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High voltage charge pumps minimise EMI

01 Apr 2016  | Tony Armstrong

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Filters are often used to reduce EMI by attenuating the strength at a certain frequency or over a range of frequencies. A portion of this energy that travels through space (radiated) is attenuated by adding metallic and magnetic shields. The part that rides on PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI cannot be eliminated but can be attenuated to a level that is acceptable by other communication, signal processing and digital components. Moreover, several regulatory bodies enforce standards to ensure compliance in both industrial and automotive systems.

Modern input filter components in surface mount technology have better performance than through-hole parts. However, this improvement is outpaced by the increased demands created by today's high frequency switching regulators. The low minimum on and off times required at higher operating frequencies result in higher harmonic content due to the faster switch transitions, thereby increasing radiated noise. However, these high switch edge rates are needed to get higher conversion efficiencies. A switched capacitor charge pump does not exhibit this behaviour since it operates at much lower switching frequencies and most importantly can tolerate slower switching transitions without degradation in efficiency.

Savvy PCB designers will make the hot loops small and use shielding ground layers as close to the active layer as possible. Nevertheless, device pin-outs, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate a minimum hot loop size. To further complicate matters, in typical planar printed circuit boards, the magnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts since the higher the harmonic frequencies are, the more effective unwanted magnetic coupling becomes.

An alternative solution to EMI issues
The tried and true solution to EMI issues is to use a shielding box for the complete circuit, and even then, a shield does not prevent coupling to sensitive circuits inside the box. Of course, this adds cost, increases required board space, makes thermal management and testing more difficult and introduces additional assembly expense. Another frequently used method is to slow down the switching edges. This has the undesired effect of reducing the efficiency, increasing minimum on, off times, and their associated dead times and compromises the potential current control loop speed.

A few years ago, Linear Technology introduced our LT8614 Silent Switcher regulator, which delivers the desired effects of a shielded box without using one, while also eliminating many of the above mentioned drawbacks. Nevertheless, in some noise applications, power supply designers simply do not like to use inductor-based regulators due to their associated EMI emissions. At the same time, the use of a linear regulator (aka LDO) may be precluded due to its relatively low conversion efficiency and need for heat sinking. As a result, they turn to a common alternative known as a charge pump.

Charge pumps have been around for decades, and they provide DC/DC voltage conversion, using a switch network to charge and discharge two or more capacitors. The basic charge pump switch network toggles between charge and discharge states of the capacitors. As shown in figure 1, C1 the "flying capacitor" shuttles charge, and C2 the "reservoir capacitor" holds charge and filters the output voltage. Additional "flying capacitors" and switch arrays enable multiple gains.

Figure 1: Simplified Charge Pump Block Diagram of a Voltage Inverter.

When switches S1 and S3 are on, or closed, and switches S2 and S4 are off, or open, the input power supply charges C1. During the next cycle, S1 and S3 are off, S2 and S4 are on, and charge transfers to C2, generating VOUT =—(V+).

However, until recently, charge pumps have had limited input and output voltage ranges, which has limited their use in industrial and automotive applications where inputs up to 40V or 60V are commonplace. However, this is now touted to change with the introduction of high voltage charge pumps from Linear Technology.

High voltage charge pumps

The LTC3245 is a buck-boost regulator that dispenses with the traditional inductor and uses a switched-capacitor charge pump instead. Its input voltage range is 2.7V to 38V, and it can be used without a feedback divider to produce one of two fixed output voltages, 3.3V or 5V, or programmed via a feedback divider to any output voltage from 2.5V to 5.5V. Maximum output current is 250mA. The LTC3245 is capable of regulating a voltage above or below the input voltage, allowing it to satisfy automotive cold crank requirements, for example. See figure 2 for its complete schematic.

Figure 2: LTC3245 Schematic Delivering a Fixed 5V Output from a 2.7V to 38V Input.

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