LM4915 Datasheet, PDF (12/18 Pages) National Semiconductor (TI) – Pseudo-Differential Mono Headphone Amplifier with Fixed 6dB Gain

LM4915
SNAS176A – MAY 2003 – REVISED MAY 2013
www.ti.com
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the bypass and power supply pins should be as close to
the device as possible. A larger half-supply bypass capacitor improves PSRR because it increases half-supply
stability.
Typical applications employ a 5V regulator with 10µF and 0.1µF bypass capacitors that increase supply stability.
This, however, does not eliminate the need for bypassing the supply nodes of the LM4915. A 1µF capacitor is
recommended for C_S. A 4.7µF capacitor is recommended for C_B. This value coupled with small input capacitors
(0.1µF to 0.47µF) gives virtually zero click and pop with outstanding PSRR performance.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the LM4915's shutdown function. Activate micro-power
shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4915's micro-power
shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The trigger point is 0.4V for a
logic-low level, and 1.8V for a logic-high level. The low 0.1µA (typ) shutdown current is achieved by applying a
voltage that is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may
increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using
a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an
external 100k. pull-up resistor between the SHUTDOWN pin and V_DD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch
connects the SHUTDOWN pin to ground, activating micro-power shutdown.
The switch and resistor ensure that the SHUTDOWN pin will not float. This prevents unwanted state changes. In
a system with a microprocessor or microcontroller, use a digital output to apply the control voltage to the
SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull-up resistor.
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 LM4915 is tolerant of external component combinations, and requires
minimal external components, consideration to component values must be used to maximize overall system
quality.
The input coupling capacitor, C_i, forms a first order high pass filter which limits low frequency response given by
f_c = 1/(2πR_iC_i). R_i is internally set to 20kΩ. 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.
In addition to system cost and size, click and pop performance is affected 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
LM4915 turns on. The slower the LM4915's outputs ramp to their quiescent DC voltage (nominally 1/2 V_DD), the
smaller the turn-on pop. Choosing C_B equal to 4.7µF along with a small value of C_I (in the range of 0.1µF to
0.47µF), should produce a virtually clickless and popless shutdown function. While the device will function
properly, (no oscillations or motorboating), with C_B equal to 1.0µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C_B equal to 4.7µF is recommended in all but the most cost sensitive
designs.
12
Submit Documentation Feedback
Product Folder Links: LM4915
Copyright © 2003–2013, Texas Instruments Incorporated