Hello,

I am using the MAX607ESA DC to DC convertor. My circuit is exactly as the one shown on P.7 of the spec. I need to send a binary high or low signal to the pin 4 input of this device. I need my high/low voltage range to be approximately 0.002 (low) to 5 (high) VDC.

The source of the signal that is to go to this input comes from a CPLD which outputs 3.3VDC only. The CPLD is the EPM240T100C5N device from Altera.

I have tried to connect the CPLD output to a 1K resistor and then to the gate of a 2n2222 transistor. The collector of the transistor connects to a 270ohm resistor and the other side of the resistor connects to a +5VDC power source.

When my CPLD ouputs a low, I get 4.9 VDC at the collector. When my CPLD is high, I get .002 VDC which is a typical low. So now everything works fine.

The moment I connect the collector to pin #4 of the DC to DC convertor input, my 4.9 VDC at the collector becomes 2.92 VDC.

Why?

All help very appreciated.
PS I am not to experienced in this sort of stuff... any guidence can be very helpfull to me. Thanking all in advance!

Ross

 

You didn't make it any easier by omitting the datasheet of the MAX607ESA.

Basically, you're trying to supply the current to run the DC-DC converter through a 270 Ohm resistor. That won't work well, as the resistor will limit the input current too much.

Do it like this instead:

Q1 turns on Q2 via the 200 Ohm resistor, which limits base current to around 20mA. The 2.2k resistor keeps Q2 turned off when Q1 turns off.
Q2 sources the current to the MAX607 pin 4. It'll handle up to perhaps 300mA current wired this way.

 

I haven't used this part, but it appears that the correct way to switch the output on and off is to provide +5V to pin 4 and switch the part on and off with pin 3. It appears you can do this directly from the CPLD.

 

Thanks SgtWookie,

Right now I can't try your circuit because I don't have Q2 in hand. I only have a 2n3904. So I will go and purchase a 2n2907 PNP transistor and try it.

I really appreciate your help. Thank you!

Ross