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HMC700LP4 / 700LP4E

v11.0411

8 GHz 16-Bit Fractional-N PLL

Charge Pump Gain

a simplified diagram of the charge pump is shown in Figure 14. charge pump up and down gains are set by

two 500ua current sources and one 1000ua current source. the current gain of the pump in radians/amp is equal to

the gain setting of this register divided by 2π. For example if both cp_UPcurrent_sel and cpDNcurrent_sel are set to

’010’ the output current of each pump will be 1ma and the phase detector gain Kp = 1ma/2π radians, or 159ua/rad.

Charge Pump Gain Trim

Each of the UP and Dn pumps may be trimmed to more precise values to improve current source matching or to allow

finer control of pump gain. the pump trim controls are 4bits, binary weighted for UP and Dn, in cp_UPtrim_sel and

Charge Pump Phase Offset

ideally the phase detector operates with zero offset, that is, the divided reference signal and the divided Vco signal

arrive at the phase detector inputs at exactly the same time. in some modes of operation, such as fractional mode,

charge pump linearity and ultimately, phase noise, is better if the Vco and reference inputs are operated with a phase

offset. normally integer mode of operation is best with no phase offset. a phase offset is implemented by adding a

constant Dc leakage to one of the charge pumps. Dc leakage may be added to the UP or Dn pumps using chp_

UPoffset_sel or chp_DNoffset_sel. these are 3 bit registers with 55µa LSB. Maximum offset is 385ua.

For best spectral performance in Fractional Mode the leakage current should be programmed to: required Leakage

current (ua) = (4E-9 + 4xtvco) x Fcomparison (Hz) x cP current (ua)

Charge Pump Operation Near the Rail

it should be noted that the charge pump is a non-ideal device. Phase locked operation with the tuning voltage very

near the positive charge pump supply voltage or very near ground will degrade the phase noise performance of the

synthesizer. Exactly how close to the supply limits that one should operate is a question of margin needed for the

limits. it should be noted that if operation is necessary very near the supply limits, for example less than 500mV

from the supply limit, then it is recommended to operate with a Dc leakage that leaks in the direction of the supply.

For example, if the charge pump supply is 5.5V and locked operation is required with a Vco tune voltage of 5.2V,

then operating with UP leakage on the charge pump will improve operation in this region. Similarly if phase locked

operation is needed, with a Vco tune voltage of say 300mV, then operating with Dn leakage is recommended.

as an example, if the main pump gain was set at 1ma, an offset of 385ua would represent a phase offset of about

(385/1000)*360 = 138degrees. normally it is sufficient to offset the pump by just slightly larger than the delta sigma

excursions for best phase noise. Best spurious operation usually occurs with Dn offset with non-inverting loop filters

and UP offset with inverting loop filters.

Figure 14. Charge Pump Gain & Offset Control

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