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# Negative Voltage Generation

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#### cmbeelby

##### New Member
I have read over the documentation for the TC7660 chip by Microchip Technology, and would like to know if I am reading this right.

Can I chain 4 or 5 of these chips together with a +5V initial input and get either -20V or -25V output ?

I need -17V for a project so I was thinking I could use these to make -20V and then use something like a LM337 to drop that down to -17V. Is this a good idea? Thanks.

#### MikeMl

##### Well-Known Member
How much current at -17V?

#### Claude Abraham

##### Member
I'd recommend a Tex Instr UC2572D IC to generate negative voltages. I've used it for years w/ great results. With the 2572, you can generate any value of output from any input. From 12V input, you can obtain -3V, -5V, -12V, -24V, -9.3V, -17.4V, etc. Are you familiar with SMPS design?

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#### cmbeelby

##### New Member
Current Requirements

How much current at -17V?

#### cmbeelby

##### New Member
Thanks

I'd recommend a Tex Instr UC2572D IC to generate negative voltages. I've used it for years w/ great results. With the 2572, you can generate any value of output from any input. From 12V input, you can obtain -3V, -5V, -12V, -24V, -9.3V, -17.4V, etc. Are you familiar with SMPS design?

I am working with +5V input right now and would like to stick with just that if possible. I am not familiar with SMPS. I read about it on Wikipedia real quick. For my project I am using a wall plug-in transformer from Radio Shack that has a selectable voltage output (it can go up to +12V at least). I set it to like 6 or 7V and then pass that through a small "power regulation" circuit I have which uses for example the 7805 to give me +5V output (it also uses capacitors to filter, a diode to prevent me hooking it up backwards, and a nice little LED to tell me its on). I believe my wall transformer generate 1000mA current (I am not near it now, and also I think that this may be a "maximum" value right? not like it always makes exactly 1A output)

I will look into the chip you suggest.

#### audioguru

##### Well-Known Member
The 7660 is made by many semiconductor manufacturers. It has a very low output current or has a high voltage drop when the load current approaches 20mA.

#### cmbeelby

##### New Member
The 7660 is made by many semiconductor manufacturers. It has a very low output current or has a high voltage drop when the load current approaches 20mA.

Well the specs do say that 10mA - 13mA is typical. 20mA is a maximum value in the table of data which gives standard operating conditions.

#### Boncuk

##### New Member
How about using a DC/DC converter?

HN Electronic components produces a large range of DC/DC converters for different input and output voltages. Input and output are isolated.

The SIM1-0524-SIL4 (1W) has an input voltage of +5V and puts out 24V/42mA. For negative voltage connect +outV to ground and -outV to the negative rail of your circuit.

The SIM2 series has maximum output voltages of 15V, but the dual outputs can be connected in series. They have 2 and 3W in a 7pin SIL package.

Attached is the data sheet for SIM1 series

Boncuk

#### Attachments

• SIM1-0505-SIL4.pdf
63 KB · Views: 297

#### cmbeelby

##### New Member
How about using a DC/DC converter?

I see you didn't read the linked PDF in my original post. The first page in large letters describes the chip as a "SUPER CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER".

HN Electronic components produces a large range of DC/DC converters for different input and output voltages. Input and output are isolated.

The SIM1-0524-SIL4 (1W) has an input voltage of +5V and puts out 24V/42mA. For negative voltage connect +outV to ground and -outV to the negative rail of your circuit.

The SIM2 series has maximum output voltages of 15V, but the dual outputs can be connected in series. They have 2 and 3W in a 7pin SIL package.

Attached is the data sheet for SIM1 series

Boncuk

Thanks for the information. I tried looking for a similar part on Digi-Key but could not find anything. Also it looks like that part would just give me a higher positive voltage so I would still need to invert that voltage somehow.

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#### Boncuk

##### New Member
Thanks for the information. I tried looking for a similar part on Digi-Key but could not find anything. Also it looks like that part would just give me a higher positive voltage so I would still need to invert that voltage somehow.

... with a slight difference. Input and output are isolated.

So using the positive output voltage inverted (+Vout to ground) results in a negative voltage.

Further the output current is higher than that of a simple charge pump.

If 1W isn't enough for your requirement there are also 2 and 3W types available.

Boncuk

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#### cmbeelby

##### New Member
... with a slight difference. Input and output are isolated.

So using the positive output voltage inverted (+Vout to ground) results in a negative voltage.

Further the output current is higher than that of a simple charge pump.

If 1W isn't enough for your requirement there are also 2 and 3W types available.

Boncuk

Okay, thanks for the clarification. I thought you were saying that this chip that you suggest was also a DC/DC charge pump--or maybe it is, but it is isolated where as the 7660 is not. I think 1W will be plenty of power. Do you know where I can find this part on Digi-Key or where I can buy it in general?

#### Boncuk

##### New Member
Do you know where I can find this part on Digi-Key or where I can buy it in general?

Sorry, I didn't check at Digikey. It's available at Reichelt Elektronik Germany. (reichelt elektronik - OnlineShop für PC-Komponenten, Elektronik - Festplatten, Mainboards, Motherboards, Karten, Kabel) May be you can find it searching for DC/DC-converters.

For +5V input and +24V output at 42mA the order number is SIM1-0524 SIL4.

If 15V are sufficient you might order SIM1-0515 SIL4.

The price is €4.70 (incl. sales tax) Minimum order is €10 at Reichelt.

Boncuk

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