TS4900 查看數據表（PDF） - STMicroelectronics

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TS4900

300mW at 3.3V SUPPLY AUDIO POWER AMPLIFIER WITH STANDBY MODE ACTIVE HIGH

STMicroelectronics 

TS4900

power and the power supply

η = P-----sP---u--O--p--U--p-T---l--y- = ---π--4-V--V---P--C-E---C-A---K--

Moreover, Cb determines the speed that the

amplifier turns ON. The slower the speed is, the

softer the turn ON noise is.

The maximum theoretical value is reached when

Vpeak = Vcc, so

The charge time of Cb is directly proportional to

the internal generator resistance 50kΩ.

Then, the charge time constant for Cb is

π-4--- = 78.5%

τb = 50kΩxCb (s)

As Cb is directly connected to the non-inverting

■ Decoupling of the circuit

input (pin 2 & 3) and if we want to minimize, in

amplitude and duration, the output spike on Vout1

Two capacitors are needed to bypass properly the

TS4900, a power supply bypass capacitor Cs and

t(s) a bias voltage bypass capacitor Cb.

c Cs has especially an influence on the THD+N in

u high frequency (above 7kHz) and indirectly on the

rod power supply disturbances.

P t(s) With 100µF, you can expect similar THD+N

performances like shown in the datasheet.

lete uc If Cs is lower than 100µF, in high frequency

so rod increases, THD+N and disturbances on the power

supply rail are less filtered.

b P To the contrary, if Cs is higher than 100µF, those

- O te disturbances on the power supply rail are more

) le filtered.

t(s so Cb has an influence on THD+N in lower frequency,

c b but its function is critical on the final result of PSRR

du O with input grounded in lower frequency.

Pro t(s) - If Cb is lower than 1µF, THD+N increase in lower

frequency (see THD+N vs frequency curves) and

te c the PSRR worsens up

le du If Cb is higher than 1µF, the benefit on THD+N in

so ro lower frequency is small but the benefit on PSRR

is substantial (see PSRR vs. Cb curve : fig.12).

Ob te P Note that Cin has a non-negligible effect on PSRR

le in lower frequency. Lower is its value, higher is the

o PSRR (see fig. 13).

Obs■ Pop and Click performance

(pin 5), Cin must be charged faster than Cb. The

charge time constant of Cin is

τin = (Rin+Rfeed)xCin (s)

Thus we have the relation

τin << τb (s)

The respect of this relation permits to minimize the

pop and click noise.

Remark : Minimize Cin and Cb has a benefit on

pop and click phenomena but also on cost and

size of the application.

Example : your target for the -3dB cut off

frequency is 100 Hz. With Rin=Rfeed=22 kΩ,

Cin=72nF (in fact 82nF or 100nF).

With Cb=1µF, if you choose the one of the latest

two values of Cin, the pop and click phenomena at

power supply ON or standby function ON/OFF will

be very small

50 kΩx1µF >> 44kΩx100nF (50ms >> 4.4ms).

Increasing Cin value increases the pop and click

phenomena to an unpleasant sound at power

supply ON and standby function ON/OFF.

Why Cs is not important in pop and click

consideration ?

Hypothesis :

• Cs = 100µF

• Supply voltage = 5V

• Supply voltage internal resistor = 0.1Ω

• Supply current of the amplifier Icc = 6mA

Pop and Click performance is intimately linked with

the size of the input capacitor Cin and the bias At power ON of the supply, the supply capacitor is

voltage bypass capacitor Cb.

charged through the internal power supply

resistor. So, to reach 5V you need about five to ten

Size of Cin is due to the lower cut-off frequency times the charging time constant of Cs (τs =

and PSRR value requested. Size of Cb is due to 0.1xCs (s)).

THD+N and PSRR requested always in lower Then, this time equal 50µs to 100µs << τb in the

frequency.

majority of application.

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