Self-Biased, High-Bandwidth, Low-Jitter 1-to-4096 Multiplier Clock

Generator PLL

Based on a presentation by:

John G. Maneatis1, Jaeha Kim1, Iain McClatchie1,Jay Maxey2, Manjusha Shankaradas2

True Circuits, Los Altos, CA1

Texas Instruments, Dallas, TX2

PDF file of JSSC paper linked at class web page.

2

Clock Generator PLLs for ASICs

Most ASICs PLLs for clock generation, but … Use different frequencies and multiplication

GraphicsProcessor

NetworkProcessor

I/OController

YourASIC

N

FREF FOUTPLL

3

Optimal PLL Design

For each FOUT and N, one must adjust loop parameters for both minimum jitter and stability

For clock generators (track input clocks)(REF = 2·FOUT/N)

Loop bandwidth :N ~ REF/20

Damping ratio : ~ 1 Third-order pole :C ~ REF/2

Circuit parameters (e.g. ICH, R) must vary with FOUT

and N!

4

Addressing Diverse Specifications

Designing a different PLL for each ASIC Easier to meet the specification, but … Verifying all designs is difficult and costly

Our Goal: One PLL design for all ASICs Only one design needs verification, but … Loop parameters must adjust automatically

to satisfy wide range of FOUT and N

5

Challenges

Self-biased PLLs [Maneatis ‘96] adjust for FOUT

Achieve fixed N/REF and indep. of PVT

But, Self-Biased PLLs do NOT adjust for N N/REF and vary with N (want fixed)

C/REF varies with N (want fixed)

This talk extends Self-Biased PLLs for wide ranges of N with a new loop filter network

6

Outline

Introduction Review of Self-Biased PLLs Pattern Jitter Issues Loop Filter Architecture Implementation of Key Circuits Measured Results Conclusions

7

Second-Order PLLs

PFDCPICH VCTL

N

CKOUT

CKREF

CKFB

UP

DN

C1

RVCOKV

2NN

N

I

O

)/s()/s(21

)/s(21N

)s(P

)s(P

1CR21

N 1

VCH C1KI

N1

N

8

Second-Order PLLs

PFDCPICH VCTL

N

CKOUT

CKREF

CKFB

UP

DN

C1

RVCOKV

)1

( RCS

IV CHCTL

Oscillation frequency is supposed to be controlled by VCTL, that is by ICH/CS + ICH*R.In Ring Oscillators, frequency is more easily controlled by current, through tail biasing voltage. want tail current to have two components: ICH/CS + ICH*R.

9

Self-Biased PLLs

PFD

CPxID

CPxID

VCOKV

=k/CB

VCTL

VBN

N

CKOUT

CKREF

CKFB

UP

DN

VFF

Replica-FeedbackBiasing

C1

1/gm ID

mg/1R DCH IxI BmVCO CgF

)1

( RCS

ICH

VBN biases the tail current in Ring Oscillator buffer stages.

So want ID to have components:

10

Self-Biased PLLs

PFD

CPxID

CPxID

VCOKV

=k/CB

VCTL

VBN

N

CKOUT

CKREF

CKFB

UP

DN

VFF

Replica-FeedbackBiasing

C1

1/gm ID

Op Amp adjust VBN so that NMOST ID matches currents in 1/gm and that from top charge pump.Notice: VCTL = VDD – ICH / sC1 dID = (VDD – VCTL)gm + ICH-ff = ICH (1/sC1 + 1)

Hence VBN will generate I-tail proportional to ICH (1/sC1 + 1)

11

Maneatis self bias generator

From 1996 Maneatis paper, which is very widely reference.PDF file linked at web page.

12

Self-Biased PLLs

With Self-Biased PLLs

N/REF and are constant with FOUT,

BUT not with N

Nx~CCNx21

1BREFN

Nx~CCNx41

B1

13

Pattern Jitter / Spurious Noise

Phase corrections every rising reference edge can cause disruptions to nearby output cycles Periodic noise pattern repeats every ref. cycle

or N output cycles

Typical causes Charge pump imbalances or leakage Jitter in reference clock (aperiodic result)

CKREF

VCTL

CKOUT

SHORT

14

Shunt Capacitor

Use third-order pole to extend disturbance with reduced amplitude over many output cycles

Problem with varying N using fixed capacitor Extended number of cycles NOT function of N Too few for large N Pattern jitter Too many for small N Instability

FILTERED

CKREF

VCTL

CKOUT

15

Proposed Loop Filter

Use switched capacitor filter network to Output scaled amplitude error signal with N

output cycle duration [Maxim ’01]

Want a simple solution using this approach that is compatible with Self-Biased PLLs

FILTERED

CKREF

VCTL

CKOUT

16

Original Filter Network

Only need to filter feed-forward path

VCTL

VBN

VFF

Replica-FeedbackBiasing

C1

CP

CP

UP

DN

1/gm

17

Sampled Feed-Forward Network

Sample phase error and generate proportional current that is held constant for N·TOUT

Sampled error is reset at end of ref. cycle Need VRST = VCTL as zero bias level

gm

VCTL

VBN

VFF

Replica-FeedbackBiasing

C1

1/gmC2

CP

CP

UP

DN

VRST

18

Complete Filter Network

Reset C2 to VCTL directly

Eliminates C1 charge pump

Equivalent feed-forward control gain

QO ~ N · QI

gm

VCTL

VBN

VFF

Replica-FeedbackBiasing

C1

1/gmC2

CPUP

DN

19

Loop Dynamics

With this new loop filter network we achieve

Need to keep N/REF and constant with N

Just scale charge pump current with 1/N (=x)

More detailed analysis will show

Both are independent of FOUT, N, and PVT!

