IEEE Solid-State Circuits Magazine - Spring 2016 - 83
Part 2: Fundamentals of the charge pump
Innovation of
Switched-Capacitor
Voltage Multiplier
Toru Tanzawa
I
n Part 1 of this series [1], a brief history of
switched-capacitor voltage multiplier was
reviewed starting with Greinacher's voltage doubler in 1914 [2]. It was concluded
that the Falke-Dickson [3], [4] parallel voltage multiplier or charge pump was best for integrated
circuits where capacitors and switching devices had
significant parasitic capacitance [5]. In this Part 2, the
structure of integrated switching devices and capacitors is reviewed. An equivalent circuit and power efficiency of the charge pump is discussed in both the ideal
and real cases in terms of threshold voltage of switching transistor and parasitic capacitance. Finally, several
design optimizations are described.
Switching Device
We have two types of devices as switching devices. The
first one is a two-terminal diode [Figure 1(a)]. The P-N
Digital Object Identifier 10.1109/MSSC.2016.2543065
Date of publication: 21 June 2016
1943-0582/16©2016IEEE
junction fabricated on a P-substrate needed a twin well
structure to eliminate substrate current as much as possible [Figure 1(b)] [7]. A Schottky barrier diode is used to
reduce the threshold voltage as well as parasitic capacitance at a small junction area with low series resistance
[Figure 1(c)] [8]. A PIN diode is fabricated in a flash memory process using a second poly layer as a mask for the
intrinsic portion of the first poly [Figure 1(d)], which had a
very low reverse current but a silicon bandgap with a large
threshold voltage [9].
A two-terminal device is also achievable by a MOSFET whose gate and drain are short together [Figure 1(e)]
[4]. To fully eliminate the diode voltage drop per stage,
there have been various design techniques to overdrive the gate of the switching transistors [Figure 1(f)],
which are going to be reviewed in Part 3 of this series
[10]-[14]. To reduce the body effect of threshold voltage, other design techniques have been developed by
controlling the body potential of the switching devices
[Figure 1(g)] [6], [15], [23], which will be reviewed in
Part 3 as well.
IEEE SOLID-STATE CIRCUITS MAGAZINE
S P R I N G 2 0 16
83
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