LM3844 DATASHEET PDF

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Welcome, Guest. Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. Author Topic: Self-made buck-converter problems! Read times. Hi lovely people of EEVblog. First of all when no load is on the output, the voltage just flies off, and is some random place between Vout and Vin always higher than Vout is set to. When loaded down with just mA, the output voltage stays without 20mV of the set voltage.

The diode and the inductor gets hot'ish. The build is on a doublesided PCB cut out with a dremel, topside with components, bottom side for groundplane. Hope you can help me with this build. The input capacitor, CIN, on the typical application circuit has to take the full switching current without the benefit of the inductor so it's worth paralleling more 22 uF. The peak no load output voltage may be too high with lots of ripple because the minimum MOSFET switch on time may be quite long.

It appears to use a one shot pulse generator. Can you provide an Oscilloscope trace of the output on no load? Hi PCHi, thank you for your answer. I just did some oscilloscope testing, and found out something quite interesting.

When I boot up the device, it works, and holds the voltage well See no-load working. Then under load see load working , the PWM is very strange. It seems to be pulsing at around 4MHz, which seems crazy to me!

Anyone have any thoughts? Buck LM no-load working. Buck LM load working. Tomorokoshi Frequent Contributor Posts: Country:.

Can you get a trace of the FB pin from when it's switched on? Hi Tomo. Do you want a read in the oscilloscope? Cuz I can't see anything other than it following the output, with the same amount of noise as the output. Did you put in Cff as shown in the example? The should be a very low value. Perhaps there is something wrong with it. Remove the capacitor and double-check the feedback resistor values. The feedback voltage node should be around 1. It's a puzzler. I don't understand the On load switching waveform but Hysteretic converters can get confused by noise so perhaps adding more input filtering and checking that the ADJ and FB pins aren't picking up noise might help.

Possibly add some temporary filtering to GND might help. I don't like the way the Typical Application Circuit is drawn. How does your PCB look like? I had massive problems with a LM circuit that I built on vero-board and that didn't work properly.

The solution was to add a ground-plane by putting some adhesive copper-foil underneath the chip. Maybe you have the same problem: The Return-Paths are too long or have too high impdances, which introduces parasitic inductance and capacitance, which in turn creates oscillations.

Tomo The datasheet recommends a pF, so that's what I'm using. It keeps it from floating around. The crazy thing is now that the frequency of the whole system is 3. This seems crazy high?! SaabFAN I have made the project on a real copper plated double layer PCB board with groundplane on the backside, so this should be optimal.

I have had the problems you are describing myself when messing around with lazy setups. Thanks for the update. It reads like it is working OK. It's never going to have a stable pulse width just by using Hysteretic control.

As it uses an external diode not bypassed with a MOSFET the efficiency reduces as the input to output voltage differential increases. The real world measured efficiency sounds reasonable.

PChi Would you consider this normal conditions? No I wouldn't consider it normal. All the little wobbles are going to be strongly dependent on the positioning of the probe ground lead. Also some Oscilloscopes aren't very good at rejecting common mode noise and the trace can be misleading. Also can you increase the sample rate? At the moment it's only My interpretation of the data sheet page 11 is that the output capacitor ESR also affects the frequency. The reality is that everything is going to alter the approximate switching frequency including layout so wont be stable.

PChi Have tried connecting the probes to different places on the ground plane, the input PSU and the output, but the readings seem to be the same. The green is the gate, and the yellow is the switching note. It looks like the gate driver is working itself too hard, and from my calculations, driving a 6nC gate at 3.

Could try to replace the chip again, to see if it is what's causing the problem. The gate pin isn't suppose to go to 0V, since it's around Vin - 5V, in order to driver higher voltages, and still protect the gate.

I'm already using a 22uF tantalum cap on the output, together with a uF electrolyctic. What would i do to change the ESR? I admit to only glancing at the data sheet and failed to understand that the PGATE voltage was limited.

Thanks for pointing that out. My guess is that the uF electrolytic capacitor will have a higher ESR than the tantalum 22 uF capacitor so removing the tantalum will increase the ESR but will that not increase the switching frequency?

My guess is that the noise on all the LM pins will all affect the comparator so it's trial and error time. If anyone else has any ideas please contribute. Seems like I have tried everything now. Have changed to chip, and the same result. Different inductor 22uH , same result. More capacitors on the output, same result. Stille running at 3. Now it's even more unstable, it would seem. A few comments about how TI expects this device to be used: Fig The controller works by switching when the waveform it sees at the FB pin goes up and down by 10mV 8.

If you have a low ESR cap, they want you to add a resistor to make the net impedance of the "output capacitor" branch resistive at the switching frequency.

The larger the ESR, the higher the switching frequency. You could take 2 of the same caps and if you put them in parallel, the switching frequency should halve.

If your Cff is too large, that will pass the Vout ripple directly to the FB pin, so the ripple seen at the output will be only 10mV peak-peak and the switching frequency will be higher. Once you have that, you can set the switching frequency by changing the inductor. There was an error while thanking. SMF 2. EEVblog on Youtube.

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