A 3-component LDO current regulator

[bikeNomad]

I've been looking at Linear Tech's LTC4059 tiny little 900mA battery charger chip (LTC information) and it looks like this might actually be a simple way to get a low-dropout current regulator in a very small package.

And it's easy to put this on a single-sided 10x10.5 mm board (or double-sided, with the bottom all ground):

I haven't tried it yet, but I think I will soon.

The circuit has an extra resistor on the EN/ line for remote on/off; if you don't need this the R1 can be eliminated and EN/ tied directly to GND.

The current is programmed using RPROG; this would also make it easy to switch between different brightness levels.

 

[Doug Owen]

Interesting idea, but I suspect you're not considering heat in that tiny (2 mm square part). The good news is the IC will self protect, the bad news is the LED will get dim when it does.

.15 Volts is low as a minimum headroom(although there are lower drop out regulators easy enough to build), but it looks like the maximum headroom you can use is 1.5 Volts or so. Looks like a pre regulator is in order?

Pretty narrow working range, IMO not suitable for typical use as is.

Doug Owen

 

[PeterB]

But seems to be ideal for a LuxIII/LiIon setup. Nice find!

 

[bikeNomad]

Actually, the part is able to put out quite a bit of current. At 5V in and 3.7V out, it can put out more than 900mA before it starts to thermally limit.

Assuming an ambient temperature of (say) 40C, and 100C/W (a guesstimate for that little board mounted to a heatsink, since they say that a 50mm square double sided board can give you 60C/W), then it can dissipate

(115-40)/100
or 0.75W
before it starts to limit. That is, it'll start to limit at 115C Tj.

So for a 3-cell light run from alkalines, with 4.5V in and 3.7V out, you could put out over 900mA! Of course, it'd only run the batteries down to where the LED stopped conducting.

Assuming a 4-cell light with Ni batteries (4.8V nom.), you could still put out 680mA.

As for maximum input voltage, it'll work up to 8V input. I don't know where you got the 1.5V figure.

The spec'd turn-ON overhead is 150mV, but it'll apparently go DOWN to 35mV if you leave the battery on (in this application).

Soldering this would probably require something like a hotplate and hot air gun with solder paste and something (maybe a thin bent wire) to hold the chip down. Any ideas?

 

[Doug Owen]

[ QUOTE ]
bikeNomad said:
As for maximum input voltage, it'll work up to 8V input. I don't know where you got the 1.5V figure.

The spec'd turn-ON overhead is 150mV, but it'll apparently go DOWN to 35mV if you leave the battery on (in this application).



[/ QUOTE ]

I got to 1.5 Volts overhead same way you got to 1.3 in your example. It seems a reasonable (but conservitive) estimate of where overtemp shutdown happens. More headroom than that and the LED is gonna dim down.....

And just 'cuz it'll turn on at .15 Volts past Vf, doesn't mean it'll make anything like the currents you're expecting. Note that Ron is 800 mili ohms at 150 mA. This means we'll only be able to pass something on that order (less than 200 mA, a small fraction of what we wanted) at this level of overhead, it'll be *dozens* of mA at .035 Volts delta, right?

And we may run afoul of the 3.75 minimum operating voltage of the part in the process. The sucker may not wanna run at all at the bottom end.

So in reality, it'll work as a flashlight if we *very carefully* match Vbatt to Vload. We need to keep the efficiency over 75% if we want 3 Watts in the load and one or less in the regulator. But only just, and only at that single point. If the battery voltage drops (as it must), the LED is sure to dim. If, OTHO, it's 'hot off the charger' the flashlight will probably go into overtemp shutdown.

Or so the back of this envelope sez...

Doug Owen