I need to lock the frequency of a PWM IC to an external CLK. The switching frequency of the PWM IC is 120Khz. The PWM IC varies in frequency with temperature. This variation in frequency causes the PWM IC to loose syncronization. I am looking for a PLL that compares the Sync frequency with the PWM frequency and adjust the PWM frequency to the Sync frequency by linearly controlling the gate of a JFET. This JFET will then be used to control the RC time constant RT input of the PWM IC.