TS4621ML STMICROELECTRONICS [STMicroelectronics], TS4621ML Datasheet - Page 34

• Manufacturer Part Number: TS4621ML
• Description: High-performance class-G stereo headphone amplifier
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Application information
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Startup phase
The TS4621ML uses different techniques to reduce the DC current consumption and offer a
pop-and-click performance close to none.
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).
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 kΩ.
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.
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.
Doc ID 023181 Rev 1
TS4621ML