LED boost driver question

There are a number of different inexpensive boost ICs of this style:

for constant current driving LED strings from e.g. LiPo battery.

Any problem with chip of that type having say two strings of different color, perhaps white and blue, and switching between them using PNP or Pfet on the high side, with a common current-sense resistor?

Reply to
bitrex
Loading thread data ...

Looks like the sense voltage is only 0.3 volts. So you could use an N chan nel device on the low side between the bottom of the LED string and the sen se resistor/sense input junction. This will provide some added protection since as the current increases the drive to the N channel device is reduced and the current will be limited.

--
  Rick C. 

  - Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Rick C

Probably it's done by PWM, so the 'sense' resistor showing zero current (when an active string is switched OFF) is just the same as the startup condition, it should handle that. But, will your high-side switch, in OFF state, handle the inductive kick of the first pulse?

You'd want any high-side switching to sequence in make-before-break fashion, including at first power-up.

Two driver chips, two inductors and two Schottky diodes, using logic into the shutdown input, seems a cleaner solution.

Reply to
whit3rd

Sure. A make-before-break switch would be preferable, and relatively slow switching (~us) so as not to dump the capacitor into the lower-voltage string. No need for P type and level shifting, the base current and shunt voltage at the low side are both small.

Heh, actually made a medical device years ago that basically did that, but at higher current (we used a 12V MeanWell supply and two independent boost controllers, one for each color) and with an MCU to set and measure the operating current. (A specific blue wavelength is used for treating infant jaundice, and the current is measured as dosage; the white was just for general illumination.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
 Click to see the full signature
Reply to
Tim Williams

Yeah I think low-side would be preferable

Reply to
bitrex

There are a few issues. Your LED strings will no doubt have quite different forward voltages. So I'd add a series resistor to the FB terminal. At the moment of switching, the voltage on cap C2 (Fig 1 in the CAT4238 datasheet) will either create excess LED current, or keep the LED string off. Inductor L is naturally a current source, and the CAT4238 has a nice internal current-mode control loop. These try to keep a constant current flowing into whatever the LED-string voltage is. But cap C2 is a voltage-source trouble maker, and you will want to minimize its value. At 40mA LED current, its voltage changes 0.04/0.22uF = 180 mV/us. You can drop C2 a lot by accepting a higher swing each cycle, especially if you add a ballast resistor dropping say 3 volts. You can also add parts to finesse your LED strings to be closer in voltage. Or use two boost converters, haha, only 75 cents for the CAT4238 and 47 cents for the Sumida inductor. OK, I get it, you're worried about space. Aha, a dual FDC6561AN MOSFET, etc., in a SOT-23-6 package, only 25 cents.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Two switchers would be ideal but yes cost is the primary concern on this lil' jerb. The two LED strings are not long, only two or three standard

5mm LEDs on each.

I had thought about hacking something out of discretes or a charge pump with a logic IC or two but some of these LED drivers are only about 40 cent in small quantity, will be more efficient, it really doesn't seem worth the hassle vs an off-the-shelf solution to bring the 2.7 volt supply up to where it needs to be.

Reply to
bitrex

nt

or

g

formatting link
~4 cents in volume

formatting link
ot-89-5-package

~20 cents

Reply to
Lasse Langwadt Christensen

Woooooooooooooow. "These prices are Kraaay-ZEEEE"

Reply to
bitrex

You could put the strings in series, and short one or the other with a small SSR or something. That allows both to be on, too.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
jlarkin

That chip appears to be a fine part, with a decent datasheet. The MT9284 has the same architecture as the CAT4238, with a current-mode loop inside, but it's a bit faster, and has a higher-current switch. I've made purchases from LCSC, easy to work with, very respectable, kind of a Chinese Digi-Key. Select an inductor from LCSC as well. And your dual MOSFET. Or, at that price level, you can use two separate converters!

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I purchased a roll of 3Peak high-speed op-amps, excellent parts at only 6 cents each. They also offered slower 36V 4MHz 15V/us op-amps, with 3mV Vos, only 1.3 cents each.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

With those prices on the boosts nowadays with what looks like good availability I don't see any reason not to use one for each string, a switching arrangement is over-complicated TOAB at that point.

The other question is whether I can tap a second diode plus cap off the inductor and run a couple low-power quad RRIO op amps off the LED booster. The current draw will be low with respect to the LEDs. I bet it won't notice.

Application is a custom "big blinky" so the PWM inputs are real handy

Reply to
bitrex

Be careful about parts from LCSC

Many are Chinese versions of known designs and the lifetime is often not guaranteed, so you design it in, get to full production and then suddenly it is discontinued

Reply to
Klaus Kragelund

From what I can tell, there are some good, reliable semi companies in the Pacific-Rim mix. No doubt, a few, competing at the sub-10-cent level, simply can not sustain the effort, and their parts may collapse. But I developed confidence in 3Peak, and aerosemi.com looks good with lots of products. They don't have an English website that I could see, so far, but the datasheet was in English, so they may have one. The MT9284 pinout matches the CAT4238, except they have an extra OVP pin to provide safety protection. In a pinch you could revert back to the CAT4238. But the parts are so cheap, you can make lifetime purchases.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

No problem at all when make before breaking strings in parallel -if anything it's a plus

NT

Reply to
tabbypurr

:

ent

or

g

ing it's a plus

you could even do with one switch, just when you turn off the lowest voltag e string the other turns on

Reply to
Lasse Langwadt Christensen

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.