5.6

Wake-up time: TWU

When standby is released to put the device ON, the bypass capacitor CB will not be charged immediatly. As CB is

directly linked to the bias of the amplifier, the bias will not work properly until the CB voltage is correct. The time to

reach this voltage is called wake-up time or TWU and typically equal to:

TWU = 0.15xCB (s) with CB in μF.

Due to process tolerances, the range of the wake-up time is:

0.12xCb < TWU < 0.18xCB (s) with CB in μF

Note:

When the standby command is set, the time to put the device in shutdown mode is a few microseconds.

5.7

Pop performance

Pop performance is intimately linked with the size of the input capacitor Cin and the bias voltage bypass capacitor

CB.

The size of CIN is dependent on the lower cut-off frequency and PSRR values requested. The size of CB is

dependent on THD+N and PSRR values requested at lower frequencies.

Moreover, CB determines the speed with which the amplifier turns ON. The slower the speed is, the softer the turn

ON noise is.

The charge time of CB is directly proportional to the internal generator resistance 150 kΩ.

Then, the charge time constant for CB is

τB = 150 kΩ x CB (s)

As CB is directly connected to the non-inverting input (pin 2 & 3) and if we want to minimize, in amplitude and

duration, the output spike on Vout1 (pin 5), CIN must be charged faster than CB. The equivalent charge time

constant of CIN is:

τIN = (Rin + Rfeed) x CIN (s)

Thus we have the relation:

τIN < τB (s)

Proper respect of this relation allows to minimize the pop noise.

Remark : Minimizing CIN and CB benefits both the pop phenomena, and the cost and size of the application.

5.8

Application : Differential inputs BTL power amplifier

The schematic on figure 98, shows how to design the TS419/21 to work in a differential input mode.

The gain of the amplifier is:

GVDIFF=2R2R1

(13)

In order to reach optimal performances of the differential function, R1 and R2 should be matched at 1% max.

Figure 98. Differential input amplifier configuration

Input capacitance C can be calculated by the following formula using the -3 dB lower frequency required. (FL is

the lower frequency required).

C≈ 12πR1FLF

(14)

Note : This formula is true only if:

FCB= 1

942000×CB Hz

(15)

is ten times lower than FL.

TS419, TS421

Application information

DS3048 - Rev 7

page 31/42