Datasheet

LM4890
SNAS138L –SEPTEMBER 2001–REVISED MAY 2013 www.ti.com
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as
possible. Typical applications employ a 5V regulator with 10 µF tantalum or electrolytic capacitor and a ceramic
bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of
the LM4890. The selection of a bypass capacitor, especially C
BYPASS
, is dependent upon PSRR requirements,
click and pop performance (as explained in the section, Proper Selection of External Components), system cost,
and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4890 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic low is placed on the
shutdown pin. By switching the shutdown pin to ground, the LM4890 supply current draw will be minimized in idle
mode. While the device will be disabled with shutdown pin voltages less than 0.5V
DC
, the idle current may be
greater than the typical value of 0.1µA. (Idle current is measured with the shutdown pin grounded).
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to
provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground
and disables the amplifier. If the switch is open, then the external pull-up resistor will enable the LM4890. This
scheme ensures that the shutdown pin will not float thus preventing unwanted state changes.
SHUTDOWN OUTPUT IMPEDANCE
For R
f
= 20k ohms:
Z
OUT1
(between Out1 and GND) = 10k||50k||R
f
= 6kΩ
Z
OUT2
(between Out2 and GND) = 10k||(40k+(10k||R
f
)) = 8.3kΩ
Z
OUT1-2
(between Out1 and Out2) = 40k||(10k+(10k||R
f
)) = 11.7kΩ
The -3dB roll off for these measurements is 600kHz
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers is critical to optimize
device and system performance. While the LM4890 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4890 is unity-gain stable which gives the designer maximum system flexibility. The LM4890 should be
used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain
configurations require large input signals to obtain a given output power. Input signals equal to or greater than
1Vrms are available from sources such as audio codecs. Please refer to the section, Audio Power Amplifier
Design, for a more complete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 7. The input coupling capacitor, C
IN
,
forms a first order high pass filter which limits low frequency response. This value should be chosen based on
needed frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers
used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to
150Hz. Thus, using a large input capacitor may not increase actual system performance.
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