9V 1Watt Led Driver for single Li-ion cells

 Recently I stumbled across the MT9284 led driver IC. The datasheet shows it can be used for strings of 7 white leds. There is a suggestion in the graphs that the maximum led current is perhaps 20mA. 

I also had some 1 watt white leds by Bridgelux, BXEN-65E-13H-9B-00-0-0. These are approximately 9V at 110ma. 

Can the MT9284 drive one or more of these? Yes. I have tested to date one Bridgelux device at currents up to 80mA. The efficiency has varied from 83% to 92%, depending on led current. 

Nice and bright! I'm going to update my collection of homebrew pocket torches. I find as I get older that the part markings are getting harder to read and a bright light source that can illuminate the part from different angles makes a big difference.

More soon.

73's

Noise in "Jelly Bean" Regulators - Part 3

I repeated the exercise with a 78L08 and noise fell from 480mVpp to 30mVpp. Two surprises - the 78L08 was less noisy than the LN6206. I had expected it would be noisier since higher output voltages tend to have more noise for the 78Lxx parts. And apart from the 78L08 being quieter than I expected my test apparatus is not perfect. 

I had expected that the capacitance multiplier would reduce noise by the same factor as it did with the LN6206 ( 30 fold, 1800mV / 60mV ). But in this case I was only getting a 16 fold reduction (480/30). I checked and the unterminated amplifier was producing ~1.5mVpp on the CRO. However, when I put a 75 ohm termination on the input BNC the noise increased to ~30mVpp. Which requires more investigation.

Still, the capacitance multiplier makes a substantial difference and will become a standard inclusion on future analogue projects.

Flush with some sort of success I put a TL431 onto the test board with no capacitors anywhere in circuit and with an approximate output of 8 volts. 

There was no change in the 30mV of noise on the CRO. So while I cannot quantify how quiet a TL431 is, it appears to be a substantial improvement on the 78L08 alone. I am also unable to determine if the TL431, with capacitors, is an improvement. Or indeed if a capacitance multiplier is worthwhile.

But I'm not beaten yet.

73's

Noise in "Jelly Bean" Regulators - Part 2

On the second attempt my low noise amplifier for measuring regulator noise worked. It's based on a dual low noise op-amp with an overall gain of 300 (x10 followed by x30) and no signal noise of a few mVpp. The very thorough noise calculator I used suggested I should have a better noise floor so further investigation is needed. 

I quickly checked a LN6206 regulator to confirm the approach and saw 1.8Vpp of amplified noise on the cro. That works out at around 6mVpp for the regulator which I might have been able to estimate with the cro itself. But now I can experiment with different noise reduction techniques and relative comparisons between regulators without having to estimate differences at that low level.

I started by trying a capacitance multiplier.  I used a 10k resistor, a 1uF tantalum and a general purpose npn transistor. That reduced the noise from the LN6206 from 1.8pp to 60mVpp. That is a large reduction in regulator noise with really no effort. 

 73's

Noise in "Jelly Bean" Regulators - Part 1

In the course of investigating the retrofit of a low noise RF pre-amp to an old 2m mobile, RT85, a random thought from nowhere made me stop and consider regulator noise. 

My goto part for 3 volt regulators in microcontroller projects is the LN6206. Cheap and reasonably low quiescent current. However, there is no specification of the output noise voltage. 

Since I had seen a TL431 on a PCB earlier in the day the device was foremost in my mind. I wouldn't need a pre-regulator to reduce the voltage to the LN6206 which is a 6V maximum input device. However, the TL431 datasheet I had also made no mention of noise.

After a bit of searching I became cynical of the claims the TL431 is low noise. I found just one objective assessment of noise which helped reassure me. Going from 50 nV/√Hz to noise in a 0-100kHz bandwidth and adjusting for gain from the 1.25V band gap voltage of the TL431 to 3V returned 38uV. Which is almost identical to the 40uV output noise voltage of the L78L33C. 

I'm not sure that qualifies as low noise, since a LT3094 is specified as 2 nV/√Hz but costs over 50 times more! I note in the web page above that an additional capacitor could be expected to reduce the output noise voltage considerably. Back of the envelope suggest under 10uV.

Can we do better? It appears so. A simple one transistor circuit after the regulator should make a large difference. 

Verification is the hardest part. I could not find a low noise amplifier circuit that I could re-create with my junk box. My attempts to build a low noise amplifier in a shielded box are yet to bear fruit. 

My whole approach to electronics is to re-use previous work until the need for improvement arises. While trawling through the internet I saw numerous statements that low noise regulators are vital to oscillator performance. This is not something I had considered beyond decoupling supply lines. So it appears the need for improvement exists!

73's