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LT1117IST Datasheet(PDF) 7 Page - Linear Technology |
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LT1117IST Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LT1117/LT1117-2.85 LT1117-3.3/LT1117-5 7 1117fd APPLICATIONS INFORMATION The LT1117 family of 3-terminal regulators are easy to use. They are protected against short circuit and thermal overloads. Thermal protection circuitry will shut down the regulator should the junction temperature exceed 165°C at the sense point. These regulators are pin compatible with older 3-terminal adjustable regulators, offer lower dropout voltage and more precise reference tolerance. Reference stability over temperature is improved over older types of regulators. Stability The LT1117 family of regulators requires an output capacitor as part of the device frequency compensation. A minimum of 10μF of tantalum or 50μF of aluminum electrolytic is required. The ESR of the output capacitor should be less than 0.5Ω. Surface mount tantalum capacitors, which have very low ESR, are available from several manufacturers. When using the LT1117 adjustable device, the adjust terminal can be bypassed to improve ripple rejection. When the adjust terminal is bypassed, the required value of the output capacitor increases. The device will require an output capacitor of 22μF tantalum or 150μF aluminum electrolytic when the adjust pin is bypassed. Normally, capacitor values on the order of 100μF are used in the output of many regulators to ensure good load transient response with large load current changes. Output capacitance can be increased without limit and larger values of output capacitance further improve stability and transient response. Protection Diodes In normal operation, the LT1117 family does not need any protection diodes. Older adjustable regulators required protection diodes between the adjust pin and the output and between the output and input to prevent over stressing the die. The internal current paths on the LT1117 adjust pin are limited by internal resistors. Therefore, even with capacitors on the adjust pin, no protection diode is needed to ensure device safety under short-circuit conditions. The adjust pin can be driven, on a transient basis, ± 25V with respect to the output without any device degradation. Diodes between input and output are not usually needed. The internal diode between the output and input pins of the device can withstand microsecond surge currents of 10A to 20A. Normal power supply cycling can not generate currents of this magnitude. Only with extremely large output capacitors, such as 1000μF and larger, and with the input pin instantaneously shorted to ground can damage occur. A crowbar circuit at the input of the LT1117 in combination with a large output capacitor could generate currents large enough to cause damage. In this case a diode from output to input is recommended, as shown in Figure 1. 1117 F01 + + D1 1N4002 (OPTIONAL) CADJ 10μF R2 R1 COUT 150μF VOUT VIN LT1117 IN ADJ OUT Figure 1 |
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