Datasheet

LM4950
www.ti.com SNAS174E –JULY 2003–REVISED MAY 2013
The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches V
DD
/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 C
BYPASS
alters the device's turn-on time. Here are some typical turn-on times for various values of
C
BYPASS
:
C
B
(µF) T
ON
(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 V
DD
may
not allow the capacitors to fully discharge, which may cause "clicks and pops".
There is a relationship between the value of C
IN
and C
BYPASS
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 C
IN
and R
i
+ R
f
to a value less than the turn-on time for a given value of C
BYPASS
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 20kΩ 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 (V
DD
= 12V).
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