NCP2815
http://onsemi.com
9
DETAIL OPERATING DESCRIPTION
Detailed Description
The NCP2815 is a stereo headphone amplifier with
NOCAP architecture. This architecture eliminates the need
to use two big external capacitors required by conventional
headphone amplifier.
The structure of the NCP2815 is composed of two true
ground amplifiers, a UVLO, a short circuit protection and a
thermal shutdown circuit. Additionally, a special circuit is
embedded to eliminate any pop and click noise that occurs
during turn on and turn off time. Version A has an external
gain selectable by two resistors, Version B has a gain of
1.5 V/V.
NOCAP
NOCAP is a patented architecture which requires only 2
small ceramic capacitors. It generates a symmetrical
positive and negative voltage which it allows the output of
the amplifiers to be biased to ground.
LongPlay Architecture
NCP2815 includes a LongPlay architecture which helps
to save battery life by reducing the quiescent current. The
charge pump frequency is reduced to 125 kHz for an output
load < 500 mW.
Current Limit Protection Circuit
The NCP2815 contains protection circuitry against shorts
to ground. The currrent is limited to 300 mA when an output
is shorted to GND and a signal appears at the input.
Thermal Overload Protection
Internal amplifiers are switched off when the temperature
exceeds 160°C, and are switched back on when the
temperature decreases below 140°C.
Under Voltage Lockout
When the battery voltage decreases below 1.4 V, the
amplifiers are turned off. The hysteresis required to turn
back on the device is 100 mV.
Pop and Click Suppression Circuitry
The NCP2815 includes a special circuit to eliminate any
pop and click noise during turn on and turn off time. The
amplifier creates an offset during these transitions at the
output which give a parasitic noise called “pop and click
noise”. The NCP2815 eliminates this problem.
Gain Setting Resistor Selection (Rin and 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 +*
Rf
Rin
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 Version A, 20 kW for
Version B).
The size of the capacitor must be large enough to couple
in the 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 a ceramic capacitor with low ESR for better
performances. An X5R / X7R capacitor is recommended.
The flying capacitor (C2) serves to transfer charge during
the generation of the negative voltage.
The PVM capacitor (C3) must be equal at least to the
flying capacitor to allow maximum transfer charge.
Table 1 suggests typical values and manufacturers:
Table 1.
Value Reference Package
Manufac-
turer
1 mF
C1005X5R0J105K 0402 TDK
1 mF
GRM155R60J105K19 0402 Murata
Lower value 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 NCP2815 is a True Ground amplifier which requires
an adequate decoupling capacitor to reduce noise and
THD + N. It is recommended to use an X5R / X7R ceramic
capacitor with a value of 1 mF and place it as close as
possible to the Vp pin.
Shutdown Function
The device enters in shutdown mode when the shutdown
signal is low. During the shutdown mode, the DC quiescent
current of the circuit does not exceed 1 mA. In this
configuration, the output impedance is 20 kW on each
output.
Layout Recommendation
Connect C1 as close as possible to the Vp pin.
Connect C2 and C3 as close as possible to the NCP2815.
Route the audio signal and SGND far away from Vp, CPP,
CPM, PVM and PGND to avoid any perturbation due to the
switching.
