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Old 4th August 2011, 07:29 PM   #131
Salas is online now Salas  Greece
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Do you have it on a near setting consuming load when taking those voltages?
Because it sounds like with 3R9 you are left without enough juice and it progressively ducks Vout max. So with 3R3 looks OK. In general, know your load exactly and then top it for CCS current setting.
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Old 4th August 2011, 08:29 PM   #132
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Do you have it on a near setting consuming load when taking those voltages?.
yes, I have put a dummy load resistor 12ohm 5W.
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Old 4th August 2011, 08:52 PM   #133
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That explains it. With 3R9 you were on the edge. It behaves normally.
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Old 4th August 2011, 09:04 PM   #134
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Let's go for the 15V!
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Old 4th August 2011, 09:14 PM   #135
Salas is online now Salas  Greece
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What is it for? The usual?
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Old 5th August 2011, 01:51 PM   #136
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is anyone using these for truly hispeed oscillator power? my clock is 100mhz and i just wonder if this is the best usage for the reg, would the output cap (a nice big film cap) compensate for the relatively high zout at such a high frequency?
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Old 5th August 2011, 01:59 PM   #137
Salas is online now Salas  Greece
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Use BJT output as in the guide if you want the highest bandwidth of flat Zo it can give. It goes higher Zo mOhm in total is the flip side.

At 100MHz even if we had an ideal voltage source I doubt that any layout and wiring would not rise impedance either on the reg or in the receiving circuit anyway. Radios, communication, and computers succeed to work on LM317 kHz bandwidth flat Zo and then shooting through the roof style, and SMPS to keep the world going, so we are in better hope with this reg I would dare guess.
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Old 5th August 2011, 02:08 PM   #138
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thanks Salas, yeah i had already decided that the bjt is the one for the job, thats what the singular lv bjt+ kit I ordered was for, its a better compromise to have flatter although a touch higher z output for this situation. i just wondered if you had seen any sims or plotted what happens at much higher frequency, as the graph only goes to 100khz, a factor of 1000x different to my load and the mosfet sim started a pretty steep upward trend towards the edge of the plot, i wondered if the bjt did the same, but later. i guess a really nice teflon cap is in order and a well bypassed XO directly across the pins with an np0

I was considering using the new lt ultrafast transient response mosfet controller LT1575, which basically does what it sounds like and drives an external mosfet as a source follower, its designed for exactly this type of load, primarily high speed CPUs, considerably better than the ol' 317

Last edited by qusp; 5th August 2011 at 02:28 PM.
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Old 5th August 2011, 02:16 PM   #139
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It should be about 1 Ohm at 100MHz with the BJT at the output. At those frequencies, if your digital circuit isn't best decoupled and awesome layout, nothing can save it. I doubt there aren't traces of the clock and multiples on rails, gnd, and on field if poking around with a 1GHz oscilloscope even on air defense grade high clock digital equipment.
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Old 5th August 2011, 02:33 PM   #140
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oh the cct is well designed and well decoupled (ackodac AKD12P PTFE version sabre dac), but it usually uses a lifepo4 battery powered AD797 based reg that is able to be placed considerably closer than i will be able to put the shunt due to physical size. I guess we'll see how i go. it may turn out better to use the shunts for the AVCC L/R and reference supplies, but use the ad797 reg for the clock.

Last edited by qusp; 5th August 2011 at 02:44 PM.
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