LM4950

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SNAS174E – JULY 2003 – REVISED MAY 2013

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

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".

There is a relationship between the value of CIN and CBYPASS that ensures minimum output transient when power

is applied or the shutdown mode is deactivated. Best performance is achieved by setting the time constant

created by CIN and Ri + Rf to a value less than the turn-on time for a given value of CBYPASS as shown in the table

above.

DRIVING PIEZO-ELECTRIC SPEAKER TRANSDUCERS

The LM4950 is able to drive capacitive piezo-electric transducer loads that are less than equal to 200nF. Stable

operation is assured by placing 33pF capacitors in parallel with the 20k

Ω feedback resistors. The additional

capacitors are shown in Figure 66.

When driving piezo-electric tranducers, sound quality and accoustic power is entirely dependent upon a

transducer's frequency response and efficiency. In this application, power dissipated by the LM4950 is very low,

typically less than 250mW when driving a 200nF piezo-electric transduce (VDD = 12V).

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