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 ().
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