About UC1863, Min fraquency range differ in datasheet.
・Page1：10kHz to 1MHz
・Page4：Please see below
Which is right?
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Operating the UC1863 below the 10 KHz minimum frequency is not a good idea in my opinion. The frequency limits are set and tested by the IC designer. Below the 10 KHz range, you cannot be sure of the accuracy of the frequency output from chip-to-chip. This is why I always stay within the manufacturers guidelines and use their guaranteed specifications.
I have had to design a low frequency VCO for a grid-tie application. Like you, I could not find any ICs with a VCO that has a low frequency capability. I learned from this experience that most of the time, VCOs below 10 KHz are implemented in software with MCUs. If you need a low frequency VCO for a medium to high volume application, I would recommend using a C2000 series MCU and implement the low frequency VCO in software.
If you don't want to do that, you can implement an analog VCO using multiplier ICs. It would be somewhat pricey.
On the other hand, if you are only designing a low number of units, you can experiment with the device to see if it will work at 1.5 KHz. And you are right about the VIO and Temp, they will have an impact on the frequency range and stability.