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Wide frequency range non-overshoot/undershoot ac coupling level shifter

IP.com Disclosure Number: IPCOM000200726D
Publication Date: 2010-Oct-26
Document File: 5 page(s) / 1M

Publishing Venue

The IP.com Prior Art Database

Abstract

AC-coupled level translation circuits are used to pass high-speed signals across voltage domain boundaries. However, the frequency range of these translators are limited. This invention presents one method by which the low-frequency end of this range may be extended.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 19% of the total text.

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Level translator circuits are commonly used to pass signals across voltage domain boundaries. Many of the traditional circuit topologies are becoming unworkable as supply voltages decrease and frequency of the signals to be level translated increase. A specific translator architecture that is not as limited by voltage headroom or signal frequency is discussed in prior art [*] and shown in Figure 1. It functions well as a general-purpose level shifter that passes CMOS signals across a voltage domain boundary in high-frequency range that is determined by RC product of R1 and C1+C2 in Figure 1; however, at low frequency, this circuit has problems as discussed next.

    Figure 4 shows simulation waveforms for the prior art circuit of Figure 1 with a low-frequency (10MHz) clock input. In the bottom waveform, it is obvious that the output clock does not mimic the input clock characteristics, and it is further observed that the afcap node shows voltage peaking which exceeds both the power and ground supply voltages, which is a gate oxide reliability concern. So what causes this voltage peaking?

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    At low frequency, the voltage across C1 and C2 in Figure 1 is fully charged, and node ls will be about mid-swing (Vs/2). When the input transitions, overshoot and undershoot on the afcap node (ignoring leakage) will be about :

Vafcap, overshoot = Vinputsignalswing + Vs/2

Vafcap, undershoot = Vs/2 - Vinputsignalswing

    For example, if the low-frequency input signal comes in on a 1V domain (Vs) and is a full-rail signal, the over/undershoot may be estimated as follows:

Vafcap, overshoot = Vinputsignalswing + Vs/2 = 1v+0.5v =1.5v

Vafcap, undershoot = Vs/2 - Vinputsignalswing = 0.5v-1v = -0.5v

    The invention in this document eliminates both voltage undershoot and overshoot. This extends the frequency range for level translation and takes care of gate oxide reliability problems

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on the internal level translator...