LM4875

SNAS042C – JANUARY 2002 – REVISED MAY 2013

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As the volume changes from minimum to maximum, RIN decrease from 110kΩ to 10kΩ. Equation 4 reveals that

the -3dB frequency will increase as the volume increases. The nominal value of Ci for lowest desired frequency

response should be calculated with RIN = 10kΩ . As an example when using a speaker with a low frequency limit

of 150Hz, Ci, using Equation 4 is 0.1µF. The 0.22µF Ci 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

CB. Since CB 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 VDD/2), the smaller the

turn-on pop. Choosing CB equal to 1.0µF, along with a small value of Ci (in the range of 0.1µF to 0.39µF),

produces a clickless and popless shutdown function. Choosing Ci 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 VDD. 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 CB alters the device's turn-on time and

the magnitude of "clicks and pops". Increasing the value of CB reduces the magnitude of turn-on pops. However,

this presents a tradeoff: as the size of CB 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

CB:

CB

TON

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 VDD may

not allow the capacitors to fully discharge, which may cause "clicks and pops". In a single-ended configuration,

the output coupling capacitor, COUT, is of particular concern. This capacitor discharges through an internal 20kΩ

resistor. Depending on the size of COUT, the time constant can be relatively large. To reduce transients in single-

ended mode, an external 1k

Ω - 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, 8

Ω 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 VDD and GND pads,

respectively. Connect a speaker with an 8

Ω 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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