Electronic Design

Amp Designers Find Novel Ways To Use Charge Pumps

Internal charge pumps are fairly common in op amps, where they're used to achieve a "rail to rail" output swing. Now, they're being used in other amplifier applications to provide output swings well beyond the range of the supply voltage as well as to solve some challenging design problems. Maxim Integrated Products now offers a pair of video amps and an audio power amp that illustrate this trend.

The MAX9509/9510 video amps operate on 1.8 V yet deliver the 2-V p-p signal (with negative sync pulses) required to drive a double-terminated load. The MAX9509 includes an internal five-pole Butterworth low-pass filter that smoothes out the step-wise output of a video digital-to-analog converter (DAC) driver. The MAX9510 doesn't include the filter.

Thanks to the negative supply generated by the internal charge pump, the chips can clamp the output video signal's black level at 0 V dc while delivering an SMPTE-170 standard 2-V p-p signal to a double-terminated transmission line. This is useful because it means there is no need for ac-coupling (a series capacitor) on the amplifier's output. Eliminating the capacitor eliminates modulation of the black level as a result of variations in average picture level.

Designed for piezoelectric speakers, the MAX9788 audio power amplifier delivers up to 20-V p-p output while running off a single lithium-ion cell that provides only 3.0 to 4.2 V. It is a class G ceramic speaker driver, which means it uses a push-pull stage like class A-B but adds a second higher-voltage power supply that only kicks in when signal peaks rise above a preset level (see the figure). Obviously, class G requires careful design to minimize nonlinearity at the transition points.

Maxim crafted this chip to meet the demands of piezoelectric speakers, which are de rigueur in today's skinny cell phones. Getting big sound out of a piezo speaker means moving a lot of air, which requires the application of large driving voltages. Additionally, electrically speaking, piezo speakers look like capacitors. They take more current at higher frequencies than at lower frequencies, which is the opposite of magnet and coil speakers.

In lots of 1000 units, the MAX9509 and MAX9510 cost $0.49 and $0.45, respectively. Pricing for the MAX9788 starts at $0.65.

Maxim Integrated Products
www.maximic.com

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