Microelectromechanical systems (MEMS) is a technology that has seen significant emergence in recent years. Consisting of miniaturized mechanical and electromechanical components that can be fabricated on a micro-scale, MEMS have found themselves an important tool in many low-power applications.
Despite its use in low-power applications, designing with MEMS can be a challenge due to its reliance on high-voltage power circuitry for proper operation. Last week, MEMS company Menlo Micro released the MM101, a new charge pump-based low-power, high-voltage driver solution for MEMS switches.
The MM101 is an 8-channel, low-voltage driver designed for MEMS applications. Image used courtesy of Menlo Micro
The device is intended as a companion to Menlo Micro’s Ideal Switch products. In 2022, the company released a slew of devices based on the technology, including a device for RF switching, and one for high power distribution. In this article we’ll take a look at charge pump circuits, their value in MEMS applications, and the new product from Menlo Micro.
Charge Pump for MEMS
One issue in the design of MEMS circuits is that these systems generally rely on relatively high and variable bias voltage for proper operation. Juxtapose this with the fact that MEMS devices are often used in low-power, low-voltage applications, and it’s clear that the real challenge lies in generating the necessary high-voltages from a low-voltage source while maintaining low power consumption.
Schematic of a simple charge pump circuit. Image used courtesy of Texas Instruments
To do this, one popular circuit is the charge pump. A charge pump circuit, or charge pump regulator, is a kind of DC-DC converter that generally consists of nothing but capacitors and switches (in other words, transistors) and work by careful timing and controlling these switches to exploit the charge transfer characteristics of capacitors. Through alternatively charging and discharging capacitors, a charge pump can increase or decrease a given input voltage to the desired level.
More efficient than a linear-dropout regulator (LDO) but less efficient than a boost converter, the charge pump converter is a relatively efficient solution for increasing voltages. While not the most efficient option available, the real strength of charge pumps is their small area.
By relying only on capacitors and transistors, both of which are easy to manufacture in standard, micro CMOS processes, charge-pump circuits offer a much more space-efficient solution than a boost converter.
In the context of MEMS circuits, charge pumps are an ideal choice for a number of reasons, but it is mainly the unique combination of high power efficiency and low area that makes them so desirable. Where MEMS circuits aim to be both small and power-efficient, the charge pump is a great solution for creating the large voltages needed for MEMS biasing.
Menlo Micro’s Low-power Solution
Menlo Micro’s new product, the MM101, was designed largely with MEMS applications in mind. To this end, it leverages charge pump circuitry in order to take low-power input voltages and convert them into high-voltage outputs. Specifically, this device is designed to take an input voltage of 5 V and can produce output voltages anywhere from 10 V to 100 V depending on external circuitry.
MM101 functional block diagram. Image used courtesy of Menlo Micro
With this high voltage output, the MM101 offers 8 high-voltage push-pull outputs, each capable of outputting up to 72 µA of current at the high voltage outputs. Additionally, the chip, which comes in a tiny 5 mm × 5 mm 32-pin QFN package, supports 32 MHz SPI for a simple communication and control scheme. More information can be found in the MM101 datasheet.
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