LM1084 Positive Regulators : Price, Application and Circuit

The LM1084 is a low dropout voltage positive regulators. The maximum voltage drop by LM1084 regulator at 5A of load current is 1.5V.  It has the same pin-out as National Semiconductor's industry standard LM317.

The LM1084 is available in an adjustable version, which can set the regulated output voltage just by using two external resistors  It is also available in three fixed voltages: 3.3V, 5.0V and 12.0V. The fixed versions intergrate the adjust resistors.

The LM1084 circuit comes with a zener trimmed bandgap reference  for current limiting and thermal shutdown purpose.

The LM1084 series is available in TO-220 and TO-263 packages. Refer to the LM1085 for the 3A version, and the LM1086 for the 1.5A version.

Figure 2. Pin Descirption

Figure 3. Symbol

Figure 4. Footprint

Figure 5. 3D Model

◆ Three variants available: 3.3V, 5.0V and Adjustable Versions

◆ Current Limiting and Thermal Protection

◆ 5A of Output Current

◆ Industrial Temperature Range −40˚C to 125˚C

◆ Line Regulation 0.015% (typical)

◆ Load Regulation 0.1% (typical)

◆ Available Package: TO-263 and TO-220

Figure 6. Simplified Schematic

Texas Instruments LM1084IS-3.3 technical specifications, attributes, parameters and parts with similar specifications to Texas Instruments LM1084IS-3.3.

Figure 9. Alternatives

Using a LM1084 is as easy as using LM317 regulator because it have the same pin configuration. Also, LM1084 comes with three variants in which two types have fixed regulated output and one is adjustable regulator. As you can see in the below diagram, this LM1084 is in adjustable mode and you can vary the regulated output voltage by changing the value of resistor R1 and R2.

Figure 10.

In fixed type regulator of LM1084, the ADJ pin is replaced by GND (Ground) pin. Check the connections for fixed type regulator below: 

Figure 11. 

Figure 12. Application Circuit

■ Post Regulator for Switching DC/DC Conveter

■ High Efficiency Linear Regulators

■ Battery Charger

Figure 13. Battery Backup Regulated Supply

A regulated battery backup supply can be generated by using two fixed output voltage versions of the part. The top regulator supplies the Line voltage during normal operation, however when the input is not available, the second regulator derives power from the battery backup and regulates it to 5 V based on the LM1084-5.0. The diodes prevent the rails from back feeding into the supply and batteries.

Figure 14. Automatic Light control

A common street light control or automatic light control circuit is designed in Figure 25 based on the LM1084- ADJ. The photo transistor conducts in the presence of light and grounds the ADJ pin preventing the lamp from turning on. However, in the absence of light, the LM1084 regulates the voltage to 1.25V between OUT and ADJ, ensuring the lamp remains on.

Figure 15. Ripple Rejection Enhancement

A very simple ripple rejection circuit is shown in Figure 24 using the LM1084-ADJ. The capacitor C1 smooths out the ripple on the output by cleaning up the feedback path and preventing excess noise from feeding back into the regulator. Please remember XC1 should be approximately equal to R1 at the ripple frequency.

Figure 16. Generating Negative Supply voltage

A quick inverting output rail or negative output rail is shown in Figure using the LM1084 fixed output part. By tying the output to GND, the GND node is at a relatively more negative potential than the output. This is then interfaced to the negative application such as an operational amplifier or any other rail needing negative voltage.

Figure 17. Remote Sensing

Remote sensing is a method of compensating the output voltage to a very precise degree by sensing the output and feeding it back through the feedback. The circuit implementing this is shown in Figure 27 using the LM1084- ADJ. The output of the regulator is fed into a voltage follower to avoid any loading effects and the output of the op-amp is injected into the top of the feedback resistor network. This has the effect of modulating the voltage to a precise degree without additional loading on the output.

Figure 18. Package TO-263

Texas Instruments (TI) is an American technology company headquartered in Dallas, Texas, which designs and manufactures semiconductors and various integrated circuits, and sells them to electronic designers and manufacturers worldwide. It is one of the top 10 semiconductor companies in global sales. The company focuses on the development of analog chips and embedded processors, which account for more than 80% of its revenue. TI also produces TI digital light processing technology and educational technology products, including calculators, microcontrollers and multi-core processors. As of 2016, the company has 45,000 patents worldwide.

Download datasheets and manufacturer documentation for Texas Instruments LM1084IS-3.3.