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LMK04803: PLL2_CP_POL and PLL1_CP_POL

Hirotaka Matsumoto
Guru 27585 points
Part Number: LMK04803

Hi all

Would you mind if we ask LMK04803?

<Question1>
On the datasheet P88, there is the description, 
"8.6.3.13.3 PLL2_CP_POL, PLL2 Charge Pump Polarity
 PLL2_CP_POL sets the charge pump polarity for PLL2. The internal VCO requires the negative charge pump
 polarity to be selected. Many VCOs use positive slope.
 A positive slope VCO increases output frequency with increasing voltage. A negative slope VCO decreases output frequency with increasing voltage."
In case of internal VCO, why does it require negative slope VCO?

<Question2>
On the datasheet P90, there is the description,
"PLL1_CP_POL sets the charge pump polarity for PLL1. Many VCXOs use positive slope.
 A positive slope VCXO increases output frequency with increasing voltage. A negative slope VCXO decreases
 output frequency with increasing voltage."
Does it mean it depends on VCXO's charge pump polarity?


Kind regards,

Hirotaka Matsumoto


  • +1
    TI__Mastermind 33790 points

    Hello Matsumoto-san,

    1. The internal VCO uses a varactor diode to tune the voltage of the tank circuit. The varactor diode orientation (which side points to Vtune, which side points to tank circuit) dictates the polarity, as we want to use the varactor in reverse-bias to tune the capacitance (and therefore the oscillation frequency) at the tank circuit. For performance-related reasons specific to our process and our tank circuit design, it makes the most sense to orient the diode so that the anode points to Vtune, and the cathode points to the tank circuit. So a decreasing Vtune voltage will increase the varactor bias voltage, which decreases the capacitance, which increases the tank circuit resonant frequency. In summary, the internal VCO tuning voltage is negative slope because this gives us the best performance with our process and circuit design. Other companies may design external VCOs with positive or negative tuning slope, and since we want to support external VCXOs with LMK04803, we support control of both positive and negative tuning slope in the charge pump with the polarity bit.
    2. VCXO manufacturers will build VCXOs with positive or negative tuning slope depending on what makes the most sense for their process and circuit design. At lower VCXO frequencies, the tuning circuit may look very different from GHz VCOs. Nearly every VCXO I've ever seen is positive slope. I'm not sure why, this would be a question for a VCXO manufacturer. Just like with the VCO case in PLL2, we support control of both positive and negative tuning slope in the charge pump of PLL1 with the polarity bit.

    For both charge pumps, set the polarity of the charge pump based on the oscillator tuning slope polarity. Positive slope, positive polarity charge pump. Negative slope, negative polarity charge pump.

    Regards,

    Derek Payne