SSM2932 Datasheet, PDF (12/16 Pages) Analog Devices – High Efficiency, Ground-Referenced

SSM2932
THEORY OF OPERATION
The SSM2932 provides a high efficiency Class-G stereo head-
phone output that is true ground-referenced; therefore, no
external coupling capacitors are required for connection to the
headphones. The headphones can be connected directly to the
headphone output pins, OUTL (Ball A3) and OUTR (Ball D3).
The headphone amplifier uses the supply provided at PVDD
(Ball A2). This supply voltage must be decoupled with a 1 μF
electrolytic capacitor, along with a 100 nF ceramic X7R capacitor.
The headphone amplifier uses Class-G architecture and generates
the required power supplies with a built-in charge pump that uses a
flying capacitor connected across CF1 (Ball B2) and CF2 (Ball C1).
The charge pump switching frequency is approximately 54 kHz
in the idle state with no input signal detected and 550 kHz when a
signal is present. The generated supply voltages are available at
CPVDD (Ball B3, positive rail) and CPVSS (Ball C2, negative rail).
The supply voltage of the headphone amplifier depends on the
input signal to the amplifier. For lower input signal levels, the
positive and negative rails are lowered, typically to ±PVDD/2.
As the signal level increases, CPVDD and CPVSS are raised to
±2.2 V. This rail switching allows the amplifier to achieve higher
efficiency.
In most typical usage conditions, the amplifier works on
the lower CPVDD and CPVSS voltages (±PVDD/2), thereby
consuming less power. In addition, because the amplifier
generates the positive and negative rails, the output amplifier
is true ground-referenced, thereby eliminating the need for
large coupling capacitors to drive the load.
For best audio performance, it is recommended that 2.2 μF,
X7R ceramic decoupling capacitors be used for CPVDD and
CPVSS. These capacitors serve as a reservoir for the headphone
amplifier.
The headphone amplifier has built-in short-circuit protection
and, therefore, shuts down in the event of a short circuit on
the headphone outputs.
The amplifier is designed to drive headphones with a mini-
mum impedance of 16 Ω. Capacitive loads of up to 150 pF
are supported.
AMPLIFIER GAIN
The SSM2932 amplifier gain can be set to either 0 dB or 6 dB
by applying the appropriate logic level to the GAIN pin (see
Table 6).
Table 6. Amplifier Gain and GAIN Pin Logic Levels
Amplifier Gain
GAIN Pin Logic Level
0 dB
Low (≤0.5 V)
6 dB
High (≥1.2 V)
Data Sheet
AMPLIFIER SHUTDOWN
Shutdown of the SSM2932 amplifier is controlled by the SD pin.
If a logic low is applied to this pin, the amplifier becomes inactive
and draws only minimal current from the supply.
Table 7. Amplifier Shutdown
Amplifier State
SD Pin Logic Level
Shutdown
Low (≤0.5 V)
Power-On
High (≥1.2 V)
HIGH OUTPUT IMPEDANCE
The SSM2932 has a HI-Z control pin that mutes the amplifier
and sets the output to a high impedance. If both HI-Z and SD
are set high, the amplifier remains in a high impedance state.
This feature allows the headphone output jack to be shared for
other functions such as video output or data transmission.
GROUND SENSE
SGND (Ball C3) is provided for sensing the dc potential at the
headphone jack. It is recommended that SGND be connected
directly to the ground pin of the headphone jack to ensure the
lowest dc offset at the amplifier output and to eliminate pop-and-
click noises when the amplifier is turned on or off. In addition,
connecting the SGND ball directly to the ground pin of the head-
phone jack helps to reduce crosstalk between the left and right
channel outputs. A dc path between the SGND pin and the
system ground must also be provided.
LAYOUT
Care must be taken to lay out PCB traces and wires properly
between the amplifier, load, and power supply. A good practice
is to use short, wide PCB tracks to decrease voltage drops and
minimize inductance. Ensure that track widths are at least 200 mil
per inch of track length for lowest DCR, and use at least 1 oz
or 2 oz copper thickness to minimize resistance. A poor layout
increases voltage drops, consequently affecting efficiency. Use
large traces for the power supply inputs and amplifier outputs
to minimize losses due to parasitic trace resistance.
Proper grounding guidelines help to improve audio perfor-
mance, minimize crosstalk between channels, and prevent
switching noise from coupling into the audio signal. The PCB
traces that connect the output pins to the load, as well as the
PCB traces to the supply pins, should be as wide as possible to
maintain the minimum trace resistances. It is also recommended
that a large ground plane be used for minimum impedances.
The SGND pin should be connected directly to the ground pin
of the headphone jack.
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