Tristate-Capable Output Buffer Implemented in Standard 2.5V CMOS Process

This paper describes high-voltage CMOS buffer architecture that uses low-voltage transistors. The voltage capability of presented architecture is nearly three times larger than the voltage capability of used MOSFET's. This buffer topology could be used to provide 3.3V compatibility of 1.2V and 1.5V digital ICs implemented in standard CMOS technology. A 7V circuit-prototype was fabricated in 0.25pm 2.5V CMOS technology. Performed measurements demonstrate stress-free operation in both active and high-impedance mode. Introduction As CMOS technology scales below 0.2pm, allowed suppIy voltages become significantly lower than previous 3.3V and 5V standards [l]. Because of economic reasons, systems usually use chips spanning several technology generat.ions. As a result, a 3.3V IC might need to interface with another IC designed to operate from lower supply voltage (such as 1.5V). This scenario presents a serious problem: The buffer circuits of the 1.5V IC neither can provide nor snstain [when in high-impedance state) a 3.3V drive. One could solve the above problem in two ways. The first approach is to use "dual-supply'' technology. Kowever, the use of technology that has two types of transistors' such as 1.5V(low-voltage) and 3.3V (high-voltage) devices, increases production cost2. The second approach is to develop buffer architectures that have high-voltage capabilities and use only low-voltage transistors. Reported to date high-volta&-&ufSers with low-voltuge transistors (HVBILVT) can be classified into two basic groups: 1) circuits with both high-voltage tolerance and high-voltage drive; 2) circuits with high-voltage tolerance and low-voltage drive. The conceptual schematics of these two types of HVB/LVT's are shown in Fig. 1. The pad driver in Fig. l(a) consists of n-channel and pchannel cascode stacks. The cascodes allow output to traverse between 0 and Vhzgh while the liyS's and Vgd's of all four transistors remain lower than l/ZVhigh [2]. Thiis, the voltage capability of Fig. I(a) pad driver is two times larger than the voltage capability of used MOSFET's. For 'Marly sub-0.2fi.m technologies are "dual-supply" technologies 2This increase could be as much as 20%. 16) Figure 1: Conceptual schematics of high-voltage tristate-capable output buffers: (a) with 2X drive and 2 X tolerance; (b) with "regular"( 1X) drive and 2X tolerance; proper operation the cascode pad driver requires two inphase input signals. Both signals must have low-voltage (1/2Vhigh) swing. These signals are provided through two "regular" inverter chains and are generated by the. levelshifter circuit. The level-shifter takes a 0-to1/2Vhigh input and produces signal that swings between 1/2Vhigh and Vhigh. Naturally, the level-shifter must be implemented in such a way that none of its transistors experiences voltage overstress. Unlike previously discussed HVB/LVT the circuit shown in Fig. l(b) is biased from the lower supply voltage. As a result its output drive is only O-to-l/PVhi,h . The structure however allows pad voltage to exceed supply (when the buffer is in tristate mode), i.e. the circuit has highvoltage tolerance (= Vh+). Three problems have been eliminated to achieve this 2 X tolerance [3]: 1) Vag overstress of n-channel transistor; 2) conduction, of p-channel transistor; 3) forward biasing of the drain-bulk pn junct i" of p-chan7ieI transistor. The first problem is resolved by using an n-channel cascode, while the second and the third are eliminated by using dynamic gate and bulk biasing (conceptually illustrated using two pairs of switches). Recently, two HVB/LVT's with beyond-2X voltage capabilities have been reported. Clark [4] has developed circuit with 3.3V drive and 5V tolerance using 2V transistors, while Singh and Salem [5] have extended the stressfree range of a cascode stack beyond supply a.nd ground with approximately one threshold voltage. Both circuits use dynamic gate biasing . In the next two Sections we will describe an HVB/LVT that has a stress-free range of nearly 3 X . Its high-voltage