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LT3085 Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LT3085 Datasheet(HTML) 10 Page - Linear Integrated Systems |
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10 / 28 page  LT3089 10 3089f For more information www.linear.com/LT3089 APPLICATIONS INFORMATION Introduction The LT3089 regulator is easy to use and has all the pro- tection features expected in high performance regulators. Includedareshort-circuitprotection,reverse-inputprotec- tion and safe operating area protection, as well as thermal shutdown with hysteresis. Safe operating area (SOA) for the LT3089 is extended, allowing for use in harsh indus- trial and automotive environments where sudden spikes in input voltage lead to high power dissipation. The LT3089 fits well in applications needing multiple rails. This new architecture adjusts down to zero with a single resistor, handling modern low voltage digital ICs as well as allowing easy parallel operation and thermal manage- ment without heat sinks. Adjusting to zero output allows shutting off the powered circuitry. Aprecision“0”TC50μAreferencecurrentsourceconnects to the noninverting input of a power operational amplifier. Thepoweroperationalamplifierprovidesalowimpedance buffered output to the voltage on the noninverting input. A single resistor from the noninverting input to ground sets the output voltage. If this resistor is set to 0Ω, zero output voltage results. Therefore, any output voltage can be obtained between zero and the maximum defined by the input power supply is obtainable. The benefit of using a true internal current source as the reference, as opposed to a bootstrapped reference in older regulators, is not so obvious in this architecture. A true reference current source allows the regulator to have gain and frequency response independent of the impedance on the positive input. On older adjustable regulators, such as the LT1086 loop gain changes with output voltage and bandwidth changes if the adjustment pin is bypassed to ground. For the LT3089, the loop gain is unchanged with output voltage changes or bypassing. Output regulation is not a fixed percentage of output voltage, but is a fixed fraction of millivolts. Use of a true current source allows all of the gain in the buffer amplifier to provide regulation, and none of that gain is needed to amplify up the reference to a higher output voltage. The LT3089 has many additional features that facilitate monitoring and control. Current limit is externally pro- grammable via a single resistor between the ILIM pin and OUT. Shorting this resistor out disables all output current to the load, only bias currents remain. The IMON pin produces a current output proportional to load current. For every 100mA of load current, the IMON pin sources 20µA of current. This can be sensed using an external resistor to monitor load requirements and detect potentialfaults.TheIMONpincanoperateatvoltagesabove OUT, so it operates even during a short-circuit condition. One additional monitoring function is the TEMP pin, a cur- rentsourcethatisproportionaltoaveragedietemperature. For die temperatures above 0°C, the TEMP pin sources a current equal to 1µA/°C. This pin operates normally during output short-circuit conditions. Programming Linear Regulator Output Voltage The LT3089 generates a 50μA reference current that flows out of the SET pin. Connecting a resistor from SET to ground generates a voltage that becomes the reference point for the error amplifier (see Figure 1). The reference voltage equals 50µA multiplied by the value of the SET pin resistor. Any voltage can be generated and there is no minimum output voltage for the regulator. 3089 F01 IN SET OUT + – LT3089 50µA RLOAD CSET RSET CIN VOUT = 50µA • RSET COUT Figure 1. Basic Adjustable Regulator |
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