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Class G ISSCC 2012

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Class-G Headphone Driver in 65nm CMOS Technology Alex Lollio 1 , Giacomino Bollati 2 and Rinaldo Castello 1 1 Università degli studi di Pavia, Pavia, Italy 2 Marvell Italia, Pavia, Italy 1
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Page 1: Class G ISSCC 2012

Class-G Headphone Driver in 65nm CMOS Technology

Alex Lollio1, Giacomino Bollati2 and Rinaldo Castello1

1Università degli studi di Pavia, Pavia, Italy

2Marvell Italia, Pavia, Italy

1

Page 2: Class G ISSCC 2012

Headphone audio amplifiersTarget application

Typical operating conditions

VIN

VHV

-VHV

Key objectives:

• Low distortion

• Low noise

• High efficiency

• Single ended• RL = 32/16 Ω• BW = 20Hz–20kHz• PO,MAX = 40mW (on 16 Ω)

Modern cellular phones incorporates MP3 music playback and users may wish to use this feature for many hours

2

Page 3: Class G ISSCC 2012

Outline

• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)

• Class-G headphone driver (architecture, switching principle, distortion analysis)

• Prototype in 65nm CMOS technology(implementation, results, comparison)

• Conclusions3

Page 4: Class G ISSCC 2012

Outline

• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)

• Class-G headphone driver (architecture, switching principle, distortion analysis)

• Prototype in 65nm CMOS technology(implementation, results, comparison)

• Conclusions4

Page 5: Class G ISSCC 2012

Class AB (Linear amplifier)

PROs: Best linearity

No EMI problems

CONs: Low efficiency

Typically the preferred solution in headphone application

Class D (Switching amplifier)

PROs: Best efficiency

CONs: Less linearity than class AB

EMI problems

Emerging solution in headphone application

Headphone audio amplifiersAlternative topologies

5

Page 6: Class G ISSCC 2012

Headphone audio amplifiersAlternative topologies

Class G: It is a linear amplifier which uses two voltage supply rails which switches to the appropriate voltage as required by the instantaneous output voltage

PROs: High efficiency but less than class D

High linearity but less than class AB

No EMI problems

CONs: It needs two voltage supply rails

VIN

VLV

VHV

-VLV-VHV

VHV

-VHV

VLV

-VLV

VOUT VOUT

6

Page 7: Class G ISSCC 2012

Class Galternative topologies

Series topology (classical)

Parallel topology

• Only one output stage

• Switches are in series with the power transistors

• Two output stages work in parallel

• No switches in series with the power transistors

• It needs a careful switching circuit design

VHV

-VHV

VLV

-VLV

VHV

VLV

-VHV

-VLV

RL

RL

This is the adopted solution7

Page 8: Class G ISSCC 2012

Class G: working principle

For Vout below the switching point the low voltage stage is active. For Vout above the switching point both the low voltage and high voltage stages drive the load (in different moments).

VHV

VLV

-VHV

-VLV

LV stageHV stage

iHV

iLV

iLV

iHV

iLV

iHV

Iout[A]Iout[A]

iLVt t

Switching point

8

Page 9: Class G ISSCC 2012

Class G: switching distortion

Distortion caused by the switching

Up to the switching point the class G linearity is the same as a class AB

Compared to class AB, class G has an additional source of distortion.

9

Switching point

Page 10: Class G ISSCC 2012

Class G: critical design choices

The implemented current based switching enables low distortion and high efficiency

• Switching point level: To achieve high efficiency, it must be as close as possible to the low voltage supply

Switching point close to low voltage supply

Switching point far from the low voltage supply

• Switching strategy: to minimize the distortion, switching must be as smooth as possible

10

Page 11: Class G ISSCC 2012

Outline

• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)

• Class-G headphone driver (architecture, switching principle, distortion analysis)

• Prototype in 65nm CMOS technology(implementation, results, comparison)

• Conclusions11

Page 12: Class G ISSCC 2012

Overall amplifier architecture

• Three stage opamp with differential input and single ended output.

• The two identical second stages, gm2, and the third stages, gm3L and gm3H, work in parallel.

