TS4621ML datasheet(34/40 Pages) STMICROELECTRONICS

34 / 40 page

Application information

TS4621ML

34/40

Doc ID 023181 Rev 1

4.5

Startup phase

The TS4621ML uses different techniques to reduce the DC current consumption and offer a

pop-and-click performance close to none.

4.5.1

Auto zero technology

During the startup phase, the differential output voltage is sensed and adjusted to 0 V

(+/-500

μV) to avoid any pop noise when the amplifier becomes operational. This also helps

to minimize extra current consumption due to the load (Icc-extra = VoutDC / Rload).

4.5.2

Input impedance

The TS4621ML requires input coupling capacitors. The usual lowest frequency used for the

headphone is close to 20 Hz. This frequency means a constant time for a first-order high-

pass filter of approximately 1 / (2 x Pi x 20) = 8 ms.

To achieve 95 % of the capacitor’s charge, it is necessary to wait 3 x 8 ms = 24 ms, which is

out of range for a device with a fast startup time.

Because of the mismatching of all input capacitors and input resistors, if it is decided to start

the TS4621ML at a time of 8 ms, a voltage difference at the inputs (multiplied by the gain)

can create a voltage step on the output and consequently a pop noise.

To avoid this issue during the starting phase, the TS4621ML accelerates the charging of the

input capacitors by reducing the input impedance to 2 k

Ω.

In such a case, for a 1

μF capacitor the 95 % charge is reached in 6 ms. As the startup time

of TS4621ML is 12 ms, there remains sufficient time to fully charge the input capacitors and

as such eliminate any pop noise.

4.6

Layout recommendations

Particular attention must be given to the correct layout of the PCB traces and wires between

the amplifier, load and power supply (in most cases, the battery of the cellular phone).

The power and ground traces are critical since they must provide adequate energy and

grounding for all circuits. Good practice is to use short and wide PCB traces to minimize

voltage drops and parasitic inductance.

A track with a width of at least 200

μm for a copper thickness of 18 μm is recommended for

bringing energy to the amplifier from the battery.

Proper grounding guidelines help improve audio performances, minimize crosstalk between

channels, and prevent switching noise from coupling into the audio signal. It is also

recommended to use a large-area and multi-via ground plane to minimize parasitic

impedance.

A multi-layer PCB board allows double or multiple ground planes to be implemented. Most of

the time, the top and bottom layers are used as ground planes and provide shielding for

tracks routed on the intermediate layers. In addition, to minimize parasitic impedance over

the entire surface, a multi-via technique that connects the bottom and top layer ground

planes together in many locations is often used.

The copper traces that connect the output pins to the load and supply pins should be as

wide as possible to minimize the trace resistances.

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