Voltage Regulator Not Providing Enough Current/Voltage

Question

I am attempting to deliver 9V @ 1.1A to a PCB circuit design using an L7809CV voltage regulator which is rated for 1.5A max.

Using a power supply, I can deliver the required current and voltage to my PCB (9V/1.1A) and all is well.

Once I turn up the voltage to ~12-14V (emulating an automotive application) and feed it into the L7809, the regulator will output 9V as expected with NO LOAD on the device. Once the load is applied, the device will only deliver 6.9V/0.1A, well below what is expected.

The device does get hot to the touched during extended use, but I would expect the correct current/voltage to (9V/1.1A) to be delivered when the device is cold. Is this not true?

Datasheet for voltage regulator: http://www.mouser.com/ds/2/389/CD00000444-249828.pdf

Answer 1

The chip die will get hot almost instantly and may go into limit before the heat conducts out to the heatsink tab.

Power dissipated in the device is \$ (V_{IN}-V_{OUT})I = (14-9)\cdot 1.1 = 5.5~W\$.

I don't see any decoupling capacitors on the regulator input and output. You may have a high frequency oscillation going on.

Even with a heat sink, can this regulator handle the the stress we're asking from it?

It can if we can keep the inner chip temperature low enough.

Figure 1. Thermal information from TI datasheet section 6.4.

I've highlighted the two relevant figures here. With no heatsink the chip temperature rise will be 22.9°C/W so you could expect a 125°C temperature rise which puts it just about on the 150°C max.

Figure 2. TO220 heatsink.

This one has a thermal resistance of 13.4°C/W. If we get good thermal conductivity between the tab and the heatsink (use thermal paste) the total resistance will be the series sum of all the thermal resistances = 13.4 + 1 = 14.4°C/W. At 5.5 W the temperature rise will now be 80°C. This is a drop of 45°C and will be a big improvement. Bigger heatsinks will reduce this further.

Answer 2

With reference to the datasheet, the thermal resistance from silicon junction to ambient is 50 °C/W. At 20°C ambient and 14 V from power supply, the silicon junction would rise to (20 + 50*(14-9)*1.1) = 295 °C. The datasheet limit for junction temperature is only 125 °C. The temperature does not need to rise to the full theoretical value of 295 °C before the thermal limit engages. AT 14 V input, the thermal limit would engage much sooner.