Help w/hv9910 LED switcher

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they seem to use it either way ... one datasheet says +15%, the other +30% (115 - 130) ... so I'd aim for 130% unless space savings of a smaller inductor is a big concern.
 
No. Ripple is NOT a factor. If you set the 9910 threshold at 500mA, the 9910 will shut off at 500mA. If the inductance is too low for the job and ripple is significant, it will lower the average current but the peak current remains fixed at 500mA by the sense resistor on the 9910.

But you DO want a significant safety margin between the peak current and the saturation current. As I say saturation point is highly variable. And another factor is that the inductance value goes down as you get anywhere near the saturation point. The inductor will act like a smaller inductance than indicated in the spec.

Hard to say, but as a rough guess I'd go with an inductor with a saturation 50% greater than what you intend to use.
 
I put on an 18pF and 10k on the current sense, and the HV9910 self destructed....

I changed the load into a 30 ohm resistor, and measured the current to start at ~0.8A and it decayed (along with an increasing pitch in the whine) to ~0.7A.... but then it just stopped working. The FET was getting quite hot for even the short couple second test runs I was doing, so I thought I killed it.. But I changed it out and still don't have anything. Everything else tests okay, so the 9910 must have bit the dust.

*sigh*

Why does such a simple circuit have so many problems? And what did I do? The IC was getting warm, but nothing that concerned me... This is the FET I'm using-

https://www.electro-tech-online.com/custompdfs/2006/12/FQPF11N40C.pdf

Is it possible the gate capacitance or charge is too high for the IC?
 
The HV9910 has none of the stability problems associated with most smps.

The gate capacitance could not have fried the HV9910.

I would be almost certain you did not wire this up correctly. Do you have a cap on Vref? Is C1 large enough to avoid excessive ripple?

You put the resistor between the Source and 9910 current sense pin, and then the cap between the current sense pin and ground, right?

10k * 18pF is 180nS, this is less than the specified 300nS.

Oh yeah. The RC network does present a minimum for the "on" time. For example, if your inductor is at 1 amp and you have a 1 amp threshold, the 9910 will still take like 300nS to turn off, during which time the inductor current increases. At 100KHz, that would be a min duty of 1/33. This will generally only show up on high ratios, as a guideline, a 1:33 voltage ratio. Driving a 7v off a 240vdc supply for example.
 
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Source -> 10k -> current sense pin and cap to ground.

It would not surprise me if I did not wire it correctly... I forgot to tie pin 5 to the other Vref pins anyways. Vref has a 2.2uF cap as specified. However it worked for a little while.... C1 was what was spec'd for 0.7A, 68uF I think.

I just downloaded the demo board manual, and it uses an IRF840 in the D2pak, which makes me think it shouldn't be putting out a lot of heat! With how quick it heated up I'd say it was close to 5 watts...

180nS is more smoothing than nothing...
 
Well something is wrong. Unless the 9910 is totally malfunctioning (bad Vdd/Vref) it should either turn the MOSFET on or off. The one you specify is rds-on of 0.5ohms, so it would be 1/4W at 1 amp.

I used it to drive a Lamina LED off of 14.6V, at a couple of amps. I used a transistor in an SO-8 pkg ("hexfet" type, low voltage MOSFET) and it ran totally cool.

I remember raising the question of the current sense pin impedance. I mean if you use a 100M ohm as part of the RC it probably won't read anything due to the leakage current draining the signal voltage away. IIRC he did say the pin's input impedance is pretty high, I think 10k should be fine.
 
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Good news! It works!

Bad news! It gets really hot!

At the suggestion of tech support, I connected the oscillator pin to the gate to select a "constant off time" mode as referenced in app note AN-50. This allows duty cycles >50% without subharmonic frequencies appearing.

Now, it's putting out the rated 1.4A, but everything is getting really hot! The inductor and diode gets warm (to be expected), the FET gets hot, and the HV9910 gets hot! When I say hot, I mean 150+F. I put the fet on a heatsink, but not sure what to do with the 9910 - or even why it's getting hot.
 
any idea what frequency it's running at? that's the only thing I can think that would make everything heat up.

high freqs increase switching losses in the transistor and diode

only reason I can think the chip should get hot is that it's somehow not wired correctly, or maybe you're taxing the internal regulator too much?

do you have any load connected to the regulator output (Vdd pin 6)

how about voltage on the feedback pin, is it within spec?
 
Wow, that's strange....... According to my $50 meter, it's output is 300+khz? It's supposed to be running at 100khz. I'll have to put it on a scope to confirm that though, it might be picking up something. Very strange.

Internal regulator is not used. (it's tied to LD and PWM_D)

The voltage at the current sense resistor is what it should be.
 
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using what FET? the HV9910 has a limited gate drive capacity. I suspect you either need to add a gate driver or change the FET.
 
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