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U. Glaeser

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15.3 Supply Voltage Management<br />

Supply Switch<br />

A supply switch is one of the most effective means to cut off power in the idle state. Actually, the subthreshold<br />

leakage current caused by reduced Vth<br />

devices is a major problem in that state. The supply switch realizes<br />

high-speed operation in the active state and low-leakage current in the standby state. The switch was<br />

originally applied to a dynamic random access memory (DRAM) to reduce its data-retention current [3].<br />

As shown in Fig. 15.1, a switching transistor Ws<br />

is inserted as the word drivers’ supply switch. The<br />

transistor width of the switch Ws<br />

is equal to the width of the driver Wd<br />

because two or more word drivers<br />

will not be on at the same time. Ws<br />

determines total leakage current in the data-retention mode. Without<br />

the switch, the total driver width is n × Wd<br />

and consumes large leakage. The supply switch is adopted to<br />

a microprocessor in [4].<br />

A supply switch designed with high-Vth<br />

MOS devices reduces leakage current, as represented by the<br />

multiple threshold-voltage CMOS (MT-CMOS) scheme [5]. The MT-CMOS circuit scheme is shown in<br />

Fig. 15.2. This technique combines two types of transistors: low-Vth<br />

transistors for high-speed switching<br />

and high-Vth<br />

transistors for low leakage. In the active state, SL is negated (low) and the high-Vth<br />

supply<br />

switch transistors Q1 and Q2 supply Vdd<br />

and GND voltage to the virtual Vdd<br />

line ( VddV)<br />

and to the virtual<br />

GND line (GNDV) respectively. The operating circuit itself is made of low-Vth<br />

transistors to accelerate<br />

the switching speed of the gate. In the idle state, SL is asserted (high) and Q1 and Q2 disconnect the<br />

VddV<br />

and GNDV to reduce leakage current when the circuits are in the idle state, with the asserted (high)<br />

SL signals. In a 0.5-µ<br />

m gate-length technology, 0.6 V Vth<br />

devices are used for high-Vth<br />

switches and 0.3 V<br />

Vth devices are used for low-Vth circuits. When the Vth difference between the two devices is 0.3 V, the<br />

switches reduce the circuit’s idle current by three orders of magnitude.<br />

FIGURE 15.1 Word driver with subthreshold-current reduction [3].<br />

© 2002 by CRC Press LLC<br />

Row Decoder<br />

D1<br />

(V ch )<br />

D2<br />

(Vch )<br />

Dn<br />

(V ch )<br />

W d<br />

I d<br />

WD1<br />

I d<br />

WD2<br />

I d<br />

WDn<br />

I s<br />

W s<br />

V ch<br />

V chl<br />

WL1<br />

WL2<br />

WLn<br />

SB

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