Another design issue with linear regulators also applies to most power supplies. You must assume that any electrolytic capacitors will short-circuit at some point during the lifetime of a product. If a short circuit occurs, you must ensure that the regulator and board do not burn or cause other damage.
You must also provide a fuse or a fusible pc trace on the input electrolytic capacitors and on any tantalum capacitors. Even if the product's wall wart cannot provide enough current to start a fire, a diligent engineer must provide for the situation in which a user uses a larger or incorrect wall wart to power a product (Figure 1).

Charge Pumps

Another type of dc/dc converter, the charge pump, can invert, double, or triple an input voltage by switching a capacitor across the input voltage to charge the capacitor. You can then switch that capacitor to sit on top of the input voltage, forming a doubler. Alternatively, you can connect the positive terminal of the capacitor to the input common, creating a voltage inverter.

Because the capacitor inherently limits the amount of current that the part can deliver, thermal problems are rare in charge pumps. They do have some drawbacks, however, including poor voltage regulation. The output changes with the input unless you employ a linear postregulator. Maxim has addressed this issue with a line of postregulator charge pumps. The switching frequency and noise of a charge pump are far less problematic than the noise from a switching converter, but the noise may still enter the signal chain.


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