NCP2811
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11
DETAIL OPERATING DESCRIPTION
Detailed Descriptions
The NCP2811 is a stereo headphone amplifier with a true
ground architecture. This architecture eliminates the need to
use 2 external big capacitors required by conventional
headphone amplifier.
The structure of the NCP2811 is basically composed of 2
true ground amplifiers, an UVLO, a short circuit protection
and also a thermal shutdown. A special circuitry is
embedded to eliminate any pop and click noise that occurs
during turn on and turn off time. The A version has an
external gain selectable by two resistor, B version has a gain
of 1.5 V/V.
NOCAPt
NOCAPt is a patented architecture which requires only
2 small ceramic capacitors. It generates a symmetrical
positive and negative voltage and it allows the output of the
amplifiers to be biased around the ground.
Current Limit Protection Circuit
The NCP2811 embed a protection circuitry against short
to ground. When an output is shorted to GND and when a
signal appears at the input, the current is limited to 300 mA.
Thermal Overload Protection
Internal amplifiers are switched off when the temperature
exceed 160°C, and will be switch on again when the
temperature decrease below 140°C.
Under Voltage Lockout
When the battery voltage decreases below 2.3 V, the
amplifiers are turned off. The hysteresis to turn on it again
is 100 mV.
Pop and Click Suppression Circuitry
The NCP2811 includes a special circuitry to eliminate any
pop and click noise during turn on and turn off time. Basic
amplifier creates an offset during these transitions at the
output which give a parasitic noise called “pop and click
noise”. The NCP2811 eliminates this problem.
Gain Setting Resistor Selection (Rin & Rf, A version
only)
R
in
and R
f
set the closed loop gain of the amplifier. A low
gain configuration (close to 1) minimizes the THD + noise
values and maximizes the signal to noise ratio.
A closed loop gain in the range of 1 to 10 is recommended
to optimize overall system performance.
The formula to calculate the gain is:
Av +*
R
f
R
in
Input Capacitor Selection
The input coupling capacitor blocks the DC voltage at the
amplifier input terminal. This capacitor creates a high−pass
filter with R
in
(externally selectable for A version, 20 kW for
B version).
The size of the capacitor must be large enough to couple
in low frequencies without severe attenuation in the audio
bandwith (20 Hz – 20 kHz).
The cut off frequency for the input high−pass filter is:
F
c
+
1
2pR
in
C
in
A F
c
< 20 Hz is recommended.
Charge Pump Capacitor Selection
Use ceramic capacitor with low ESR for better
performances. X5R / X7R capacitor is recommended.
The flying capacitor (C2) serves to transfer charge during
the generation of the negative voltage.
The CPVM capacitor (C3) must be equal at least to the
CFly capacitor to allow maximum transfer charge. The
CPVM value must not exceed 1 m F. Higher capacitor value
can damage the part.
Table 4 suggests typical value and manufacturer:
Table 4.
Value Reference Package Manufacturer
1 mF
C1005X5R0J105K 0402 TDK
1 mF
GRM155R60J105K19 0402 Murata
Lower value of capacitors can be used but the maximum
output power is reduced and the device may not operate to
specifications.
Power Supply Decoupling Capacitor (C1)
The NCP2811 is a True Ground amplifier which requires
the adequate decoupling capacitor to reduce noise and
THD+N. Use X5R / X7R ceramic capacitor and place it
closed to the CPVDD pin. A value of 1 mF is recommended.
Shutdown Function
The device enters in shutdown mode when shutdown signal
is low. During the shutdown mode, the DC quiescent current
of the circuit does not exceed 500 nA. In this configuration,
the output impedance is 10 kW on each output.
Output Resistor for Capacitive Drive Capability
Under normal operation, NCP2811 maximum direct
capacitive load is in the 80 pF range. If, for any reason, high
value capacitive loads should be connected to NCP2811
outputs, an additional 10 W resistor should be placed
between the NCP2811 output and the capacitive load to
ensure amplifier stability.
Layout Recommendation
Connect C1 as close as possible of the Vp pin.
Connect C2 and C3 as close as possible of the NCP2811.
Route audio signal and AGND far from Vp, CPP, CPM,
PVM and PGND to avoid any perturbation due to the
switching.
