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Browse Prior Art Database

Differential Charge Pump

IP.com Disclosure Number: IPCOM000119676D
Original Publication Date: 1991-Feb-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 75K

Publishing Venue

IBM

Related People

Gillingham, RD: AUTHOR [+3]

Abstract

A charge-pump circuit is described which has differential logic inputs and drives a loop filter with differential current pulses. The circuit is applicable to phase-locked loops or delay-locked loops used for clock generation or other control systems.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Differential Charge Pump

      A charge-pump circuit is described which has differential
logic inputs and drives a loop filter with differential current
pulses.  The circuit is applicable to phase-locked loops or
delay-locked loops used for clock generation or other control
systems.

      Referring to the figure, +CU and -CU are one pair of
differential digital inputs and +CD, -CD are another.  A reference
voltage, NREFL, biases all of the DC current-source transistors,
Q5-Q12 and their corresponding bias resistors, R5-R12.  During the
majority of the time there is neither a charge-up or a charge-down
pulse.  During this time, Q2 and Q3 are turned off, and R13 and R14
pull-up nodes +FILT and -FILT.  For a charge-up pulse, +CU goes high
and -CU goes low.  The constant current in the Q6 collector is
switched through Q3 to pull down the -FILT node. Similarly, a
charge-down pulse switches the constant Q5 collector current through
Q2 to discharge the +FILT node.

      There would be a differential current due to the different bias
at nodes +FILT and -FILT; however, the feedback loop alters the
voltages at the Q13 and Q14 emitters to keep the DC voltage drops
across R13 and R14 constant.  This also keeps the R13 and R14
currents constant.

      The loop works as follows.  The differential voltage developed
across the filter is applied to a differential stage, Q15-Q16.  The
Q15-Q16 collector voltages are feed-back to the bases of Q13-Q14.
Therefore, if the +FILT voltage rises, Q16 carries more current, Q15
carries less current.  The base and emitter of Q14 fall while the
base and emitter of Q13 rise.  If the gain of the loop is one, the
voltag...