Nx~REFN Nx~NxQQ~ IO

1B CC21

REFN

21B CCC41

20

Complete Self-Biased CGPLL

PFD

CP1

CP2

VCO

Charge Pump Bias Gen.(Prog. 1/N Current Mirror)

VFS1

VFS2

VCTL

VBN

VBC

VBC

ClockDivider

CKOUT

CKREF

CKFB

Divide Ratio(N=1...4096)

UP

DN

VBP

gm(VFS1+VFS2)

VFF 1/gm

Loop Filter

Replica-FeedbackVCO Bias Gen.

C2

C1

C2

21

Self-Biased Filter Network

CP1

CP2

VFS1

VFS2

VFF

VCTL

C2

C1

C2

S1

S2

SelectControl

UP

DN

en

en

VBN

VBN

22

Filter Network Reset Switches

Can switch to VCTL independent of voltage level

VBN

VBR VCTL

SEL_B SEL

VCTLVFS

VCTL

VDD+VCTL

sel_boosted

23

Filter Network Reset Switches

When un-selected, sel = 0, sel_B = 1 (VDD). V_B = VDD, V_D = VDD + V_CTL, V_A = 0, V_C = V_BR = V_CTL.

V_CA charged to V_CTL

VBN

VBR VCTL

SEL_B SEL

VCTLVFS

VCTL

VDD+VCTL

sel_boosted

A B

C D

24

Filter Network Reset Switches

When selected, sel_B = 0, sel = 1 (VDD). V_A = VDD, V_C = VDD + V_CTL, V_B = 0, V_D = V_BR.

V_DB charged to V_BR or V_CTL

VBN

VBR VCTL

SEL_B SEL

VCTLVFS

VCTL

VDD+VCTL

sel_boosted

A B

C D

25

Inverse-Linear Current Mirror

Need to generate ICH = ID / N

Use switches to adjust device size on input side For N=1~4096, need 12 binary weighted legs Need size range of 2048:1 Too much area!

IOUTIIN

VBD

S0S1S2S3S4S5

x2x4 x1x8x16x32

26

Multi-Stage Linear CM

Solution to size problem with LINEAR control Use multiple device groups operating at

different but ratioed current densities Can have large ranges using small devices

IOUTIIN

VBDS3S4S5

8:1

S0S1S2

x2x4 x1x2x4 x1

27

Multi-Stage Inverse-Linear CM

Just diode connect multi-stage linear current source and use as input side of current mirror Can output gate bias of any device group Stable as long as gain blocks reduce currents

IOUTVBD

S3S4S5

8:1

S0S1S2

x2x4 x1x2x4 x1

IIN

28

Complete Current Mirror

VBN

IOUT

VBC

8:18:18:1

S9S10S11 S6S7S8 S3S4S5

IIN

E3

E3

E2

E2

(N = 1 … 4096)

IOUT

IIN 1

N

S0S1S2

x2x4 x1 x1

VBD

29

Voltage-Controlled Oscillator

VFF

VCTL

VBN VBN

VBP

VBP

VI-VI+VO+VO-

VTAIL

Buffer StageVCO Replica-Feedback Bias Generator

11-Stage Ring Oscillator

VBN

VBP

CK+

CK-

CK+

CK-

VREP

30

V

VDS

Higher VDD

V

IL IHID

V

V

VTAIL

VREP

time

Buffer Tail Node Matching

31

Modified VCO

VFF

VCTL

VBN VBN

VBP

VBP

VI-VI+VO+VO-

VTAIL(SHARED)

Buffer StageVCO Replica-Feedback Bias Generator

11-Stage Ring Oscillator

VBN

VBP

VTAIL

CK+

CK-

CK+

CK-

32

Static Supply Sensitivity

32

33

PLL Implementation

Process Technology 0.13m N-well CMOS

Nominal Supply Voltage 1.5V (designed for 1.2V)

Total Occupied Area 0.38 x 0.48mm2

VCO Frequency Range 30 ~ 650 MHz

Multiplication Factor Range N = 1 ~ 4096

Power Dissipation 7mW @ 240 MHz, 1.5V

34

Measured Jitter vs N (240MHz)

35

PLL Jitter Measurement Summary

36

Conclusions

Proposed PLL achieves wide N and FOUT range

PLL is self-biased with constant loop dynamics (N/REF, ), independent of N, FOUT , and PVT

Sampled feed-forward network suppresses pattern jitter with effective C that tracks REF

Achieves relatively constant period jitter of less than 1.7% as N is scaled from 1 to 4096

37

References J. Maneatis et al., “Self-biased high-bandwidth low-jitter 1-to-4096

multiplier clock generator PLL,” in IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, Feb. 2003, pp. 424–425.

J. Maneatis, “Low-jitter process-independent DLL and PLL based on self-biased techniques,” IEEE J. Solid-State Circuits, vol. 31, pp. 1723–1732, Nov. 1996.

A. Maxim et al., “A low-jitter 125–1250 MHz process-independent 0.18 m CMOS PLL based on a sample-reset loop filter,” in IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, Feb. 2001, pp. 394–395.

T. C. Lee and B. Razavi, “A stabilization technique for phase-locked frequency synthesizers,” in Symp. VLSI Circuits Dig. Tech. Papers, June 2001, pp. 39–42.

J. Maneatis and M. Horowitz, “Precise delay generation using coupled oscillators,” IEEE J. Solid-State Circuits, vol. 28, pp. 1273–1282, Dec. 1993.