Electronic Design

Power Supply Write-Protects SmartVoltage Memories

Second-generation FlashFile memory devices developed by Intel, which employ the company’s SmartVoltage technology, can operate with a 5-V Vpp supply in addition to the established 12 V Vpp. This dual-operating voltage causes a problem in write-protecting the memory.

One write-protection method for flash memories is simply to turn off the 12-V Vpp supply. Since most systems derive this supply from 5 V using a boost converter, turning off the converter only lowers the supply to 5 V minus a diode drop¾not 0 V. Such protection was effective for the earlier 12-V—only flash memories, but not for the SmartVoltage memory with its 5-V operating capability.

The circuit shown solves this problem. IC2 is a step-up dc-dc converter that boosts 5 V to 12 V, and IC1 is a high-side switch that serves as a power switch and current limiter (see the figure). Therfore, when IC1 is turned off by applying a TTL-logic “1” to pin 3, the memory supply Vpp goes to 0 V. IC1 limits the current (between its IN and OUT terminals) according to the value R2 = 1240/Ilimit, where R2 is in ohms and Ilimit is in amperes (with a maximum of 1 A). A value of 1.5 kΩ, for example, will set the limit near 1 A. If lower peak-load currents are needed, the R2 value should be higher.

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