Datasheet LT3088 (Analog Devices) - 9

LT3088
APPLICATIONS INFORMATION Introduction Programming Linear Regulator Output Voltage
The LT3088 regulator is easy to use and has all the pro- The LT3088 generates a 50µA reference current that flows tection features expected in high performance regulators. out of the SET pin. Connecting a resistor from SET to Included are short-circuit protection, reverse-input protec- ground generates a voltage that becomes the reference tion and safe operating area protection, as well as thermal point for the error amplifier (see Figure 1). The reference shutdown with hysteresis. Safe operating area (SOA) for voltage equals 50µA multiplied by the value of the SET the LT3088 is extended, allowing for use in harsh indus- pin resistor (Ohm’s Law). Any voltage can be generated trial and automotive environments where sudden spikes and there is no minimum output voltage for the regulator. in input voltage lead to high power dissipation. The LT3088 fits well in applications needing multiple rails. IN LT3088 This new architecture adjusts down to zero with a single CIN resistor, handling modern low voltage digital ICs as well 50µA as allowing easy parallel operation and thermal manage- + ment without heat sinks. Adjusting to zero output allows – shutting off the powered circuitry. SET OUT A precision “0” TC 50µA reference current source connects VOUT = 50µA • RSET to the noninverting input of a power operational amplifier. CSET RSET COUT RLOAD The power operational amplifier provides a low impedance buffered output to the voltage on the noninverting input. 3088 F01 A single resistor from the noninverting input to ground
Figure 1. Basic Adjustable Regulator
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 Table 1 lists many common output voltages and the clos- the input power supply is obtainable. est standard 1% resistor values used to generate that output voltage. The benefit of using a true internal current source as the reference, as opposed to a bootstrapped reference in older
Regulation of the output voltage requires a minimum load
regulators, is not so obvious in this architecture. A true
current of 2mA.
For true zero voltage output operation, reference current source allows the regulator to have gain return this 2mA load current to a negative output voltage. and frequency response independent of the impedance on
Table 1. 1% Resistors for Common Output Voltages
the positive input. On older adjustable regulators, such as
VOUT (V) RSET (kΩ)
the LT1086 loop gain changes with output voltage and 1 20 bandwidth changes if the adjustment pin is bypassed to 1.2 24.3 ground. For the LT3088, the loop gain is unchanged with 1.5 30.1 output voltage changes or bypassing. Output regulation 1.8 35.7 is not a fixed percentage of output voltage, but is a fixed 2.5 49.9 fraction of millivolts. Use of a true current source allows 3.3 66.5 all of the gain in the buffer amplifier to provide regulation, 5 100 and none of that gain is needed to amplify up the reference to a higher output voltage. 3088fb For more information www.linear.com/LT3088 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information Typical Performance Characteristics Block Diagram Applications Information Revision History Typical Application Related Parts