CD74HC4046A frequency drift with temp

Jaison Joy

Other Parts Discussed in Thread: CD74HC4046A

Hello,

I did a prototype of CD74HC4046A (DIP package) for one FSK application with center frequency 30KHz. It was found working well. But while testing in the final PCB with 16TSSOP package, it is found the frequency is drifting. And when operated continuously for 7 hours it is found an increase in frequency of 1.4 KHz @ room temperature.

It is interesting to see the frequency drift when a hot object is holding near to CD74HC4046APW.

Please suggest any solution to have a constant operating frequency in the specified temp range (-55 to 125 degreeC) since the device has to work in industrial environments.

Thanks

Jaison

over 11 years ago

0 C. Ryan Land over 11 years ago

TI__Genius 9860 points

Hi Jaison,

What resistor value are you using for R1?

-Ryan

0 Jaison Joy over 11 years ago in reply to C. Ryan Land

Prodigy 185 points

Hi Ryan,

R1 is 80K. (R2 = 8.2K and C1=333nF, just for your information)

Another thing is, for testing purpose I am keeping the board continuously ON (sending FSK data @2400bps). Will that cause the IC go saturated or increase its temperature?

Thanks

Jaison

0 Jaison Joy over 11 years ago in reply to Jaison Joy

Prodigy 185 points

Hello Ryan,

I kept CD74HC4046 (TSSOP package) ON for two days, and it is found malfunctioning, becuase there was a big change in frequency. But when I touched the IC, it started working, as if my finger cooled the IC.

By the way, C1 is 33nF (Previously I typed 333nF by mistake).

Looking for a solution soon.

Thanks

Jaison

0 C. Ryan Land over 11 years ago in reply to Jaison Joy

TI__Genius 9860 points

Hi Jaison,

I apologize for the late reply. Have you tuned your circuit so that the value of R1 is lowered to around 3K? Figure 22 on the datasheet shows a more flat performance of frequency across temperature using an R1 around 3K.

If this does not solve your problem, then I will look for another part that has better performance across temperature.

-Ryan

0 Jaison Joy over 11 years ago in reply to C. Ryan Land

Prodigy 185 points

Hi Ryan,

I will try out R1 value and get back to you.

Thanks

Jaisn

0 Jaison Joy over 11 years ago in reply to Jaison Joy

Prodigy 185 points

Hi Ryan,

Tried R1 with different lower values down to 3K. Still the same problem. Also CD74HC4046(TSSOP pakage) is getting heated up.

Also removed R2 and tested, still no improvement.

Power supply is 5V and the device consumes around 100mA.

Will HCT variant perform better than this HC type?

0 Jaison Joy over 11 years ago in reply to Jaison Joy

Prodigy 185 points

Jaison Joy said:

Power supply is 5V and the device consumes around 100mA.

It was because of pin1 and 2 short, it was hardly visible in this TSSOP.

But still frequency drift issue is there. VCO out is given to +/-12V supplied filter and driver sections. Will that cause that issue?

0 Jaison Joy over 11 years ago in reply to Jaison Joy

Prodigy 185 points

Hello Ryan,

Could you please help me find another part that has better performance across temperature?

Thanks,

Jaison

0 C. Ryan Land over 11 years ago in reply to Jaison Joy

TI__Genius 9860 points

It might be a better idea to use a temperature compensated frequency oscillator crystal. I looked at Euroquartz and the EQXO-1000GW has good stability over operating temperature range.

0 Jaison Joy over 11 years ago in reply to C. Ryan Land

Prodigy 185 points

How can I use crystal oscillator with CD74HC4046A ?

0 C. Ryan Land over 11 years ago in reply to Jaison Joy

TI__Genius 9860 points

Hi Jaison,

You can use the crystal instead of the VCO as the input to the PLL of the part. The temperature stability of the crystal should help stabilize your FSK application.

www.ti.com/lit/an/scaa088/scaa088.pdf

www.ti.com/lit/an/scha003b/scha003b.pdf

0 Jaison Joy over 10 years ago in reply to C. Ryan Land

Prodigy 185 points

Thanks Ryan,

I will study this and will implement it.

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CD74HC4046A frequency drift with temp