Fairchild Semi has obsoleted a 74hc4046A chip on a legacy product of ours and I need a drop in replacement. I have tried to drop in the TI SN74hc4046A chip but now the VCO runs at roughly 287 KHz rather than 153.6 KHz. Nothing else has changed in the circuit. Both datasheeta are closely comparable. What differences are there between the two chips that may casue this problem?
The oscillator may may work require an R/C adjustment to match the old frequency. They could also try the SN74LV4046 to see if the oscilator comes closer to matching the frequency of the fairchild part.
As I recall, even though they have the same part number, the Fairchild 74HC4046A is different than all the other vendor's 74HC4046A. It has differences in several key specifications. The specific one that I remember is that its maximum frequency is lower. There may not be an exact drop in replacement for it. You will likely need to adjust the R and C as Chris has suggested.
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