Hi! The combination of a boosting DC/DC converter and a constant- current regulator in a single IC seems popular for driving strings of LEDs, especially for a low voltage input (say 12 volt) and a long string of LEDs which need a high current. Has anybody got any recommendations on what chip to use ? It is difficult to find these among the dozens of similar IC's with multiple channels, dimming, too low current or too low output-voltage etc.
So far I have one from Maxim called 16818 and a National part, LM5000 as candidates..
If you need the power (V & I), you should build it out of discrete components. Power transistors need room to cool and Inductors and capacitors are hard to build in IC. Anyway, I am using 3Q, 2C and 1L booster circuit in an IC, but I don't need high power.
yes I need at least 500-700 mA for approx 1 ms long flashes, driving a chain of LEDs with a total forward voltage of maybe around 40-60 V. The circuits I've looked at have had the switching regulation controlled by an IC but then used discrete external inductors, capacitors and MOSFET's for switching the inductor and dimming the load. That setup sounds fine by me. I just have trouble finding a useable application note describing a similar setup - most application notes are for driving single or maybe 2-3 white LEDs at a couple of dozen mA (obviously since white LEDs are a major booming market!), or they are buck regulators where the input voltage already is 40-60 V.
I wonder if a switching integrated regulator with a switch frequency of say 700 kHz can ramp up and control such a short flash in a stable manner. I guess it would, since
1 ms would be 700 switching cycles and it shouldn't take that many cycles before it was up enough in voltage over the output capacitor to give the needed current.
I have often needed a bucking LED driver instead. We have the typical application in which a microcontroller controls a set of parallel LEDs in a 7-segment 3 digit display.
The typical design has a lot of power loss in the resistors in series with the LEDs since we have to take VF and supply tolerance into consideration. What I would like to have is a chip that controls the
3digit 7-segment and that controls a buck converter to set the total current in the LEDs. So if x LEDs are on, the current control is set to x current.
Anybody know if such a device exists? It would boost the efficiency and discard of all the resistors too.
I ran some simulations. Using 10mH inductor and 10uF capacitor, it would take 500 ms to ram from 12V to 50V. I use base current of 8 mA on a NPN power transistor and no output load.
Depending on your duty cycle, you would need to drive the transistor harder.
You might think that at first, but remember--the stored energy is (1/2)*L*i^2.
For a given peak current a smaller inductor value stores less energy, but it can be charged proportionally faster, so you just increase the switching frequency proportionally and there's no difference in energy delivered.
What a lower value _does_ give you is many fewer turns on your inductor, allowing fatter wire. Fatter wire cuts losses.
More importantly, fewer turns means you can push more current without saturating your core, allowing you to increase the i^2 term a bunch.
So, smaller inductors allow one to *increase* power.
I should've proposed a driver too, since duty cycle matters, not just frequency, and your 2n3055 isn't going to like being driven 100x faster!
Now this hysteretic converter is complete crap right off the top of my head--so don't go putting it in any airplanes--but it'll switch fast enough to illustrate the point at hand:
PS: I don't know if this would be a problem or not. The ZTX1048A Collector to Emitter break down voltage is only 17.5V at 10mA. How would you model this in spice?
I have only a bit of basic knowledge of Spice due to doing more work that is better done than simulated.
I suspect a reduced C-E breakdown voltage as a function of current (as in higher) is due to "forward bias second breakdown". This is a phenomenon of bipolar transistors where power dissipation capability is reduced as a result of higher C-E voltage, even if such higher C-E voltage is well within the device's C-E forward bias voltage limit.
In my past experience of reading power transistor datasheets, power dissipation capability tends to be impaired by "forward bias second breakdown" when C-E voltage is above something like 30-50 volts (varies somewhat from one device to another). This gets "less bad" when the bipolar transistor in question has "hometaxial" structure rather than "epitaxial", though "hometaxial" has appeared to me less-desirable by being slower and "epitaxial" is more common.
The forward-bias second-breakdown phenomenon also has tolerance for higher instantaneous power dissipation at "offending" C-E voltages if this has to be withstood for pulses (generally a few milliseconds or less, better still if fractional millisecond or several microseconds) at a low duty cycle. Transistors with "forward bias second breakdown" vulnerability do often do well as switchers since in such duty they tend to only need to dissipate high power with high C-E voltage for only microsecond ballpark with low duty cycle, and otherwise do their best at approximating an open or a short.
Breakdown of 30 to 50 V is more typical. But the ZTX1048A will breakdown as low as 17.5V, which is not a problem for my need (6V to
15V). However, it will indeed be a problem beyond that. Anyway, I would rather use a 2N2222A with higher breakdown voltage. Unfortunately, neither one is available in SOT-23, so back to the drawing (layout) board. My current board is using BC847B, which peak at 12V in simulations.
For higher power, the ZXT1053AK (4A) works nicely with 470uH inductor. There are something in the Zetex parameters that make them work well with small inductors. I haven't figure it out yet, but the simulations are good.
We want a nice parabolic curve to reach high voltage quickly, so we can shut down the circuit ASAP. For 25mA load, The ZXT1053AK 470uH reaches 50V in 100ms and 60V in 300ms.
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