LM4901MMX Datasheet, PDF (13/25 Pages) Texas Instruments – 1.6 Watt Audio Power Amplifier with Selectable Shutdown Logic Level

LM4901
www.ti.com
SNAS139F – DECEMBER 2001 – REVISED DECEMBER 2001
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4901 contains shutdown circuitry that is used to
turn off the amplifier's bias circuitry. In addition, the LM4901 contains a Shutdown Mode pin, allowing the
designer to designate whether the part will be driven into shutdown with a high level logic signal or a low level
logic signal. This allows the designer maximum flexibility in device use, as the Shutdown Mode pin may simply
be tied permanently to either V_DD or GND to set the LM4901 as either a "shutdown-high" device or a "shutdown-
low" device, respectively. The device may then be placed into shutdown mode by toggling the Shutdown pin to
the same state as the Shutdown Mode pin. For simplicity's sake, this is called "shutdown same", as the LM4901
enters shutdown mode whenever the two pins are in the same logic state. The trigger point for either shutdown
high or 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.
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 LM4901 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4901 is unity-gain stable which gives the designer maximum system flexibility. The LM4901 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 1
Vrms 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 1. The input coupling capacitor, C_i,
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 100 Hz to
150 Hz. 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.
Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value.
Bypass capacitor, C_B, is the most critical component to minimize turn-on pops since it determines how fast the
LM4901 turns on. The slower the LM4901's outputs ramp to their quiescent DC voltage (nominally 1/2 V_DD), the
smaller the turn-on pop. Choosing C_B equal to 1.0 µF along with a small value of C_i (in the range of 0.1 µF to
0.39 µF), should produce a virtually clickless and popless shutdown function. While the device will function
properly, (no oscillations or motorboating), with C_B equal to 0.1 µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C_B equal to 1.0 µF is recommended in all but the most cost sensitive
designs.
Copyright © 2001, Texas Instruments Incorporated
Product Folder Links: LM4901
Submit Documentation Feedback
13