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Improved Slew Rate Controlling Pre-Driver

IP.com Disclosure Number: IPCOM000127756D
Publication Date: 2005-Sep-13
Document File: 3 page(s) / 88K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for an improved pre-driver with two “kicker” circuits that help stabilize buffer output delay (TCO) effects. Benefits include reducing the time it takes to reach the threshold voltage.

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Improved Slew Rate Controlling Pre-Driver

Disclosed is a method for an improved pre-driver with two “kicker” circuits that help stabilize buffer output delay (TCO) effects. Benefits include reducing the time it takes to reach the threshold voltage.

Background

Figure 1 shows a typical slew rate controlled buffer. The output transistor gate (nDRV) is driven by a number of tri-state invertors (tri-nots) of varying strength. The slew rate is controlled by selecting the appropriate combination of tri-nots with the DSR and USR control bits. This controls how fast the nDRV is pulled up or down, thereby controlling how fast the transistor is turned on and off.

Note. While an N-type driver is shown, the technique applies to P-type driver transistors as well. 

Figure 2 shows the nDRV waveforms for various selections of tri-nots. For a given silicon process corner, voltage and temperature (PVT), a faster rising nDRV voltage causes a faster slew rate on the pad. Slow PVT corners require higher drives on the nDRV to maintain a desired slew rate. Fast PVT corners require less. With fast corners the gate drive may become so low that the nDRV does not reach the rail before the next bit. This causes the gate to discharge at a different timing than if the pre-driver had remained high for two or more bits. The change in timing affects the TCO and makes it data dependent. Figure 2 also shows another effect of slow pre-driver waveforms: the rising part of the pre-driver voltage below the driver transistor threshold (VTN) has little effect on the pad. A slow rise on the nDRV pushes out the buffer TCO. It is preferred that the slew rate control and the TCO are independent.

General Description

Figure 3 shows the disclose...