LM4940

SNAS219C – OCTOBER 2003 – REVISED MAY 2013

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SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires

amplification and desired output transient suppression.

As shown in Figure 26, the input resistor (RIN) and the input capacitor (CIN) produce a high pass filter cutoff

frequency that is found using Equation 5.

fC = 1/2πRiCi

(5)

As an example when using a speaker with a low frequency limit of 50Hz, Ci, using Equation 5 is 0.159µF. The

0.39µF CINA shown in Figure 26 allows the LM4940 to drive high efficiency, full range speaker whose response

extends below 30Hz.

Output Coupling Capacitor Value Selection

The capacitors COUTA and COUTB that block the VDD/2 output DC bias voltage and couple the output AC signal to

the amplifier loads also determine low frequency response. These capacitors, combined with their respective

loads create a highpass filter cutoff frequency. The frequency is also given by Equation 5.

Using the same conditions as above, with a 4

Ω speaker, COUT is 820µF (nearest common valve).

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consideration should be paid to value of CBYPASS, the

capacitor connected to the BYPASS pin. Since CBYPASS determines how fast the LM4940 settles to quiescent

operation, its value is critical when minimizing turn-on pops. The slower the LM4940's outputs ramp to their

quiescent DC voltage (nominally VDD/2), the smaller the turn-on pop. Choosing CBYPASS equal to 10µF along with

a small value of CIN (in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As

discussed above, choosing CIN no larger than necessary for the desired bandwidth helps minimize clicks and

pops.

OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE

The LM4940 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this

discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode

is deactivated.

As the VDD/2 voltage present at the BYPASS pin ramps to its final value, the LM4940's internal amplifiers are

configured as unity gain buffers and are disconnected from the AMPA and AMPB pins. An internal current source

charges the capacitor connected between the BYPASS pin and GND in a controlled 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 applied to the BYPASS pin.

The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches VDD/2. As soon as

the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are

reconnected to their respective output pins. Although the BYPASS pin current cannot be modified, changing the

size of CBYPASS alters the device's turn-on time. Here are some typical turn-on times for various values of

CBYPASS:

CB (µF)

TON (ms)

1.0

120

2.2

120

4.7

200

10

440

12

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