Flash EPROM is becoming increasingly popular, in spite of their being harder to reprogram than EEPROMs. That difficulty is, however, countered by their lower price, greater density, and higher programming speed.
Maxim, a specialist manufacturer of all sorts of small convertors, produces a special IC for generating the necessary programming voltage of 12 V at 50 mA: the MAX732.
The MAX732 contains virtually everything to make a mini switch-mode power supply. its input voltage requirement is 5 V, from which it produces an output of 12 V. Since that output is needed only during programming, it may be disabled via the shut-down input (pin 1).
A problem encountered in producing switch-mode supplies is the availability of certain passive components, particularly the inductor. What is used here is the Sumida type CD54-470KC, which is available from a number of retailers and also from Maxim dealers as Type MAX-1.001. if neither of these can be obtained, however, a triac choke may be used, but that will lower the efficiency of the converter to some extent. It is always possible to add or remove turns if the inductance is incorrect. Bear in mind that the inductance is proportional to the square of the number of turns.
Diode D1 must be a Type 1N5187 as indicated or equivalent, note that a 1N4001 is not last enough.
The prototype delivered 12 V and an output current, Io, of up to 200 mA, more than enough for a flash-EPROM. The current drawn from the 5 V supply was about 2.4Io.
Use a single earthing point and decouple the IC directly at its points.
EPROM
An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power supply has been turned off and back on is called non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to a strong ultraviolet light source (such as a mercury-vapor lamp). EPROMs are easily recognizable by the transparent fused quartz (or on later models resin) window on the top of the package, through which the silicon chip is visible, and which permits exposure to ultraviolet light during erasing.