• Only the low voltage stage gm3L is supplied by the low voltage rail ±VLV. The rest of the circuit is supplied by the high voltage rail ±VHV

gm2

gm2

gm1

-gm3L

-gm3H

Switching stage

R2

R1

R1R2 RL

CM2

CM2CM1

VOUT

Main path

12

Page 13: Class G ISSCC 2012

Amplifier architecture: main path

First stage

Input pairs

gm1

1313

VO

VLV

-VLV

VHV-VHV

Floating battery

VHV

VHV

-VHV

RL

Page 14: Class G ISSCC 2012

Second stage

Amplifier architecture: main path

gm2

14

Floating battery ref: Renirie, Langen, Huijsing, 1995

VO

VLV

-VLV

VHV-VHV

Floating battery

VHV

VHV

-VHV

RL

Page 15: Class G ISSCC 2012

Amplifier architecture: main path

Third stage

LV stage

gm3L

HV stage

gm3H15

RL

VO

VLV

-VLV

VHV

-VHV

-VHV

Floating battery

VHV

VHV

Page 16: Class G ISSCC 2012

Amplifier architecture: switching stageconceptual schematic

PMOS switching

stage

NMOS switching

stage

RL

VO

VOVLV - VTH

-VLV + VTHVO

VLV

-VLV

VHV

-VHV

-VHV

Floating battery

VHV

VHV

Page 17: Class G ISSCC 2012

Amplifier architecture: switching stageconceptual schematic

PMOS switching

stage

RL

VO

VOVLV - VTH

-VLV + VTHVO

VLV

-VLV

VHV

-VHV

-VHV

Floating battery

VHV

VHV

Page 18: Class G ISSCC 2012

Switching principle details

• Switching point sensing is in voltage domain. A differential pair compares the output voltage to the switching point voltage VLV-VTH

• The switching between the high voltage and low voltage output stage is current based.The switching circuit injects all its bias current into the gate of the MOS to be switched off.

VOUT

LV stage

HV stage

iJH

iJL

18

VOUT VLV - VTH

VHV

-VHV

-VLV

VLV

VHV

VHV

IBIAS

PMOS switching stage

Page 19: Class G ISSCC 2012

Output currents during switching

t

Iout[A]

Ou

tpu

t c

urr

en

ts

iLV

iHV

t

VLV -VTH

VLV

Vout[V]

• When VOUT is lower than the switching point (VLV-VTH) the switching circuit enables the LV stage and disables the HV stage

• When VOUT is higher than the low voltage supply VLV only the HV stage drives the load

• When VOUT is between VLV-VTH and VLV both stages drive the load

19

Page 20: Class G ISSCC 2012

Switching distortion:Amplifier model during the switching

• We use a simplified model of the amplifier during the switching.

This current is used to represent the disturbance generated by the switching stage.

gm1 gm2 -gm3

20

RL

VOUT

R1

R1

R2CM1

CM2

IJ

Page 21: Class G ISSCC 2012

gm2f

Cm2s

f

s1

/fs

gm2

1iVout

T

2

T

TJ

Design criteria for distortion reduction

• From the model of previous slide we obtain the equation:

21

Where21

1T

RR

R

Cm1

gm1f

time

ΔVOUT

VOUT

195u 200u

300m

305m

To minimize the distortion at the output we have to minimize ΔVout.

• Lower iJ value means better linearity and lower switching speed

• Higher amplifier bandwidth, fT, means higher linearity

Switching point

Page 22: Class G ISSCC 2012

Outline

• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)

• Class-G headphone driver (architecture, switching principle, distortion analysis)

• Prototype in 65nm CMOS technology(implementation, results, comparison)

• Conclusions22

Page 23: Class G ISSCC 2012

Chip micrograph

• 65nm CMOS process

• 0.14mm2 active area per channel

• Voltage supplies:

High voltage rail ±1.4V

Low voltage rail ±0.35V

• Switching point 50mV under the low voltage supply

• Max load capacitance 1nF

23

Page 24: Class G ISSCC 2012

Measurement results:Power dissipation versus output power

Fin=1kHzRL=32Ω

24

Page 25: Class G ISSCC 2012

Measurement results:THD+N and efficiency versus output power

• Sinusoidal input signal (fin=1kHz)• About 6dB extra distortion due to switching

