NCP4894
http://onsemi.com
13
Gain−Setting Resistor Selection (R
in
and R
f
)
R
in
and R
f
set the closed−loop gain of both amplifiers.
In order to optimize device and system performance, the
NCP4894 should be used in low gain configurations.
The low gain configuration minimizes THD + noise
values and maximizes the signal to noise ratio, and the
amplifier can still be used without running into the
bandwidth limitations.
A closed loop gain in the range from 2 to 5 is
recommended to optimize overall system performance.
An input resistor (R
in
) value of 22 kW is realistic in most
applications, and doesn’t require the use of a very large
capacitor C
in
.
Input Capacitor Selection (C
in
)
The input coupling capacitor blocks the DC voltage at
the amplifier input terminal. This capacitor creates a
high−pass filter with Rin, the cut−off frequency is given by
fc +
1
2*P *R
in
*C
in
.
The size of the capacitor must be large enough to couple
in low frequencies without severe attenuation. However a
large input coupling capacitor requires more time to reach
its quiescent DC voltage (VP/2) and can increase the
turn−on pops.
An input capacitor value between 0.1 m and 0.39 mF
performs well in many applications (With R
in
= 22 kW).
Bypass Capacitor Selection (Cby)
The bypass capacitor Cby provides half−supply filtering
and determines how fast the NCP4894 turns on.
This capacitor is a critical component to minimize the
turn−on pop. A 1.0 mF bypass capacitor value
(C
in
= < 0.39 mF) should produce clickless and popless
shutdown transitions. The amplifier is still functional with
a 0.1 mF capacitor value but is more susceptible to “pop and
click” noises.
Thus, a 1.0 mF bypassing capacitor is recommended.
Figure 31. Demonstration Board Schematic
+
−
VP
INM
VP
VP
R
8W
OUTA
OUTB
VMC
BRIDGE
INP
BYPASS
20 kW
1 mF
1 mF
SHUTDOWN
CONTROL
20 kW
1 mF
C4
R2
C2
+
−
20 kW
20 kW
R1C1
R5
VM
BYPASS
BYPASS
SD SELECT
SD MODE
C3
1 mF
VP
J4J5R3
100 kW
J10
J1
VP
GND
J2
J3
