LM4819 datasheet(12/27 Pages) TI

12 / 27 page

LM4819, LM4819MBD

SNAS133D – FEBRUARY 2001 – REVISED MARCH 2013

www.ti.com

Besides affecting system cost and size, Ci has an effect on the LM4819's click and pop performance. When the

supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero

to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor's value. Higher value

capacitors need more time to reach a quiescent DC voltage (usually 1/2 VDD) when charged with a fixed current.

The amplifier's output charges the input capacitor through the feedback resistor, RF. Thus, selecting an input

capacitor value that is no higher than necessary to meet the desired -3dB frequency can minimize pops.

As shown in Figure 1, the input resistor (Ri) and the input capacitor, Ci produce a -3dB high pass filter cutoff

frequency that is found using Equation 5.

f-3dB = 1/(2 πRiCi) (Hz)

(5)

As an example when using a speaker with a low frequency limit of 150Hz, Ci, using Equation 5 is 0.063µF. The

0.39µF Ci shown in Figure 1 allows the LM4819 to drive a high efficiency, full range speaker whose response

extends down to 20Hz.

Besides optimizing the input capacitor value, the bypass capacitor value, CB requires careful consideration. The

bypass capacitor's value is the most critical to minimizing turn-on pops because it determines how fast the

LM4819 turns on. The slower the LM4819's outputs ramp to their quiescent DC voltage (nominally 1/2VDD), the

smaller the turn-on pop. While the device will function properly (no oscillations or motorboating), with CB less

than 1.0µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of CB equal to or

greater than 1.0µF is recommended in all but the most cost sensitive designs.

Bypass Capacitor Value Selection

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

connected to the BYPASS pin. Since CB determines how fast the LM4819 settles to quiescent operation, its

value is critical when minimizing turn-on pops. The slower the LM4819's outputs ramp to their quiescent DC

voltage (nominally 1/2VDD), 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 click-less and pop-less shutdown function. As discussed above,

choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops.

Optimizing Click and Pop Reduction Performance

The LM4819 contains circuitry that minimizes turn-on and shutdown transients or "clicks and pops". For this

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

deactivated. While the power supply is ramping to it's final value, the LM4819's internal amplifiers are configured

as unity gain buffers. An internal current source charges the voltage of the bypass capacitor, CB, connected to

the BYPASS pin in a controlled, linear manner. Ideally, the input and outputs track the voltage charging on the

bypass capacitor. The gain of the internal amplifiers remains unity until the bypass capacitor is fully charged to

1/2VDD. As soon as the voltage on the bypass capacitor is stable, the device becomes fully operational. Although

the BYPASS pin current cannot be modified, changing the size of the bypass capacitor, CB, alters the device's

turn-on time and 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 (Ton) increases. There is a

linear relationship between the size of CB and the turn on time. Below are some typical turn-on times for various

values of CB:

TON

0.01µF

20ms

0.1µF

200ms

0.22µF

440ms

0.47µF

940ms

1.0µF

Submit Documentation Feedback

Product Folder Links: LM4819 LM4819MBD

LM4819 Datasheet(PDF) 12 Page - Texas Instruments

LM4819 Datasheet(HTML) 12 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL