SSM2932

Data Sheet

Rev. 0 | Page 12 of 16

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)

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.