In integrated circuit memory devices, a high voltage source or generator may be required for word line pumping or to supply a programming voltage in nonvolatile memories. The generator provides an output voltage greater than that required by a load device. A working voltage is determined by a limiting circuit connected in parallel with the high voltage generator and the load device. In a continuous operation, an output of a high voltage generator provides current sufficient to supply a load device at a limit voltage. Any current not used by the load device is shunted to ground by an output clamp current and is lost.
In reference to FIG. 1, a prior art high voltage generator 100 runs with a high logic level applied to an oscillator enable input 101. Once enabled, the oscillator 105 generates a clock signal at an oscillator output 107. The clock signal applied to a charge pump 110 generates a high voltage supply at a charge pump output 112. As the voltage generator 100 operates, a charge pump current 115 flows to supply a memory array 120 with an array current 125. The remainder of the charge pump current 115 is a clamp current 135, which flows through a high voltage clamp 130. The voltage at the charge pump output 112 rises until the high voltage clamp 130 reaches a limit voltage.
The high voltage clamp 130 is composed of a plurality of reverse biased zener diodes 140 connected in series with a plurality of forward biased zener diodes 145. With the clamp current 135 flowing through the high voltage clamp 130, the zener diodes 140, 145 reach device limit voltages and establish a clamp voltage as the high voltage supply at the charge pump output 112. Any current not used by the load device is shunted to ground through the high voltage clamp 130.
With reference to FIG. 2, a waveform diagram 200 of a typical high voltage generator includes an oscillator enable signal 201 transitioning to a high logic level at the oscillator enable input 101 (FIG. 1) at an oscillator enable time 250. The high logic level at the oscillator enable input 101 causes the oscillator 105 to generate a clock signal 207. The charge pump 110 receives the clock signal 207 and produces a high voltage supply 212. The high voltage supply 212 starts at a voltage between zero volts and the supply voltage and rises to the clamp voltage limit controlled by the high voltage clamp 130. The clamp voltage limit is reached at a high voltage clamp time 270, which defines a high voltage ramp time 260. Operation continues in a high voltage clamp region 280 for as long as a high logic level is maintained on the oscillator enable input 301. In the high voltage clamp region, the high voltage clamp 130 shunts to ground any current not necessary to supply the array current 125 and a minimum of the clamp current 135 for regulation.
Various attempts to improve regulation of a magnitude of high voltage output can be found. For example, U.S. Pat. No. 6,577,514 to Shor et al. describes an apparatus for providing a constant boosted voltage at the output of a charge pump. Further, U.S. Pat. No. 6,724,241 to Bedarida et al. describes a variable charge pump circuit to minimize voltage ripples of the pumped output.
An object of the invention is to devise a capability to sense when a desired high voltage supply level is attained, then suspend power generation and thus save power.
What is needed is a way of avoiding unused power generation. It is desirable to sense when sufficient power has been generated to sustain a high-voltage supply and maintain a level of optimal operation near that power delivery point.