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U. Glaeser

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FIGURE 15.12 VT-CMOS block diagram [24].<br />

FIGURE 15.13 Switched substrate-impedance scheme [26].<br />

same time. This method is the variable threshold-voltage CMOS (VT-CMOS) scheme [24]. The substrate<br />

bias to the n-type well of a pMOS transistor is called V bp and the bias to the p-type well of an nMOS<br />

transistor is called V bn. The voltage between V dd and V bp, or between GND and V bn is defined by ∆V bb.<br />

∆V bb controls V th as described by Eq. (15.3). This V bb control system raises CMOS circuit performance by<br />

compressing V th fluctuation in the active state, and reduces subthreshold leakage current by raising the<br />

MOS device V th in the idle state.<br />

The system diagram of VT-CMOS is shown in Fig. 15.12. The control circuit enables the leakage<br />

current monitor (LCM) and the self-substrate bias (SSB) circuit in the active state. The LCM measures<br />

the leakage current of MOS devices. The SSB forces the leakage current to be constant at a given value.<br />

For example, suppose V th is designed at 0.1 ± 0.1 V initially. Applying a 0.4 V ∆V bb increases the V th to<br />

0.2 V ± 0.05 V. Therefore, the V th fluctuation is compensated from 0.1 to 0.05 V. In this way the V bb<br />

control system reduces V th fluctuation [25].<br />

The SSB and the substrate charge injector (SCI) operate in the idle state. The SSB applies large ∆V bb<br />

to reduce leakage current. The SCI enables V bb to drive the substrate quickly and accurately. ∆V bb becomes<br />

about 2 V and then the V th is 0.5 V. The usage of the SSB and SCI results in low subthreshold leakage in<br />

the idle state. When applied to a discrete cosine transform processor, it occupies only 5% of the area.<br />

The substrate-bias current of V bb control is less than 0.1% of the total current, a small power penalty.<br />

The Switched substrate-impedance (SSI) scheme [26], as shown in Fig. 15.13, is one solution for<br />

preventing the V bb noise. SSI distributes switch cells throughout a die that function as V bb supplies.<br />

© 2002 by CRC Press LLC<br />

Leakage<br />

current<br />

Monitors<br />

(LCM)<br />

VBC macro<br />

vsub VBCG<br />

v bp<br />

Sleep<br />

Control<br />

VBCP VBCN VBCI<br />

cbp<br />

vbn cbn<br />

cbpr<br />

VBCR<br />

cbnr<br />

v bp<br />

cbp<br />

cbn<br />

v bn<br />

v well<br />

v dd<br />

v ss<br />

Self-substrate bias<br />

(SSB)<br />

Substrate charge<br />

Injector (SCI)<br />

vbbenb<br />

vbbenbr<br />

Standby<br />

controller<br />

v ddq<br />

v ss<br />

Switch cells Standard cells<br />

Logic circuit<br />

V bb<br />

v dd<br />

v ss

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