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MAX9724A

60mW, DirectDrive, Stereo Headphone Amplifier with Low RF Susceptibility and Shutdown

Maxim Integrated 

60mW, DirectDrive, Stereo Headphone Amplifier

with Low RF Susceptibility and Shutdown

Detailed Description

The MAX9724A/MAX9724B stereo headphone ampli-

fiers feature Maxim’s patented DirectDrive architecture,

eliminating the large output-coupling capacitors

required by conventional single-supply headphone

amplifiers. The device consists of two 60mW Class AB

headphone amplifiers, undervoltage lockout

(UVLO)/shutdown control, charge pump, and compre-

hensive click-and-pop suppression circuitry (see the

Functional Diagram/Typical Operating Circuits). The

charge pump inverts the positive supply (VDD), creat-

ing a negative supply (PVSS). The headphone ampli-

fiers operate from these bipolar supplies with their

outputs biased about PGND (Figure 1). The benefit of

this PGND bias is that the amplifier outputs do not have

a DC component. The large DC-blocking capacitors

required with conventional headphone amplifiers are

unnecessary, conserving board space, reducing sys-

tem cost, and improving frequency response. The

MAX9724A/MAX9724B feature an undervoltage lockout

that prevents operation from an insufficient power sup-

ply and click-and-pop suppression that eliminates audi-

ble transients on startup and shutdown. The

MAX9724A/MAX9724B also feature thermal-overload

and short-circuit protection.

DirectDrive

Conventional single-supply headphone amplifiers have

their outputs biased about a nominal DC voltage (typi-

cally half the supply) for maximum dynamic range.

Large-coupling capacitors are needed to block this DC

bias from the headphone. Without these capacitors, a

significant amount of DC current flows to the head-

phone, resulting in unnecessary power dissipation and

possible damage to both headphone and headphone

amplifier.

Maxim’s patented DirectDrive architecture uses a

charge pump to create an internal negative supply volt-

age, allowing the MAX9724A/MAX9724B outputs to be

biased about GND. With no DC component, there is no

need for the large DC-blocking capacitors. The

MAX9724A/MAX9724B charge pumps require two

small ceramic capacitors, conserving board space,

reducing cost, and improving the frequency response

of the headphone 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 off-

set. However, the offsets of the MAX9724A/MAX9724B

are typically 1.5mV, which, when combined with a 32Ω

load, results in less than 47µA of DC current flow to the

headphones.

VOUT

VDD

VDD/2

VDD

GND

CONVENTIONAL DRIVER-BIASING SCHEME

VOUT

VDD

GND

2VDD

-VDD

DirectDrive BIASING SCHEME

Figure 1. Conventional Driver Output Waveform vs.

MAX9724A/MAX9724B Output Waveform

Charge Pump

The MAX9724A/MAX9724B 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).

RF Susceptibility

Modern audio systems are often subject to RF radiation

from sources like wireless networks and cellular phone

networks. Although the RF radiation is out of the audio

band, many signals, in particular GSM signals, contain

bursts or modulation at audible frequencies. Most ana-

log amplifiers demodulate the low-frequency envelope,

adding noise to the audio signal. The architecture of

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