Datasheet

LM4875
SNAS042C –JANUARY 2002–REVISED MAY 2013 www.ti.com
As the volume changes from minimum to maximum, R
IN
decrease from 110kΩ to 10kΩ. Equation 4 reveals that
the -3dB frequency will increase as the volume increases. The nominal value of C
i
for lowest desired frequency
response should be calculated with R
IN
= 10kΩ . As an example when using a speaker with a low frequency limit
of 150Hz, C
i
, using Equation 4 is 0.1µF. The 0.22µF C
i
shown in Figure 1 is optimized for a speaker whose
response extends down to 75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of the bypass capacitor
C
B
. Since C
B
determines how fast the LM4875 turns on, its value is the most critical when minimizing turn-on
pops. The slower the LM4875's outputs ramp to their quiescent DC voltage (nominally V
DD
/2), the smaller the
turn-on pop. Choosing C
B
equal to 1.0µF, along with a small value of C
i
(in the range of 0.1µF to 0.39µF),
produces a clickless and popless shutdown function. Choosing C
i
as small as possible helps minimize clicks and
pops.
CLICK AND POP CIRCUITRY
The LM4875 contains circuitry that minimizes turn-on and shutdown transients or "clicks and pops". For this
discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated.
While the power supply is ramping to its final value, the LM4875's internal amplifiers are configured as unity gain
buffers. An internal current source changes the voltage of the bypass pin in a controlled, linear manner. Ideally,
the input and outputs track the voltage applied to the bypass pin. The gain of the internal amplifiers remains unity
until the voltage on the bypass pin reaches 1/2 V
DD
. As soon as the voltage on the bypass pin is stable, the
device becomes fully operational and the gain is set by the external voltage applied to the DC Vol/SD pin.
Although the bypass pin current cannot be modified, changing the size of C
B
alters the device's turn-on time and
the magnitude of "clicks and pops". Increasing the value of C
B
reduces the magnitude of turn-on pops. However,
this presents a tradeoff: as the size of C
B
increases, the turn-on time increases. There is a linear relationship
between the size of CB and the turn-on time. Shown below are some typical turn-on times for various values of
C
B
:
C
B
T
ON
0.01µF 3ms
0.1µF 30ms
0.22µF 65ms
0.47µF 135ms
1.0µF 280ms
In order eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching V
DD
may
not allow the capacitors to fully discharge, which may cause "clicks and pops". In a single-ended configuration,
the output coupling capacitor, C
OUT
, is of particular concern. This capacitor discharges through an internal 20kΩ
resistor. Depending on the size of C
OUT
, the time constant can be relatively large. To reduce transients in single-
ended mode, an external 1kΩ - 5kΩ resistor can be placed in parallel with the internal 20kΩ resistor. The tradeoff
for using this resistor is increased quiescent current.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figure 28 through Figure 30 show the recommended two-layer PC board layout that is optimized for the SOIC-8
packaged LM4875 and associated external components. Figure 31 through Figure 33 show the recommended
two-layer PC board layout for the VSSOP packaged LM4875. Both layouts are designed for use with an external
5V supply, 8Ω speakers, and 8Ω - 32Ω headphones. The schematic for both recommended PC board layouts is
Figure 1.
Both circuit boards are easy to use. Apply a 5V supply voltage and ground to the board's V
DD
and GND pads,
respectively. Connect a speaker with an 8Ω minimum impedance between the board's -OUT and +OUT pads.
For headphone use, the layout has provisions for a headphone jack, J1. When a jack is connected as shown,
inserting a headphone plug automatically switches off the external speaker.
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