Leakage reduction techniques

Mohammad Sharifkhani

Introduction

• Leakage current is important in– Standby mode: no T. activity– Active mode: Static units: (e.g., SRAM cells)– Active mode: Non-critical path logics

• It is the current that does not do anything for us

The lower the better

Power components (revisit)

• Speed

• Energy Battery lifetime

• Instantaneous power Package, cooling

The leakage power is a function of Vth, VDD and transistor size.

Power Down

• Complexity:– Cellular phone: checks

base-station every sec. in cell waiting mode partitioning the design

• Floating output nodes between 0.7V/0– High

leakage in subsequent stage pull-downer area, power

Power Down

• 100uSec settle time for power up. Windows-CE demands 1uSec

• On board level:– Turn off: All input signals to a chip 0;

otherwise short through ESD. Then turn off chip VDD

– Turn on: Reverse order. Except for the active low inputs which may disrupt the operation (e.g., CE, OE)

– Un-conventional power up/down methodology

Power Down

• High Vth Sleep T.– Multi-Threshold MTCMOS

• Both VDD and Virtual VDD Area

• Sizing Sleep T.:– Speed: virtual VDD bounce

Large enough– Discharge pattern virtual

VDD bounce Delay– VDD↓ Larger W/L; @

VDD=0.7 super cut-off T. SCCMOS

Power Down

• Example Input Pattern: 8x8 Multiplier speed penalty <5%– Second pattern W/L=60, First pattern W/L=170

• Right pattern is hard to find– In consistent with conventional CMOS design

Power Down

• High Vt Sleep T.– Does not operate at Vdd<0.7V

• Super cut-off transistor (SCCMOS)– Instead of high-Vt transistor, a regular

transistors is used for Sleep T.– The gate of the Sleep T. is connected to

Vdd+0.4V during cut-off– Operates at lower voltages (<0.7V)

Layout

Standard Cell implementation

Multiple Vt

• Multiple Vt is a common standard today

• It can be used in – Static CMOS– Domino

• It can be used as– In block level (Sleep T.)– Circuit level

Dual Vt for Domino

Preserving State

• Virtual supply collapse in sleep mode will cause the loss of state in registers

• Putting the registers at nominal VDD would preserve the state– These registers leak

• Can lower VDD in sleep– Some impact on robustness, noise and SEU

immunity

Low-leakage FF w. Sleep

High Vt

Stacking

Stacking

Stacking

3 Challenges

• High standby current in low Vth

• IDDQ testing failure

• Degradation of worst case speed due to Vth variation @ low Vth– Vth scaling to keep delay

constant: for 3V => 2V change 25% Vth reduction is needed

Vth controlling

• Solves all three problems together– Variable threshold CMOS (VTCMOS)– Can be used as a low-voltage (low active power)

method

• Two main blocks:– Leakage current monitor (LCM)– Self substrate bias (SSB)

• Two major schemes:– Self-adjusting threshold voltage (SAT)– Standby Power Reduction (SPR)

VTCMOS

• The body of the T. is the key controlling knob.– All properties of CMOS is carried over

(unlike Power Down) – Much smaller current flows through

Substrate. – Slow turn off, Fast turn on

• <0.1um Sec. good for Windows-CE

Self-Adjusting Threshold-Voltage Scheme (SATS)

SATS Experimental Results

VTCMOS

VTCMOS (LCM+SSB)

• Two sets of circuits:– Feedback control of SSB, LCM– Switch between Standby and Active

• LCM controls SSB when ILeak and IRef don’t match

• Controls the average Vth on the chip

VTCMOS (LCM+SSB)

• SSB is turned on and off intermittently and sets Vth– Real time and process independent

• LCM design criteria

• Large ILCM High power

• Too small ILCM slows LCM response speed; Vth unevenness and longer periods of Vth control hence error

VTCMOS (LCM+SSB)

• Typical values:– ILCM = 1uA, when ILeakage chip = 1mA,

XLCM =0.1%.– Given Vb=0.2V and S (the slope of

leakage current)= 0.1V/Dec W LCM T. = 0.001% of W leakage, chip

• Design of Vb– W1,2 @ subthreshold

VTCMOS (LCM+SSB)

