LDO or Charge Pump? Choosing Between Linear and Switched-Capacitor Regulation

LDO or Charge Pump? Choosing Between Linear and Switched-Capacitor Regulation

ersaelectronics.com

When a design needs a small, low-power regulated rail, the choice often narrows to two candidates: a low-dropout linear regulator or a switched-capacitor converter, commonly called a charge pump. Both use no inductor, both are available in small packages, and both are inexpensive, but they behave very differently under the hood. A charge pump can step up, step down, or even invert the input voltage, while an LDO can only step down, so the two are not direct competitors in every application. This article compares them on efficiency, noise, size, and flexibility, and explains when one is clearly the better choice.

1. How a Charge Pump Works

A charge pump uses flying capacitors and switches to move charge between rails. In its simplest voltage-doubling form, a flying capacitor is first connected across the input, charged to the input voltage, and then reconnected in series with the input to deliver roughly twice the voltage at the output. More advanced configurations can divide the input by two, generate a negative rail from a positive one, or produce an adjustable output through fractional conversion ratios. The switches operate at a fixed or variable switching frequency, typically in the range of a few hundred kilohertz to a few megahertz, and the output is smoothed by an output capacitor. Because no inductor is used, the solution can be small and cheap, but the output is inherently choppy.

1. Efficiency

An LDO operating on a small input-to-output differential can be very efficient, approaching Vout/Vin, which may be 90 percent or better. A charge pump's efficiency depends on its conversion ratio. In a perfect voltage doubler, the output current is half the input current and the theoretical efficiency approaches 100 percent when the output voltage is exactly twice the input, but real parts lose energy in the switches and capacitors, and efficiency falls off when the output moves away from the ideal ratio. A step-down charge pump with a 2:1 ratio can also be efficient, but for ratios that do not match the flying-capacitor configuration, the pump must regulate by linear control or by skipping cycles, and efficiency degrades sharply. An LDO is usually more efficient than a charge pump when the input is already close to the desired output.

1. Ripple and Noise

This is the most important practical difference. An LDO output is quiet: it contains no switching ripple, only broadband noise from the reference and amplifier, typically in the range of tens of microvolts. A charge pump output contains periodic ripple at the switching frequency, with amplitude set by the output capacitor, the load current, and the switch resistance. Even with large output capacitors, ripple of several millivolts is common, and the fast edges of the switching waveform generate high-frequency spikes. For analog, audio, RF, or precision measurement rails, a charge pump is almost never acceptable on its own, although it is sometimes followed by a small LDO to clean up the rail.

1. Voltage Range and Function

An LDO can only produce a lower voltage than its input. A charge pump can step up, step down, or invert, which makes it uniquely useful in several situations: generating a negative supply for an op-amp or audio stage from a single positive rail; producing a boosted gate-drive voltage; or creating a higher voltage for an LCD bias or LED string. In these applications there is no linear alternative that can do the job without an inductor, and the charge pump wins by default. For simple step-down rails near the input voltage, the LDO is the more natural choice.

1. Size, Cost, and External Components

Both technologies avoid inductors. An LDO typically needs only two small ceramic capacitors and costs very little. A charge pump needs the input and output capacitors plus one or two flying capacitors, which adds board area and bill-of-materials cost, although the components are still small and cheap. The integrated circuit itself is slightly more complex because of the switch matrix and clock generation. For a single small step-down rail, the LDO solution is usually the smaller and cheaper of the two.

1. Load Current and Power Level

Charge pumps are typically used at low to moderate currents, up to a few hundred milliamperes, because the switch and capacitor losses grow with current. At higher currents, inductor-based switching converters take over, and LDOs remain competitive only when the voltage difference is tiny. For loads below about 100 mA, both are practical, and the decision comes down to noise and function rather than power handling.

1. When Each One Wins

A charge pump wins when the required rail is higher than the input, inverted, or otherwise outside the reach of a linear regulator, and the ripple can be tolerated or filtered. An LDO wins when the rail must be quiet, the input is close to the output, and the current is small to moderate. A common compromise is to use a charge pump to create an intermediate rail and then follow it with an LDO that cleans up the ripple, combining the voltage flexibility of the pump with the cleanliness of the linear stage.

Summary

• A charge pump can step up, step down, or invert the input, while an LDO can only step down.
• LDOs deliver far lower output noise and ripple; charge pumps inherently produce switching ripple.
• LDOs are more efficient when the input is close to the desired output; charge pumps are efficient only near their ideal conversion ratio.
• Charge pumps are the default choice when a negative or boosted rail is needed without an inductor.
• For quiet step-down rails at low current, the LDO is usually the simpler, smaller, and cheaper solution.