NCP4894
http://onsemi.com
12
APPLICATION INFORMATION
Detailed Description
The NCP4894 audio amplifier can operate under 2.6 V
until 5.5 V power supply. It delivers 320 mW rms output
power to 4.0 W load (VP = 2.6 V) and 1.0 W rms output
power to 8.0 W load (VP = 5.0 V).
The structure of the NCP4894 is basically composed of
two identical internal power amplifiers. Both are externally
configurable with gain−setting resistors R
in
and R
f
(the
closed−loop gain is fixed by the ratios of these resistors).
The load is driven differentially through OUTA and OUTB
outputs. This configuration eliminates the need for an
output coupling capacitor.
Internal Power Amplifier
The output PMOS and NMOS transistors of the amplifier
were designed to deliver the output power of the
specifications without clipping. The channel resistance
(R
on
) of the NMOS and PMOS transistors does not exceed
0.6 W when they drive current.
The structure of the internal power amplifier is
composed of three symmetrical gain stages, first and
medium gain stages are transconductance gain stages to
obtain maximum bandwidth and DC gain.
Turn−On and Turn−Off Transitions
A cycle with a turn−on and turn−off transition is
illustrated with plots that show both single ended signals on
the previous page.
In order to eliminate “pop and click” noises during
transitions, output power in the load must be slowly
established or cut. When logic high is applied to the
shutdown pin, the bypass voltage begins to rise
exponentially and once the output DC level is around the
common mode voltage, the gain is established slowly
(20 ms). Using this turn−on mode, the device is optimized
in terms of rejection of “pop and click” noises.
A theoretical value of turn−on time at 25°C is given by
the following formula.
C
by
: bypass capacitor
R: internal 150 k resistor with a 25% accuracy
T
on
= 0.95 * R * C
by
The device has the same behavior when it is turned−off
by a logic low on the shutdown pin. During the shutdown
mode, amplifier outputs are connected to the ground.
However, to totally cut the output audio signal, you only
need to wait for 20 ms.
Shutdown Function
The device enters shutdown mode once the SD SELECT
and SD MODE pins are in the same logic state. This brings
flexibility to the design, as the SD MODE pin must be
permanently connected to VP or GND on the PCB. If the
SD SELECT pin is not connected to the output of a
microcontroller or microprocessor, it’s not advisable to let
it float. A pulldown or pullup resistor is then suitable.
During the shutdown state, the DC quiescent current has a
typical value of 10 nA.
Current Limit Circuit
The maximum output power of the circuit
(Porms = 1.0 W, VP = 5.0 V, R
L
= 8.0 W) requires a peak
current in the load of 500 mA.
In order to limit the excessive power dissipation in the
load when a short−circuit occurs between both outputs, the
current limit in the load is fixed to 800 mA.
Thermal Overload Protection
Internal amplifiers are switched off when the
temperature exceeds 160°C, and will be switched on again
only when the temperature decreases below 140°C.
The NCP4894 is unity−gain stable and requires no
external components besides gain−setting resistors, an
input coupling capacitor and a proper bypassing capacitor
in the typical application.
Both internal amplifiers are externally configurable (R
f
and R
in
) with gain configuration.
The differential−ended amplifier presents two major
advantages:
− The possible output power is four times larger (the
output swing is doubled) as compared to a single−ended
amplifier under the same conditions.
− Output pins (OUTA and OUTB) are biased at the same
potential VP/2, this eliminates the need for an output
coupling capacitor required with a single−ended
amplifier configuration.
The differential closed loop−gain of the amplifier is
given by A
vd
+ *
R
f
R
in
+
V
orms
V
inrms
. V
orms
is the rms value of
the voltage seen by the load and V
inrms
is the rms value of
the input differential signal.
Output power delivered to the load is given by
P
orms
+
(Vopeak)
2
2*R
L
(Vopeak is the peak differential
output voltage).
When choosing gain configuration to obtain the desired
output power, check that the amplifier is not current limited
or clipped.
The maximum current which can be delivered to the load
is 500 mA I
opeak
+
V
opeak
R
L
.
