Datasheet

LM4906, LM4906LDBD, LM4906MMBD
www.ti.com SNAS191E –APRIL 2003–REVISED MAY 2013
TURNING ON THE LM4906
The power supply must first be applied before the application of an input signal to the device and the ramp time
to V
DD
must be less than 4ms, otherwise the wake-up time of the device will be affected. After applying V
DD
, the
LM4906 will turn-on after an initial minimum threshold input signal of 7mV
RMS
, resulting in a generated output
differential signal. An input signal of less than 7mV
RMS
will result in a negligible output voltage. Once the device
is turned on, the input signal can go below the 7mV
RMS
without shutting the device off. If, however, SHUTDOWN
or V
DD
is cycled, the minimum threshold requirement for the input signal must first be met again, with V
DD
ramping first.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4906 contains shutdown circuitry that is used to
turn off the amplifier's bias circuitry. The device is placed into shutdown mode by toggling the Shutdown pin
Low/ground. The trigger point for shutdown low is shown as a typical value in the Supply Current vs Shutdown
Voltage graphs in the Typical Performance Characteristics section. It is best to switch between ground and
supply for maximum performance. While the device may be disabled with shutdown voltages in between ground
and supply, the idle current may be greater than the typical value of 0.1µA. In either case, the shutdown pin
should be tied to a definite voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry, which
provides a quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction
with an external pull-up resistor (or pull-down, depending on shutdown high or low application). This scheme
ensures that the shutdown pin will not float, thus preventing unwanted state changes.
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.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, C
i.
A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
1/2 V
DD
). This charge comes from the output via the feedback and is apt to create pops upon device enable.
Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be
minimized.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
Power Output 1 Wrms
Load Impedance 8Ω
Given: Input Level 1 Vrms
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply
rail can be easily found.
Extra supply voltage creates headroom that allows the LM4906 to reproduce peaks in excess of 1W without
producing audible distortion. At this time, the designer must make sure that the power supply choice along with
the output impedance does not violate the conditions explained in the Power Dissipation section.
The gain of the LM4906 is internally set at either 6dB or 12dB.
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Product Folder Links: LM4906 LM4906LDBD LM4906MMBD