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U. Glaeser

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FIGURE 10.25 Push-pull charge pump.<br />

FIGURE 10.26 Current mirror charge pump.<br />

The switches for this charge pump are best placed away from the output toward the supply rails in order<br />

to minimize charge injection from the supply rails to the control voltage. The opposite configuration<br />

can inject charge from the supply rails through the capacitance at the shared node between the series<br />

devices.<br />

A current mirror charge pump is shown in Fig. 10.26. This charge pump tends to have the lowest<br />

input offset due to balanced charge injection. In the limit that a current mirror has infinite output<br />

impedance, it will mirror exact charge quantities; however, because the DN current pulse is mirrored to<br />

the output, it will occur later and have a longer tail than the UP current pulse, which is switched directly<br />

to the output. This difference in current pulse shape will lead to some disturbance to the control voltage.<br />

Another combined approach for the charge pump and loop filter involves using an amplifier-based<br />

voltage integrator. This approach is difficult to implement in most IC processes because it requires<br />

floating capacitors. Any of the above approaches can be modified to work in a “bang-bang” mode, where<br />

the output charge magnitude is fixed independent of the phase error. This mode of operation is sometimes<br />

used with digital feedback loops when it is necessary to cancel the aperture offset of a high-speed interface<br />

receiver [11]; however, it makes the loop control very nonlinear and commonly produces dither jitter,<br />

where the output phase, once in lock, alternates between positive and negative errors.<br />

© 2002 by CRC Press LLC<br />

DN<br />

I REF<br />

V DD<br />

V DD<br />

V CS<br />

DN<br />

UP UP<br />

DN<br />

UP<br />

V CTRL<br />

V CTRL

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