MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER
MP7720 Rev. 2.1 www.MonolithicPower.com 5
1/22/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2010 MPS. All Rights Reserved.
OPERATION
The MP7720 is a single-ended Class D audio
amplifier. It uses the Monolithic Power Systems
patented Analog Adaptive Modulation
TM
to
convert the audio input signal into pulses.
These pulses drive an internal high-current
output stage and, when filtered through an
external inductor-capacitor filter, reproduce the
input signal across the load. Because of the
switching Class D output stage, power
dissipation in the amplifier is drastically reduced
when compared to Class A, B or A/B amplifiers
while maintaining high fidelity and low distortion.
The amplifier uses a differential input to the
modulator. PIN is the positive input and NIN is
the negative input. The common mode voltage
of the input is set to half the DC power supply
input voltage (V
DD
/2) through the resistive
voltage divider formed by R2 and R3. The input
capacitor C1 couples the AC signal at the input.
The amplifier voltage gain is set by the
combination of R1 and R4 and is calculated by
the equation:
1R
4R
AV
−
=
The output driver stage uses two 180m
N-Channel MOSFETs to deliver the pulses to
the LC output filter which in turn drives the load.
To fully enhance the high-side MOSFET, the
gate is driven to a voltage higher than the
source by the bootstrap capacitor between SW
and BS. While the output is driven low, the
bootstrap capacitor is charged from V
DD
through
an internal circuit on the MP7720. The gate of
the high-side MOSFET is driven high from the
voltage at BS, forcing the MOSFET gate to a
voltage higher than V
DD
and allowing the
MOSFET to fully turn on, reducing power loss in
the amplifier.
Pop Elimination
The capacitor C9 passes only AC currents to
the load. To insure that the amplifier passes low
frequency signals, the time constant of
C9*R
LOAD
needs to be long. However, when EN
is asserted, the capacitor charges over a long
period and in a normal amplifier can result in a
turn on and/or turn off “pop.” The MP7720
includes integrated circuitry that eliminates the
turn on and turn off pop associated with the
charging of the AC coupling capacitor.
Short Circuit/Overload Protection
The MP7720 has internal overload and short
circuit protection. The currents in both the high-
side and low-side MOSFETs are measured and
if the current exceeds the 5.0A short circuit
current limit, both MOSFETs are turned off. The
MP7720 then restarts with the same power up
sequence that is used for normal starting to
prevent a pop from occurring after a short
circuit condition is removed.
Mute/Enable Function
The MP7720 EN input is an active high enable
control. To enable the MP7720, drive EN with a
2.0V or greater voltage. To disable the amplifier,
drive it below 0.4V. While the MP7720 is
disabled, the VDD operating current is less than
5µA and the output driver MOSFETs are turned
off. The MP7720 requires approximately 500ms
from the time that EN is asserted (driven high)
to when the amplifier begins normal operation.
