LDO6C voltage regulator

[Packhorse]

I am building a Maglite based dive light using a SST-90 and a LDO6C voltage regulator wired up with a thermistor for thermal feed back. Battery pack for testing is a 2P3S Li Ion 7.4 v 7500ma pack. I have come across 2 issues. When I measure the current in to the voltage regulator is is the same as the current out. I have used an inline current meter and a DC clamp meter (not at the same time) and the result was about the same. So is the regulator simply burning off the excess voltage or is it a switch mode regulator and the meters cant get a proper reading off the input to the reg? The second issue I have is that I am using a MOSFET and reed as the power switch as I have with all my other cannister light builds. But the MOSFET is getting hot while with all my other builds ( AMC 7135 drivers or KD 2.8 amp P7 drivers) the MOSFET has remained cool. Is this because the LDO6C is switching? Is there is simple solution? Would putting a capacitor across the MOSFET help at all?

LDO10C data sheet ( same as LDO6c except only 6 amp)

MOSFET data sheet

 

[KillingTime]

If your mosfet is getting hot, then it's operating in it's linear region, meaning it's not fully switched on. 7.4V at the gate might not be enough to turn it fully on. Measure supply voltage at the LD06C, not at the battery. The difference between the two is what your mosfet is 'dropping' before the switching reg.

Try to post a picture of your schematic so we can see what your circuit looks like.

 

[Packhorse]

I tried it with a 12v NiMh pack and the same result.

I have a 50k resistor across the Gate and Source with the reed connected to Vin and Gate. Source goes to Neg in and Drain goes directly to the LED and Com of LDO6C

I have swapped the MOSFET out for another of the same and it was the same problem. I then tried a IRLZ34NPBF and it runs warm instead of hot.

I will try putting a cap across the FETs output tomorrow and see if that helps.

 

[HarryN]

Hi, I answered before I looked at the data sheet, so I will try again. It is a DC-DC voltage regulator, and should be making a voltage conversion the way you are intending. The efficiency is very dependent on the input voltage on the unit I tested. What is the output voltage you are setting the regulator for?

I bought some of the emerson regulators (not that exact model) and the comment about it not needing external capacitance is not exactly true when you read the more detailed data sheets. TI and others make similar units, and all need caps from what I can tell.

There is also a sort of mixed aspect - both high and low frequencies needed to be damped out, so almost 2 different caps were needed IIRC.

It looks like that mosfet needs at least 6-8 volts over the switched voltage to turn on properly, so it is running in linear mode right now.

 

[Packhorse]

Output voltage is about 3.7-4 volt. Or at least was. Although it was working fine the night before I tried it again and it instantly blew a SST-90.

I give up!

 

[KillingTime]

Have you checked the SST-90?

Unless it was very badly heatsinked, the SS reg should have tripped into hiccup protection mode (>6A) and the SST-90 can handle this (I regularly hit the hiccup mode on the LD010C (about 12A) when I was testing my SST-50 / Copper heat sink - no probs).

FYI, if you look at my original LD010C post, you'll see I used a 25W resistor mounted to PC heatsink (fan cooled). This is the way to go when you're building new prototypes. Saves you blowing expensive emitters.......

http://i67.photobucket.com/albums/h316/goodproducts101/CPF/IMG_0923_.jpg