LM1084: Adjustable Voltage Regulator Querries

[blackshadow]

I'm designing a system where i needed to create adjustable regulated voltage output. i was using LM1084.

My maximum load requirement is 3A.

My Vinput=15V and set outputs at 9.5V and 5.5V.

My load for 5.5V is 3A while for 9.5V, it is 1A.

During my setup for 5.5V/2.5A, i realized that initially, i was able to maintain the load but after few seconds, the Load current starts to decrease and the LM1084 is overheating. I don't have a heatsink but only a copper with 2x the dimension of LM1084.

Here are my questions:

1. what is the typical Heat Sink setup for my application with 15V input and a load of 3A maximum?

2. what is the maximum Input to Output Voltage Differential?

 

[Diver300]

From the data sheet:-

Maximum Input to Output Voltage Differential
LM1084-ADJ 29V
LM1084-12 18V
LM1084-3.3 27V
LM1084-5.0 25V

With a 15 V input, a 5.5 V output and a 3 A load, the power is

(15 - 5.5) * 3 = 28.5 W

The output section has a thermal resistance of 2.7 °C/W, so it will be 77 °C hotter than the tab. If you are running in air at 30 °C, and the output section at its limit of 150 °C, you have 120 °C difference. 77 °C is internal, leaving 43 °C in the heat sink, so you need a heat sink that is rated at 43/28.5 = 1.5 °C/W

Something like:-
FISCHER ELEKTRONIK|SK 04/100 SA|HEAT SINK, 100MM | Farnell United Kingdom

You really should consider a buck regulator, as it will be far more efficient, and you will need a far smaller heat sink, if any. What is the application?

 

[blackshadow]

From the data sheet:-


With a 15 V input, a 5.5 V output and a 3 A load, the power is

(15 - 5.5) * 3 = 28.5 W

The output section has a thermal resistance of 2.7 °C/W, so it will be 77 °C hotter than the tab. If you are running in air at 30 °C, and the output section at its limit of 150 °C, you have 120 °C difference. 77 °C is internal, leaving 43 °C in the heat sink, so you need a heat sink that is rated at 43/28.5 = 1.5 °C/W

Something like:-
FISCHER ELEKTRONIK|SK 04/100 SA|HEAT SINK, 100MM | Farnell United Kingdom

You really should consider a buck regulator, as it will be far more efficient, and you will need a far smaller heat sink, if any. What is the application?

Thank you so much for your informative answers. Well understood.

This is actually a circuit module of my Power Supply Sequencing and Current Limiter Circuit.
I needed 5 different output voltages whose load can reach 3A.
I'm currently on my prototype stage.

Where did you get the thermal resistance of 2.7 C/W ?

I will come back at you on other circuit modules of my project. Thank you again.

 

[Diver300]

Where did you get the thermal resistance of 2.7 C/W ?

From the data sheet:-
**broken link removed**
The last electrical characteristic is the thermal resistance.