MAX9728AETC-T Datasheet, PDF (7/19 Pages) Maxim Integrated Products – 60mW, DirectDrive, Stereo Headphone Amplifiers with Shutdown

60mW, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Detailed Description
The MAX9728A/MAX9728B stereo headphone ampli-
fiers feature Maxim’s DirectDrive architecture, eliminat-
ing the large output-coupling capacitors required by
conventional single-supply headphone amplifiers.
These devices consist of two 60mW Class AB head-
phone amplifiers, undervoltage lockout (UVLO)/shut-
down control, charge pump, and comprehensive
click-and-pop suppression circuitry (see the Functional
Diagram/Typical Operating Circuits). The charge pump
inverts the positive supply (V_DD), creating a negative
supply (PV_SS). The headphone amplifiers operate from
these bipolar supplies with their outputs biased about
SGND (Figure 1). The benefit of this SGND bias is that
the amplifier outputs do not have a DC component. The
large DC-blocking capacitors required with convention-
al headphone amplifiers are unnecessary, conserving
board space, reducing system cost, and improving fre-
quency response. The MAX9728A/MAX9728B feature
an undervoltage lockout that prevents operation from
an insufficient power supply and click-and-pop sup-
pression that eliminates audible transients on startup
and shutdown. The MAX9728A/MAX9728B also feature
thermal-overload and short-circuit protection.
DirectDrive
Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typical-
ly half the supply) for maximum dynamic range. Large-
coupling capacitors are needed to block this DC bias
from the headphone. Without these capacitors, a signifi-
cant amount of DC current flows to the headphone,
resulting in unnecessary power dissipation and possible
damage to both headphone and headphone amplifier.
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage, allowing
the MAX9728A/MAX9728B outputs to be biased about
SGND. With no DC component, there is no need for the
large DC-blocking capacitors. The MAX9728A/
MAX9728B charge pumps require two small ceramic
capacitors, conserving board space, reducing cost,
and improving the frequency response of the head-
phone amplifier. See the Output Power vs. Load
Resistance and Charge-Pump Capacitor Size graph in
the Typical Operating Characteristics for details of the
possible capacitor sizes. There is a low DC voltage on
the amplifier outputs due to amplifier offset. However,
the offsets of the MAX9728A/MAX9728B are typically
1.5mV, which, when combined with a 32Ω load, results
in less than 47µA of DC current flow to the head-
phones.
V_OUT
V_DD
V_DD/2
V_DD
SGND
CONVENTIONAL DRIVER-BIASING SCHEME
V_OUT
V_DD
SGND
2V_DD
-V_DD
DirectDrive BIASING SCHEME
Figure 1. Conventional Driver Output Waveform vs.
MAX9728A/MAX9728B Output Waveform
Charge Pump
The MAX9728A/MAX9728B feature a low-noise charge
pump. The 270kHz switching frequency is well beyond
the audio range and does not interfere with audio sig-
nals. The switch drivers feature a controlled switching
speed that minimizes noise generated by turn-on and
turn-off transients. The di/dt noise caused by the para-
sitic bond wire and trace inductance is minimized by
limiting the switching speed of the charge pump.
Although not typically required, additional high-fre-
quency noise attenuation can be achieved by increas-
ing the value of C2 (see the Functional Diagram/Typical
Operating Circuits).
Click-and-Pop Suppression
In conventional single-supply audio amplifiers, the out-
put-coupling capacitor contributes significantly to audi-
ble clicks and pops. Upon startup, the amplifier charges
the coupling capacitor to its bias voltage, typically half
the supply. Likewise, on shutdown, the capacitor is dis-
charged. This results in a DC shift across the capacitor,
which appears as an audible transient at the speaker.
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