NCP2993
http://onsemi.com
10
APPLICATION INFORMATION
Detailed Description
The NCP2993 audio amplifier can operate under 2.5 V
until 5.5 V power supply. With less than 1% THD + N, it
can deliver up to 1.35 W RMS output power to an 8.0
load (V
P
= 5.0 V). If application allows to reach 10%
THD + N, then 1.65 W can be provided using a 5.0 V
power supply.
The structure of the NCP2993 is basically composed of
two identical internal power amplifiers; the first one is
externally configurable with gain−setting resistors R
in
and
R
f
(the closed−loop gain is fixed by the ratios of these
resistors) and the second is internally fixed in an inverting
unity−gain configuration by two resistors of 20 k. So 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 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
When a shutdown low level is applied, the output level
is tied to Ground on each output after 10 s.
With T
ON
= GND, turn on time is set to 30 ms. With T
ON
= V
P
, turn on time is set to 15 ms. To avoid any pop and click
noises, R
in
* C
in
< 2.4 ms with T
ON
= GND and R
in
* C
in
< 1.2 ms with T
ON
= Vp. The electrical characteristics are
identical with the 2 configurations. This fast turn on time
added to a very low shutdown current saves battery life and
brings flexibility when designing the audio section of the
final application.
NCP2993 is a zero pop noise device when using a
single−ended or differential audio input configuration.
Shutdown Function
The device enters shutdown mode when shutdown signal
is low. During the shutdown mode, the DC quiescent
current of the circuit does not exceed 100 nA. In this
configuration, the output impedance is 8.5 k on each
output.
Current Limit Circuit
The maximum output power of the circuit (P
orms
=
1.0 W, V
P
= 5.0 V, R
L
= 8.0 ) 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, the current limit in the
load is fixed to 1.1 A. The current in the four output MOS
transistors are real−time controlled, and when one current
exceeds 1.1 A, the gate voltage of the MOS transistor is
clipped and no more current can be delivered.
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 fewer than 140°C.
The NCP2993 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.
The first amplifier is externally configurable (R
f
and
R
in
), while the second is fixed in an inverting unity 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 V
P
/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
+ 2*
R
f
R
in
+
V
orms
V
inrms
.
Output power delivered to the load is given by
P
orms
+
(Vopeak)
2
2*R
L
(V
opeak
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
.
Gain−Setting Resistor Selection (R
in
and R
f
)
R
in
and R
f
set the closed−loop gain of the amplifier.
In order to optimize device and system performance, the
NCP2993 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 24 k is realistic in most
of applications, and doesn’t require the use of a too 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
