Datasheet LT3089 (Analog Devices) - 10

LT3089
APPLICATIONS INFORMATION Introduction
The LT3089 has many additional features that facilitate The LT3089 regulator is easy to use and has all the pro- monitoring and control. Current limit is externally pro- tection features expected in high performance regulators. grammable via a single resistor between the ILIM pin and Included are short-circuit protection, reverse-input protec- OUT. Shorting this resistor out disables all output current tion and safe operating area protection, as well as thermal to the load, only bias currents remain. shutdown with hysteresis. Safe operating area (SOA) for The IMON pin produces a current output proportional to the LT3089 is extended, allowing for use in harsh indus- load current. For every 100mA of load current, the IMON trial and automotive environments where sudden spikes pin sources 20µA of current. This can be sensed using an in input voltage lead to high power dissipation. external resistor to monitor load requirements and detect The LT3089 fits well in applications needing multiple rails. potential faults. The IMON pin can operate at voltages above This new architecture adjusts down to zero with a single OUT, so it operates even during a short-circuit condition. resistor, handling modern low voltage digital ICs as well One additional monitoring function is the TEMP pin, a cur- as allowing easy parallel operation and thermal manage- rent source that is proportional to average die temperature. ment without heat sinks. Adjusting to zero output allows For die temperatures above 0°C, the TEMP pin sources a shutting off the powered circuitry. current equal to 1µA/°C. This pin operates normally during A precision “0” TC 50μA reference current source connects output short-circuit conditions. to the noninverting input of a power operational amplifier.
Programming Linear Regulator Output Voltage
The power operational amplifier provides a low impedance buffered output to the voltage on the noninverting input. The LT3089 generates a 50μA reference current that flows A single resistor from the noninverting input to ground out of the SET pin. Connecting a resistor from SET to sets the output voltage. If this resistor is set to 0Ω, zero ground generates a voltage that becomes the reference output voltage results. Therefore, any output voltage can point for the error amplifier (see Figure 1). The reference be obtained between zero and the maximum defined by voltage equals 50µA multiplied by the value of the SET the input power supply is obtainable. pin resistor. Any voltage can be generated and there is no minimum output voltage for the regulator. 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 IN LT3089 reference current source allows the regulator to have gain CIN and frequency response independent of the impedance on 50µA 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 SET OUT ground. For the LT3089, the loop gain is unchanged with VOUT = 50µA • RSET output voltage changes or bypassing. Output regulation CSET RSET COUT RLOAD is not a fixed percentage of output voltage, but is a fixed fraction of millivolts. Use of a true current source allows 3089 F01 all of the gain in the buffer amplifier to provide regulation,
Figure 1. Basic Adjustable Regulator
and none of that gain is needed to amplify up the reference to a higher output voltage. 3089f 10 For more information www.linear.com/LT3089 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts