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UC1525A: UC1525A

John Haines
Prodigy 10 points
Part Number: UC1525A

While updating a very early design for the 20 pin version surface mount version of this device, I found a small but important difference in from the 16 pin dual inline package in how the device performed. 

I basically had a finished PCB without the outputs of the IC being connected and operating with an 12 V lab supply. When operating at about 100KHz oscillator frequency, the outputs both exhibited a extra 150nS pulse.

All input signals appeared normal. The PWM amplifier was biased so the outputs repained on. So this was basically set up as a breadboard.

After carefully searching for I stumbled across the problem. 

It appears that the engineer in the early 1980's decided to dutifully bypass all unused pins to ground. This included to oscillator output pin which was bypassed with 100pF.After removing the capacitor the IC returned to normal operation. This was surprising since this signal directly drove the input to a flip flop that was supposed to latch the output and prevent double pulsing.

In addition, no differences in the internal oscillator were noted on the Test Data sheet for the part. (SLUS191D Feb. 97)  

/cfs-file/__key/communityserver-discussions-components-files/196/uc3525a.pdf

Now I am wondering what is the difference between the two parts? The Dual inline package had no problems with the 100 pF capacitor on the Oscillator output pin while the 20 pin surface mount package exhibits this double pulsing. 

At least this should be added as a note to the data sheet?

  • 0
    TI__Genius 14580 points

    Thanks for the details. I am looking into it, and get back to you.

  • 0 in reply to Sean Xu
    TI__Genius 14580 points

    Hello, I have a quick question about double pulse you observed. 

    Did you test  both16 pin and 20pin devices?  Was the waveform you provided taken from which device? 

  • 0 in reply to Sean Xu
    Prodigy 10 points

    The waveform was from the 20 pin device. The 16 pin device was in use in products for many years with the same value components so I didn't breadboard the 16 pin device again. 

  • 0 in reply to John Haines
    TI__Genius 14580 points

    From D/S, there is no difference shown between the two packages. I will align your question internally. It may take some time to dig out the root cause, I am going to close it temporarily and keep you posted when I get feedback. Thanks.

  • 0 in reply to Sean Xu
    TI__Genius 14580 points

    Hello, John,

    I got more details about 20 pins and 16 pins.

    Per designer, the two packages use the same die and design, and normally they shouldn’t generate double pulses. From your previous message, you used breadboard to test 20 pin, do you think it is possible the grounding is not solid? The cap instead causes decoupling problem? If possible, you can make a small PCB and solder the device on the board and do further tests.

  • 0 in reply to Sean Xu
    Prodigy 10 points

    Well, I said breadboard, but it was just a new PCB I was running on the bench with a lab supply with no other connections. It has a ground plane on the bottom. All the ground leads on the PCB are less than an inch long so it shouldn't be an issue.

    How about the sync pin? should I leave it open as well? The old design connected it to ground directly. It says in one of the questions here that it should be left open? 

    I'll mess around with it some more and see what I can find. Putting the PCB into a finished power supply seems to work now but there are some stability issues to deal with. 

  • 0 in reply to John Haines
    TI__Genius 14580 points

    Ok, thanks for your clarification. SYNC can be open. It is input pin with high impedance, being tied to ground won't draw much current.

  • +1
    Prodigy 10 points

    Finally got the board working and operating in the power supply. So there is a definite difference between the surface mount and dual inline package IC's. Adding the 100 pF on the osc. output pin to ground makes the double pulse as shown in the attached picture and this is repeatable. No bouncing grounds or anything going on. 

  • 0 in reply to John Haines
    TI__Genius 14580 points

    Thanks for your update, and I gave your comments to IC designer, if he find anything interesting, I will let you know.