25

Page 26: Class G ISSCC 2012

Measurement results:THD+N versus frequency

RL=32ΩBW= 20Hz – 20 kHz

26

Page 27: Class G ISSCC 2012

Measurement results: Spectrum at different output power

PO=20mWFin=1kHz

PO=1mWFin=1kHz

27

Page 28: Class G ISSCC 2012

Performance summary and comparison

Parameter This workJSSC 09

[1]

ESSCIRC 06

[2]

Technology 65nm 130nm 65nm

Supply voltage±1.4V

±0.35V

±1V

±0.6V2.5V

Quiescent power (per

channel)0.41mW 1.2mW 12.5mW

Peak load power (16Ω) 90mW 40mW 53.5mW

THD+N @ PRMS (32Ω)-80dB @

16mW

-84dB @

10mW

-68dB @

27mW (16Ω)

SNR A-weighted 101dB92dB (un-

weighted)-

FOM=

Peak load power

Quiescent power

219.5 33.3 4.328

Page 29: Class G ISSCC 2012

Performance summary and comparison

Parameter This workMAX9725

[3]

TPA6141

[4]

LM48824

[5]

Supply voltage

1.4V with two

charge pumps + 1

buck

1.5V with a

charge pump

3.6V with 1

charge pump +

1 buck

3.6V with 1

charge pump +

1 buck

Quiescent power (per

channel)

0.41mW + 0.5mW

(2 CPs + 1 buck)1.57mW 2.16mW 1.62mW

PSUP @ PL=0.1mW 0.87mW + 0.6mW - 4.5mW 3.24mW

PSUP @ PL=0.5mW 1.63mW + 0.8mW - 7.2mW 5.58mW

Peak load power

(16Ω)90mW 50mW 50mW 74mW

THD+N @ PRMS

(32Ω)-80dB @ 16mW -84dB @12mW -80dB @20mW -69dB@20mW

SNR A-weighted 101dB 92dB 105dB 102dB

FOM=

Peak load power

Quiescent power

90 31.8 23.2 45.629

Page 30: Class G ISSCC 2012

Outline

• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)

• Class-G headphone driver (architecture, switching principle, distortion analysis)

• Prototype in 65nm CMOS technology(implementation, results, comparison)

• Conclusions30

Page 31: Class G ISSCC 2012

Conclusions

• A class-G headphone driver has been presented. It shows 50% less power consumption than the competitors.

• The class-G amplifier is very suitable in low voltage systems which require high efficiency and low distortion.

• A class-G headphone prototype with charge pumps and a buck converter is in progress

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Page 32: Class G ISSCC 2012

References

[1] Vijay Dhanasekaran; Jose Silva-Martinez; Edgar Sanchez-Sinencio, "Design ofThree-Stage Class-AB 16Ohm Headphone Driver Capable of Handling Wide Range of Load Capacitance," Solid-State Circuits, IEEE Journal of , vol.44, no.6, pp.1734-1744, Jun 2009.

[2] P. Bogner, H. Habibovic and T. Hartig, ‘‘A High Signal Swing Class AB EarpieceAmplifier in 65nm CMOS Technology,’’ Proc. ESSCIRC, pp.372-375, 2006.

[3] Maxim, ‘‘1V, Low-Power, DirectDrive, Stereo Headphone Amplifier with Shutdown,’’ Rev. 3; 8/08, accessed on Jul. 7, 2009 < http://datasheets.maximic.com/en/ds/MAX9725.pdf>

[4] Texas Instrument, ‘‘Class-G Directpath Stereo Headphone Amplifier,’’ 3/09, accessed on Jul. 7, 2009 < http://focus.ti.com/lit/ds/symlink/tpa6141a2.pdf>

[5] National Semiconductor ”Class G Headphone Amplifier with I2C Volume Control,” August 31,2009, accessed on Jan. 25, 2010 < http://www.national.com/ds/LM/LM48824.pdf >

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