• Hence:– XLCM set by transistor ration; independent of

VDD, Temp., process variation – Consider deriving Vb using a resistive V.

divider:• Dependency on all above

– @ corners X does not vary more than 15% sufficient tolerance to keep Vth in check

Substitution in the previous equation yields:

VTCMOS (SPR)

• There is an Standby Signal• Active: M1 on VBB:VSS• Standby: M1 off

– First: D1 off (N2 turns on M1)– SSB pumps out current (N2 v.

drops)… until M1 turns off– D1 only turn on (SSB connected

to VBB) when N2<-0.7V (M1 is OFF)

– When D1 is on; SSB pulls down VBBn

– M3?– M3 prevents M2 from over

voltage (drain of M2 < VSS+Vth)

VTCMOS (SPR)

• Going back to Active:– M2 turns on (St’by low)– LCM, SSB disabled– N2 charges up through M2, M3– M1 turns on

• Over voltage on M1 gate • Reliability

– D2-4 is to solve it • Clamp: VGS <1.8V (N2 v.

restricted)

– When VBB is VSS –Vth: NMOS cuts the clamp; M1 gate jump to VDD

VTCMOS (Performance/Penalty)

• Vth controllability– Sample: 40 chips

• Some with Vth=0.15• Some with Vth=0.05• Two gray distributions

– Each: 3 scenario• Vth conv.• Vth active mode• Vth standby mode

VTCMOS (Performance/Penalty)

• Standard deviation↓– +/- 0.1V +/-0.05V– Raise average Vth to

0.25 @Stby

• Temp. Dependency Reduction ↓– Mobility ↓ as T ↑ – 2mV/oC 0.7mV/oC

VTCMOS (Temp.)

• @ Sub 1V, Vth is small (speed)– Leakage current may dominate

– T↑ , I↑ T↑ , I↑ : Positive feedback– A leakage monitor is necessary

VTCMOS (Power/Area)• IVTCMOS = ISSB + ILCM

– ISSB independent of switching activity only a function of impact ionization (VDD and W chip) ISSB can be 1% of total power;

• Active: I pwell very small (<0.1% of total power)– ILCM must be large

enough (3uA) (VBB control)

• Energy consumption due to substrate V. variation– 5nF for 10mm2; 3V voltage variation

0.05uJoul– Suppose every 25mSec 2uW (Good only if

not very frequent)

• Area penalty– VTCMOS 1%– Separation of substrates 6% (substrate

contacts)

VTCMOS (Power/Area)

Substrate noise

• Variation of the VBB can influence the substrate noise– Noise affecting the SRAM stability– Noise affecting the jitter

• Same substrate for a DLL and an SRAM• SRAM shmoo plot no significant

differnce• DLL jitter almost the same

– In inverter chain only one inverter is operating

VTCMOS (within die Vth var.)

• In deep submicron, Vth is a VDrain than the body, especially for shorter devices

• Shorter devices small Vth, less affected by VTCMOS remain small Vth

• Longer devices higher Vth, affected more by VTCMOS– Vth variation increases

VTCMOS (within die Vth var.)• Measurement: true

leakage current for a given nominal Vth

• Note: the leakage controlled pretty well

• The variance of I leakage is not too much– Leakage is an

exponential function of Vth Vth variation reflected in leakage

– The std of leakage is not increased Vth variation is negligible

Inter-die variation

Inter-die variation with ABB

Experimental Results

Experimental Results

Design example I

• TX3900– 32bit RISC– Variable Supply (VS) DC-DC

converter– Vth control (SAT+SPR)– Placed at the corners

Design Example I

• Measurement– 1.3V @10MHz – 1.9V @40MHz– 20mW @ DC-DC converter

(85% efficiency)

• @ 20MHz– Conv. CMOS: 150 MIPS/W– VTCMOS: 480 MIPS/W

Design Example II

• SPR+SAT

• Distributed supply switches