NCP2823 Series
http://onsemi.com
9
DETAIL OPERATING DESCRIPTION
General Description
The basic structure of the NCP2823A/B is composed of
one analog pre−amplifier, a pulse width modulator and an
H−bridge CMOS power stage. The first stage is externally
configurable with gain−setting resistor Ri and the internal
fixed feedback resistor Rf (the closed−loop gain is fixed by
the ratios of these resistors). The load is driven differentially
through two output stages. The differential PWM output
signal is a digital image of the analog audio input signal. The
human ear is a band pass filter regarding acoustic
waveforms, which the typical cut off values are 20 Hz and
20 kHz. Thus, the user will hear only the amplified audio
input signal within the frequency range. The switching
frequency and its harmonics are fully filtered. The inductive
parasitic element of the loudspeaker helps to guarantee a
superior distortion value.
Power Amplifier
The output PMOS and NMOS transistors of the amplifier
have been designed to deliver a maximum output power
before clipping. The channel resistance (Ron) of the NMOS
and PMOS transistors is typically 0.3 W.
Gain Selection
The preamplifier stage amplifies the input signal. The
gain is fully configurable by external resistors.
The gain setting is given by the following equation:
Av +
300 kW
Ri
(eq. 1)
Turn On and Turn Off Transitions
In order to reduce “pop and click” noises during transition,
the output power in the load must not be established or cutoff
suddenly. When logic high is applied to the Enable pin, the
internal biasing voltage rises quickly and, 4 ms later, once
the output DC level is around the common mode voltage, the
gain is established slowly (5.0 ms). Thus, the total turn on
time to get full power to the load is 7.4 ms (typical). The
device has the same behavior when it is turned−off by a logic
low on the Enable pin. No power is delivered to the load 4 ms
after a falling edge on the shutdown pin. Due to the fast turn
on and off times, the shutdown signal can be used as a mute
signal as well.
Shutdown Function
The device enters shutdown mode when the Enable signal
is low. During the shutdown mode, the DC Shutdown
current of the circuit does not exceed 1 mA.
The NCP2823A/B has an internal resistor (R
PLD
=
250 kW) connected between GND and Enable. The purpose
of this resistor is to eliminate any unwanted state changes
when the Enable pin is floating.
30 kHz Built−in Low Pass Filter
This filter allows connecting directly a DAC or a CODEC
to the NCP2823 input without increasing the output noise by
mixing frequency with the DAC/CODEC output frequency.
Consequently, optimized operation with DACs or CODECs
is guaranteed without additional external components.
Power Supply Bypassing
The NCP2823 requires a correct decoupling of the power
supply in order to guarantee the best operation in terms of
audio performances. To achieve these performances, it is
necessary to place a 4.7 mF low ESR ceramic capacitor as
close as possible to the PVDD pin in order to reduce high
frequency transient spikes due to parasitic inductance (see
Layout considerations).
Input Capacitors C
in
Thanks to its fully differential architecture the NCP2823
does not require input capacitors. However, it is possible to
use input capacitors when the differential source is not
biased or in single ended configuration. In this case it is
necessary to take into account the corner frequency which
can influence the low frequency response of the NCP2823.
The following equation will help choose the adequate input
capacitor.
f
C
+
1
2 @ p @ Ri @ C
in
(eq. 2)
Over Current Protection
This protection allows detecting an over current in the
H−Bridge. When the current is higher than 2A for the
NCP2823B or 1A for the NCP2823A, the H−Bridge is
positioned in high impedance. When the short circuit is
removed or the current is lower, the NCP2823 goes back to
normal operation. This protection avoids over current due to
a bad assembly (Output shorted together, to V
DD
or to
ground).
Layout Recommendations
For Efficiency and EMI standpoints, it is strongly
recommended to use Power and ground plane in order to
reduce parasitic resistance and inductance.
For the same reason, it is recommended to keep the output
traces short and well shielded in order to avoid them to act
as antenna.
