MP7720 查看數據表（PDF） - Monolithic Power Systems
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MP7720 Datasheet PDF : 11 Pages
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MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER
The quality factor (Q) of the LC filter is important.
If this is too low, output noise will increase, if this
is too high, then peaking may occur at high
signal frequencies reducing the passband
flatness. The circuit Q is set by the load
resistance (speaker resistance, typically 4Ω or
8Ω). The Q is calculated as:
Q
=
ω0
×
L
R
=
2π ×
f0
×
L
R
ω0 is the characteristic frequency in radians per
second and f0 is in Hz. Use an LC filter with Q
between 0.7 and 1.
The actual output ripple and noise is greatly
affected by the type of inductor and capacitor
used in the LC filter. Use a film capacitor and an
inductor with sufficient power handling capability
to supply the output current to the load. The
inductor should exhibit soft saturation
characteristics. If the inductor exhibits hard
saturation, it should operate well below the
saturation current. Gapped ferrite, MPP,
Powdered Iron, or similar type toroidal cores are
recommended. If open or shielded bobbin ferrite
cores are used for multi-channel designs, make
sure that the start windings of each inductor line
up (all starts going toward SW pin, or all starts
going toward the output) to prevent crosstalk or
other channel-to-channel interference.
Output Coupling Capacitor
The output AC coupling capacitor C9 serves to
block DC voltages and thus passes only the
amplified AC signal from the LC filter to the
load. The combination of the coupling capacitor,
C9 and the load resistance results in a first-
order high-pass filter. The value of C9 should
be selected such that the required minimum
frequency is still allowed to pass. The output
corner frequency (-3dB point), fOUT, can be
calculated as:
fOUT
=
1
2 × π × RLOAD
× C9
Set the output corner frequency (fOUT) at or
below the minimum required frequency.
The output coupling capacitor carries the full
load current, so a capacitor should be chosen
such that its ripple current rating is greater than
the maximum load current. Low ESR aluminum
electrolytic capacitors are recommended.
Input Coupling Capacitor
The input coupling capacitor C1 is used to pass
only the AC signal at the input. In a typical
system application, the source input signal is
typically centered around the circuit ground,
while the MP7720 input is at half the power
supply voltage (VDD/2). The input coupling
capacitor transmits the AC signal from the
source to the MP7720 while blocking the DC
voltage. Choose an input coupling capacitor
such that the corner frequency (fIN) is less than
the passband frequency. The corner frequency
is calculated as:
fIN
=
1
2 × π × R1× C1
Power Source
For maximum output power, the amplifier circuit
requires a regulated external power source to
supply the power to the amplifier. The higher
the power supply voltage, the more power can
be delivered to a given load resistance,
however if the power source voltage exceeds
the maximum operating voltage of 24V, the
MP7720 may sustain damage. The power
supply rejection of the MP7720 is excellent
(typically 60dB), however noise at the power
supply can get to the output, so care must be
taken to minimize power supply noise within the
pass-band frequencies. Bypass the power
supply with a large capacitor (typically
aluminum electrolytic) along with a smaller 1µF
ceramic capacitor at the MP7720 VDD supply
pins.
Circuit Layout
The circuit layout is critical for optimum
performance and low output distortion and
noise. Place the following components as close
to the MP7720 as possible:
Power Supply Bypass, C5
C5 carries the transient current for the switching
input stage. To prevent overstressing of the
MP7720 and excessive noise at the output,
place the power supply bypass capacitor as
close to pins 6 (VDD) and 8 (PGND) as
possible.
MP7720 Rev. 1.9
www.MonolithicPower.com
7
3/13/2006
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