1/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

CML (Current-Mode Logic) driver– Current Steering

– Both side are terminated by 50Ω

– Used in most high-performance serial link

Vinp

Vbias

Vinn

Vdd

Zo

Zo

2Zo

ZoZoRXP

RXN

TXP

TXN

2/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Output driver Specification

– 50Ω

– V_outp = V_outn = 200mV

– Vdiff_pk_pk = 400mV

– Input MOSFET

• Length : 180n

• Width : 20u

– Source MOSFET

• Length : 1u

• Width : 750u

– Vbias

• Voltage : 0.7

– Input_source

• rand_bit_stream

• Tperiod : 1n

• Seed : 1

• Vlogic _high : 1.8

• Vlogic _low : 0

• Vrise : 20p

• Vfall : 20p

– Mtline

• Type of Input : Tline

• Impedance : 50ohm

• Delay Time : 150p

3/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Simulation result

– Receiver output voltage

– Differential peak-peak voltage : 400mV

Rx_outp

diff_out

Rx_outn

400mV

200mV

-200mV

4/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Differential termination

– Impedance matching with differential termination : 2ZO

Vinp

Vbias

Vinn

Vdd

Zo

Zo

2Zo

ZoZoRXP

RXN

TXP

TXN

¼Ⅰ

¾ Ⅰ

Ⅰ

Vd

+

-

_1

_ 0

_

_

( / 4)(2R)

( / 4)(2R)

R

d

d

d pp

d pp

V

V

V

VI

R

5/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Simulation result

– Total RMS current : 8.0 mA (Vdiff_pp 400mV condition)

– Termination RMS current : 2.0 mA (Vdiff_pp 400mV condition)

Total current

Termination current

6/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Simulation result

– Transmission line mismatching

Transmission Line : 50ohm Transmission Line : 75ohm

Reflection

(time delay :300ps)

7/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Replica circuit

– Output swing voltage control

– Negative feedback

Circuit Structure Alternative Circuit Structure

① Vout ↑

③ Id ↑

④ Vout ↓

②Vbias↑

Vdd

VrefAMP

+

-

Vbias

Vdd

Vref

+_

Vss

VCVS circuit 이용Gain : 1000

Max voltage : 1.8

Min voltage : 0

LPF 연결

1Kohm

1nF

Vin+

VCVS

Vout+ - Vout- = Gain * (Vin+ - Vin-)

Vin-

Vout+Vout+

Vout-

Vbias

8/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Schematic design

9/26고속 SERIAL INTERFACE 회로및시스템

Current-Mode Driver

Simulation result

V_diff_pp : 400mV

Total RMS current : 16.0 mA

V_diff_pp : 500mV

Total RMS current : 20.0 mA

10/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Voltage mode driver

Zo

Zo

2Zo

RXP

RXN

TXP

TXN

DP

DN

VZN

VZP

VDRV

M1

M2

M3 M5

M4

M6 Ron = 1

μ𝑛𝐶𝑜𝑥𝑊/𝐿(𝑉𝑔𝑠−𝑉𝑡ℎ)

11/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Output driver Specification

– 50Ω

– Vdiff_outp = 200mV

– Vdiff_pk_pk = 400mV

– Input MOSFET

• Length : 180n

• Width : 30u

– Current source MOSFET

• Length : 180n

• Width : 800u

– VZP bias

• Voltage : 935mV

– VZN bias

• Voltage : 495mV

– VDRV

• Voltage : 400mV

12/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Simulation result

– Output swing voltage

Rx_outp

Diff_out

Rx_outn

200mV

300mV

100mV

400mV

200mV

-200mV

13/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Differential termination

– Impedance matching with differential termination : 2ZO

_1

_ 0

_

_

( / 2)

( / 2)

4

d s

d s

d pp s

d pp

V V

V V

V V

VI

R

14/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Simulation result

– Total RMS current : 2.0 mA (Vdiff_pp 400mV condition)

– Termination RMS current : 2.0 mA (Vdiff_pp 400mV condition)

Total current

Termination current

15/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Replica circuit

Circuit Structure Alternative Circuit Structure

VDD

VDD

VDRV

2ZO

3/4VDRV

1/4VDRVVZN

VZP

M1

M2

M4

M6

ZO

ZO

+_

VDD

VDD

VDRV

2ZO

3/4VDRV

1/4VDRV

VZN

VZP

M1

M2

M4

M6

ZO

ZO

Vss

Vin+

Vin-

+_

Vin+

Vin-

Vout+

Vout-

Vss

Vout+

Vout-

1nF

1Kohm

1nF

1Kohm

16/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Reference voltage circuit

Circuit Structure Alternative Circuit Structure

VMDRVReplica DRV

VDDVDD

VDRV

REF_VDRV

VMDRVReplica DRV

VDD

VDRV

+_

REF_VDRV

Vss

Vin+

Vin-

Vout+

Vout-

1nF

1Kohm

17/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Schematic design

18/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Simulation result

V_diff_pp : 400mV

Total RMS current : 4.0 mA

V_diff_pp : 500mV

Total RMS current : 5.0 mA

19/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Simulation result

– Transmission line mismatching

Transmission Line : 50ohm Transmission Line : 75ohm

Reflection

(time delay :300ps)

20/26고속 SERIAL INTERFACE 회로및시스템

Voltage-Mode Driver

Simulation result

– Compare current mode and voltage mode current.

– Current mode output driver

• RMS current : 16.0 mA (Vdiff_pp 400mV condition)

– Voltage mode output driver

• RMS current : 4.0 mA (Vdiff_pp 400mV condition)

Current mode output driver Voltage mode output driver

21/26고속 SERIAL INTERFACE 회로및시스템

Exercise

Current mode and voltage mode output driver

– Design Vpk_pk 600mV Current and voltage mode driver with 50ohm

impedance.

• Derive mosfet size.

• Plot the output waveform and current waveform.

• Compare and analyze between current mode and voltage mode output

driver.

• Plot the eye diagram in case of transmission line impedance 75ohm and

50ohm. (Differential Rx termination 50ohm condition)

• Plot the eye diagram in case of differential rx termination 75ohm and

50ohm. (Transmission line impedance 50ohm condition)

22/26고속 SERIAL INTERFACE 회로및시스템

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Q & A