Datasheet

LM4862
www.ti.com SNAS102F –MAY 1997–REVISED MAY 2013
POWER SUPPLY BYPASSING
As with any power 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. As displayed in Typical Performance Characteristics, the effect of a larger half supply bypass capacitor
is improved PSSR due to increased half-supply stability. Typical applications employ a 5V regulator with 10 μF
and a 0.1 μF bypass capacitors which aid in supply stability, but do not eliminate the need for bypassing the
supply nodes of the LM4862. The selection of bypass capacitors, especially C
B
, is thus dependant upon desired
PSSR requirements, click and pop performance as explained in PROPER SELECTION OF EXTERNAL
COMPONENTS, system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4862 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. The shutdown feature turns the amplifier off when a logic high is placed on the
shutdown pin. The trigger point between a logic low and logic high level is typically half supply. It is best to switch
between ground and supply to provide maximum device performance. By switching the shutdown pin to V
DD
, the
LM4862 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin
voltages less than V
DD
, the idle current may be greater than the typical value of 0.7 μA. In either case, the
shutdown pin should be tied to a definite voltage because leaving the pin floating may result in an unwanted
shutdown condition.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which
provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch
that when closed, is connected to ground and enables the amplifier. If the switch is open, then a soft pull-up
resistor of 47 kΩ will disable the LM4862. There are no soft pull-down resistors inside the LM4862, so a definite
shutdown pin voltage must be applied externally, or the internal logic gate will be left floating which could disable
the amplifier unexpectedly.
AUTOMATIC SWITCHING CIRCUIT
As shown in Figure 3, the LM4862 and the LM4880 can be set up to automatically switch on and off depending
on whether headphones are plugged in. The LM4880 is used to drive a stereo single ended load, while the
LM4862 drives a bridged internal speaker.
The Automatic Switching Circuit is based upon a single control pin common in many headphone jacks which
forms a normally closed switch with one of the output pins. The output of this circuit (the voltage on pin 5 of the
LM4880) has two states based on the position of the switch. When the switch inside the headphone jack is open,
the LM4880 is enabled and the LM4862 is disabled since the NMOS inverter is on. If a headphone jack is not
present, it is assumed that the internal speakers should be on and the external speakers should be off. Thus the
voltage on the LM4862 shutdown pin is low and the voltage on the LM4880 shutdown pin is high.
The operation of this circuit is rather simple. With the switch closed, R
P
and R
O
form a resistor divider which
produces a gate voltage of less than 50 mV. The gate voltage keeps the NMOS inverter off and R
SD
pulls the
shutdown pin of the LM4880 to the supply voltage. This shuts down the LM4880 and places the LM4862 in its
normal mode of operation. When the switch is open, the opposite condition is produced. Resistor R
P
pulls the
gate of the NMOS high which turns on the inverter and produces a logic low signal on the shutdown pin of the
LM4880. This state enables the LM4880 and places the LM4862 in shutdown mode.
Only one channel of this circuit is shown in Figure 3 to keep the drawing simple but a typical application would be
a LM4880 driving a stereo headphone jack and two LM4862's driving a pair of internal speakers. If a single
internal speaker is required, one LM4862 can be used as a summer to mix the left and right inputs into a mono
channel.
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 LM4862 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
Copyright © 1997–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM4862