TS421-2 datasheet(28/32 Pages) STMICROELECTRONICS

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TS419-TS421

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The efficiency is the ratio between the output

power and the power supply

The maximum theoretical value is reached when

Vpeak = Vcc, so

s Decoupling of the circuit

Two capacitors are needed to bypass properly the

TS419/TS421. A power supply bypass capacitor

CS and a bias voltage bypass capacitor CB.

CS has particular influence on the THD+N in the

high frequency region (above 7kHz) and an

indirect influence on power supply disturbances.

With

1µF,

you

can

expect

similar

THD+N

performances to those shown in the datasheet.

In the high frequency region, if CS is lower than

1µF, it increases THD+N and disturbances on the

power supply rail are less filtered.

On the other hand, if CS is higher than 1µF, those

disturbances on the power supply rail are more

filtered.

CB has an influence on THD+N at lower

frequencies, but its function is critical to the final

result of PSRR (with input grounded and in the

lower frequency region).

If CB is lower than 1µF, THD+N increases at lower

frequencies and PSRR worsens.

If CB is higher than 1µF, the benefit on THD+N at

lower frequencies is small, but the benefit to PSRR

is substantial.

Note that CIN has a non-negligible effect on PSRR

at lower frequencies. The lower the value of CIN,

the higher the PSRR.

s Wake-up Time: T

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.

s 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 150k

Ω..

Then, the charge time constant for CB is

B = 150kΩxCB (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:

IN = (Rin+Rfeed)xCIN (s)

Thus we have the relation:

IN < 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.

s 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:

In order to reach optimal

performances of the differential function, R1 and

R2 should be matched at 1% max.

Vcc

ply

sup

PEAK

OUT

VDIFF

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