6. Pump Control Circuits

This chapter is devoted to individual circuit block, i.e., pump regulators, oscillators, level shifters, and voltage references, to realize on-chip high-voltage generator together with charge pumps.

Section 6.1 presents pump regulators. Some of the pump output voltages need to be varied to adjust them to the target voltages. This can be done with the voltage gain of the regulator or the reference voltage changed. The voltage divider which is a main component of the regulator has to have small voltage coefficient and fast transient response enough to make the controlled voltage linear to the trim and stable in time. A regulator for a negative voltage has a circuit configuration different from that for a positive voltage. State of the art is reviewed.

Section 6.2 deals with oscillators. Without an oscillator, the charge pump never works. In order to make the pump area small, process, voltage, and temperature variations in oscillator frequency need to be done as small as possible. There is the maximum frequency at which the output current is maximized. If the oscillator is designed to have the maximum frequency under the fastest conditions such as fast process corner, high supply voltage, and low temperature, the pump output current is minimum under the slowest conditions such as slow process, low supply voltage, and high temperature. It is important to design the oscillator with small variations for squeezing the pump area.

Section 6.3 reviews level shifters. The level shifter shifts the voltage for logic high or low of the input signal to a higher or lower voltage of the output signal. Four types of level shifters are discussed (1) high-level NMOS level shifter, (2) high-level CMOS level shifter, (3) high-voltage depletion NMOS + PMOS level shifter, and (4) low-level CMOS level shifter. The trade-offs between the first three high-voltage shifters are mentioned. The negative voltage can be switched with the low-level shifter. As the supply voltage lowers, operation margins of the level shifters decrease. As the supply voltage lowers, the switching speed becomes slower, eventually infinite, i.e., the level shifter does not work. Some design techniques to lower the minimum supply voltage at which the level shifters are functional are shown.

Section 6.4 provides voltage references. Variations in regulated high voltages increase by a factor of the voltage gain of the regulators from those in the reference voltages. Reduction in the variations in voltage references is a key to make the high voltages well controlled. Some innovated designs for low supply voltage operation are presented as well.

Figure 6.1 shows on-chip high-voltage generator system and each component circuit block discussed in each section of this chapter. The charge pump inputs the supply voltage (V _DD) and the clock, which is generated by the oscillator, and outputs a voltage (V _PP) higher than the supply voltage or a negative voltage. The pump regulator enables the charge pump when the absolute value of the output voltage of the charge pump is lower than the target voltage on the basis of the reference voltage V _REF, or disables it otherwise. The output voltage of the pump is determined by the reference voltage and the voltage gain of the regulator. To vary the pump output voltage, either reference voltage or voltage gain of the regulator is varied. The generated high or negative voltage is transferred to a load through high- or low-level shifters. The level shifters are controlled by the input supply voltage. The load is capacitive, resistive, or both.