NCP2809 Series
http://onsemi.com
16
Bypass Capacitor Selection (C
bypass
)
The bypass capacitor C
by
provides half−supply filtering
and determines how fast the NCP2809 turns on.
A proper supply bypassing is critical for low noise
performance and high power supply rejection ratio.
Moreover, this capacitor is a critical component to
minimize the turn−on pop noise. A 1.0 F bypass capacitor
value should produce clickless and popless shutdown
transitions. The amplifier is still functional with a 0.1 F
capacitor value but is more sensitive to “pop and click”
noises.
Thus, for optimized performances, a 1.0 F ceramic
bypassing capacitor is recommended.
Without Output Coupling Capacitor
As described in Figure 42, the internal circuitry of the
NCP2809 device eliminates need of heavy bypassing
capacitors when connecting a stereo headset with 3
connecting points. This circuitry produces a virtual ground
and does not affect either output power or PSRR.
Additionally, eliminating these capacitors reduces cost and
PCB place.
However, user must take care to the connection between
pin REF_I and ground of the headset: this pin is the ground
reference for the headset. So, in order to improve
crosstalk performances, this pin must be plugged
directly to the middle point of the headset connector.
With Output Coupling Capacitor
However, when using a low cost jack connector (with
third connection to ground), the headset amplifier requires
very few external components as described in Figure 43.
Only two external coupling capacitors are needed. The
main concern is in output coupling capacitors, because of
the value and consequently the size of the components
required. Purpose of these capacitors is biasing DC voltage
and very low frequency elimination. Both, coupling
capacitor and output load form a high pass filter. Audible
frequency ranges from 20 Hz to 20 kHz, but headset used
in portable appliance has poor ability to reproduce signals
below 75 or 100 Hz. Input coupling capacitor and input
resistance also form a high pass filter. These two first order
filters form a second order high pass filter with the same
−3 dB cut off frequency. Consequently, the below formula
must be followed:
1
2 R
in
C
in
[
1
2 R
L
C
out
(eq. 2)
As for a loudspeaker amplifier, the input impedance
value for calculating filters cut off frequency is the
minimum input impedance value at maximum output
volume.
To obtain a frequency equal to when frequency is 5 times
the cut off frequency, attenuation is 0.5 dB. So if we want
a ±0.5 dB at 150 Hz, we need to have a –3 dB cut off
frequency of 30 Hz:
f−
3dB
w
1
2 R
L
C
out
(eq. 3)
C
out
w
1
2 R
L
f−
3dB
(eq. 4)
With R
L
= 16 , and f
−3dB
= 30 Hz formula (4) shows that
C
out
≥ 330 F.
With C
out
= 220 F, ±0.5 dB attenuation frequency will
be 225 Hz with a –3.0 dB cut off frequency of 45 Hz.
Following this, the input coupling capacitor choice is
straightforward. Using formula (2) input coupling
capacitor value would be 68 nF for a 220 F output
coupling capacitor and 100 nF for a 330 F output coupling
capacitor.
When using the NCP2809 with this configuration, pins
REF_I and OUT_I must be left unconnected
(see Figure 43).
Optimum Equivalent Capacitance at Output Stage
Cellular phone and wireless portable device designers
normally place several Radio Frequency filtering
capacitors and ESD protection devices between the outputs
and the headset connector. Those devices are usually
connected between amplifier outputs and ground, or
amplifier output and virtual ground. Different headsets
with different impedance can be used with NCP2809. 16,
32 and 64Ohm are standard values. The extra impedance
resulting of parasitic headset inductance and protections
capacitance can affect sound quality.
In order to achieve the best sound quality, we suggest the
optimum value of total equivalent capacitance:
• Between each output terminal to the virtual ground
should be less than or equal to 100pF
• Between each output terminal to the ground should be
less than or equal to 100pF.
This total equivalent capacitance consists of the radio
frequency filtering capacitors and ESD protection device
equivalent parasitic capacitance. Because of their very low
parasitic capacitance value, diode based ESD protection
are preferred.
If for some reason the above requirements cannot be met,
a series resistor between each NCP2809 output and the
protection device can improve amplifier operation. In
order to keep dynamic output signal range, the resistor
value should be very small compared to the loudspeaker
impedance. For example, a 10Ohm resistor for a 64Ohm
loudspeaker allows up to 400pF parasitic capacitance load.
