Universal programmer VPP Actual VPP Voltage vs Rated 25v - TOP853

[weirdaljr]

Hello,

I wanted to see what you thought about this, and see if anyone has one to verify my finds.   I have a TOP853 eprom programmer (among others, this is a cheaper one) that is rated for 25v chips, and had 100s of them in its list.  For the example I will say a 2716-25v.  After noticed my programmer has a lot of problem programming these I probed the programming pin (VPP Pin 21 on the 2716) voltage while programming and found it to be only at ~20.5v while programming.  First thinking it was due to it using the USB bus power I tried again with a USB powered hub and it got to 20.7v.  Now I know some 25v chips may program at 21v but not all, but I was more concerned with why the programmer was not outputting the correct voltage.   Next I opened it up as I was told it had unused connections on the board for a external power supply, and their was.  I connected that to my variable supply and started at 5v and tried up to 5.6v (didnt want to go higher not knowing if the circuit could handle it) and that got the programming vpp up to 20.8v trying to program a 25v chip.   For a comparison I tried testing it on a 21v chip and the voltage is the same (20.6-20.8v depending on power source).

My question is this.   Is this programmer defective (it does program most chips >25v) ?  Or is this the way it was designed claiming it can program 25v chips, but actually only supplies them a VPP of 20.7v hoping it works?

Can anyone else with a top853 verify theirs only outputs ~20.7v for VPP on when trying to program any 21v & 25v chips?

[amyk]

You could see what type of circuit it uses to generate the programming voltage, and measure the output voltage with no load; its impedance might be too high to supply enough current at 25V. The 2716s require ~5mA continuous on the 25V rail, with pulses up to 30mA when programming, and nearly 100mA on the 5V.