing. Load regulation will be It is important to give careful consideration to all sources limited by the resistance of the wire connecting the regulator of thermal resistance from junction to ambient. For to the load. The data sheet specification for load regulation the SOT-223 package, which is designed to be surface is measured at the output pin of the device. Negative side mounted, additional heat sources mounted near the device sensing is a true Kelvin connection, with the bottom of the must also be considered. Heat sinking is accomplished output divider returned to the negative side of the load. using the heat spreading capability of the PC board and Although it may not be immediately obvious, best load its copper traces. The thermal resistance of the LT1117 is regulation is obtained when the top of the resistor divider 15°C/W from the junction to the tab. Thermal resistances (R1) is returned directly to the output pin of the device, from tab to ambient can be as low as 30°C/W. The total not to the load. This is illustrated in Figure 3. Connected thermal resistance from junction to ambient can be as low as shown, RP is not multiplied by the divider ratio. If R1 as 45°C/W. This requires a reasonable sized PC board with were connected to the load, the effective resistance between at least one layer of copper to spread the heat across the the regulator and the load would be: board and couple it into the surrounding air. R2 R + 1 R × R P , P =Parasitic Line Resistance R1 1117fd 8