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Using PWM (TL494) to regulate the positive volt doubler with ICL7660

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fuji

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I'm using the ICL7660 charge pump converter to increase from 9v to 18v. I achieved this making the positive volt doubler circuit shown in the datasheet. I'm using 2 1n5817 schottky diodes because of the low voltage drop, so it came around 17 volts on the output of the volt doubler circuit.

My problem is, when checking the output voltage of 17v with my multimeter, the voltage fluctuates up and down, not staying at 17v accurately. It randomly changes voltages and current above and below 17v. This happens only when I apply the load.

How can I stabalize the voltage output to stay at 17 volts on the output of the voltage doubler circuit at the same time I have a load to it? I was thinking to use a TL494 PWM for this issue, but I am not sure if I should place the PWM before or after the charge pump.

Any ideas would be appreciated.

For some reason the picture for the volt doubler of the ICL7660 is not uploading. It is in the datasheet though.
https://www.intersil.com/content/dam/Intersil/documents/icl7/icl7660.pdf

tl494
https://www.ti.com/lit/ds/symlink/tl494.pdf
 
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What load current are you looking for?, the 7660 is only very low current, and not regulated in any way.

I don't really see how a TL494 would help you?.

I need at least 20mA load current.

Then what is the reason that the output voltage (17v) of the volt doubler keeps fluctuating when I apply a load to it? How can I stabalize this?
 
All charge pump voltage converters have a relatively high output impedance, so what you basically have is 17 V source followed an output resistor. That resistance and your load (another effective resistance or impedance) form a voltage divider. As your load draws more current, the voltage at the divider node (the output of the 7660) drops as it should.

If you want a true regulated output with varying load current, you'll probably have to shift to some other boost regulator circuit. You can search for 'boost regulator circuit' and get tons of examples. Or you can buy small modules from China for $2-$3 already assembled and tested. Or, use a 555 to drive a two-stage Cockroft-Walton generator followed by a linear regulator.

ak
 
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All charge pump voltage converters have a relatively high output impedance, so what you basically have is 17 V source followed an output resistor. That resistance and your load (another effective resistance or impedance) form a voltage divider. As your load draws more current, the voltage at the divider node (the output of the 7660) drops as it should.

If you want a true regulated output with varying load current, you'll probably have to shift to some other boost regulator circuit. You can search for 'boost regulator circuit' and get tons of examples. Or you can buy small modules from China for $2-$3 already assembled and tested. Or, use a 555 to drive a three-stage Cockroft-Walton generator followed by a linear regulator.

ak

Thats a very good answer and I really like the cockroft-walton generator circuit. Looks pretty simple to build. I'll take this one into consideration, but can I also use the TL494 and build a boost regulator out of it? I have used the mc34063 a while ago and it was not reliable for me since its a pretty old dc/dc buck/boost regulator and it still gave me voltage fluctuations as well.
 
That is surprising. The 34063 is one of the most used small regulator parts in the world. Another option is a National Semi (now TI) Simple Switcher. Both are easier to use than the 494 IMO.

ak
 
That is surprising. The 34063 is one of the most used small regulator parts in the world. Another option is a National Semi (now TI) Simple Switcher. Both are easier to use than the 494 IMO.

ak

Yes thats correct. As I remember it is mostly used in chargers, and vehicle chargers for USB appliances and phones etc. It's more accurate on higher currents, but not low currents, and frequency only goes up to 100khz.

I'll take a look at some Simple Switchers. Thanks.
 
I recently designed and built a buck using a TL494 and 3 paralleled bipolar 2n3906 transistors to get around 300mA at 15V from a 36V supply.
Compared to dedicated buck boost chips the '494 requires more external passive parts to handle soft start, voltage and current control. It does oscillate faster than the 34063 and can use smaller inductors.
The '494 does lend it self to a wider array of PWM apps though, so it's a more gen purp. chip. Recently I have started acquiring TL594s as an updated chip with 1% references etc.
 
The 7660 switched capacitor ic allthough I havent done any calc's I would have thought would supply 20ma, the 7660 isnt regulated so if you switch on and off a 20ma load you will get voltage changes, if the nature of the load is constant then the circuit might be ok, but if you circuit load nature is changing then you'll need a version on the circuit that is regulated.
The problem is if you want 17v from a 9v supply, esp a battery there isnt enough voltage headroom for a regulator circuit.

It sounds like if you have the knowledge to go ahead and use a tl494, but not as a switched capacitor bosster, as a switched mode booster using an inductor, or as mentioned the staple power supply diet ic the mc34063, the latter is regulated, however in voltgage mode it can be difficult to design, a current mode ic like a uc3842 might be simpler esp for a one off circuit.
 
Your 20 mA out at 17 Volts, Probably pulls 45 to 50 mA from your 9 volt battery.
With the 20 mA Load applied to your 17 Volts, What Voltage is your battery at?
Measure it.
 
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