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.
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Pump Control Circuits
Section 4.1 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 4.2 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 4.3 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 4.4 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. T. Tanzawa, On-chip High-Voltage Generator Design, Analog Circuits and Signal Processing, DOI 10.1007/978-1-4614-3849-6_4, # Springer Science+Business Media New York 2013
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4 Pump Control Circuits
On-chip high-voltage generator (4.3) Level shifter
(2, 3) Pump
(4.2) Oscillator
VPP
Load ILOAD
clk clk_cp
flg
VMON (4.1) Pump Regulator
VREF (4.4) Voltage Reference
Fig. 4.1 Block diagram of on-chip high-voltage generator
Figure 4.1 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
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