Automotive DirectDrive Headphone Amplifiers

with Output Protection and Diagnostics

8

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Detailed Description

The MAX13330/MAX13331 headphone amplifiers fea-

ture Maxim’s DirectDrive architecture, eliminating the

large output-coupling capacitors required by conven-

tional single-supply headphone amplifiers. The devices

consists of two Class AB headphone amplifiers, under-

voltage lockout (UVLO), low-power shutdown control,

comprehensive click-and-pop suppression, output

short-circuit/ESD protection and output short-circuit

diagnostics.

These devices can drive loads as low as 8

Ω, and deliv-

er up to 120mW per channel into 16

Ω and 135mW into

32

Ω. The MAX13330 features a fixed gain of -1.5V/V,

and the MAX13331 features a programmable gain con-

figured with external resistors. The headphone outputs

feature ±15kV Human Body Model ESD protection, and

enhanced short-circuit protection to ground or battery

tic output provides the status of the MAX13330/

MAX13331 during operation as a fraction of the analog

supply voltage.

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 the headphone and the head-

phone amplifier.

Maxim’s DirectDrive architecture uses a charge pump

to create an internal negative-supply voltage, allowing

the MAX13330/MAX13331 outputs to be biased about

SGND (Figure 1). With no DC component, there is no

need for the large DC-blocking capacitors. Instead of

two large (220µF, typ) tantalum capacitors, the

MAX13330/MAX13331 charge pump requires 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

graph

in

the

Typical

Operating

Characteristics for details of the possible capacitor

sizes. There is a low DC voltage on the amplifier out-

puts due to amplifier offset. However, the output offset

of the MAX13330 is typically ±2.5mV which, when com-

bined with a 32

Ω load, results in less than ±78µA of DC

current flow to the headphones. Previous attempts to

eliminate the output-coupling capacitors involved bias-

ing the headphone return (sleeve) to the DC-bias volt-

age of the headphone amplifiers.

This method raises some issues:

•

The sleeve is typically grounded to the chassis.

Using this biasing approach, the sleeve must be

isolated from system ground, complicating product

design.

•

During an ESD strike, the amplifier’s ESD structures

are the only path to system ground. Thus, the ampli-

fier must be able to withstand the full ESD strike.

•

When using the headphone jack as a line out to

other equipment, the bias voltage on the sleeve

may conflict with the ground potential from other

equipment, resulting in possible damage to the

amplifiers.

GND

CONVENTIONAL DRIVER-BIASING SCHEME

DirectDrive BIASING SCHEME

GND

Figure 1. Conventional Driver Output Waveform vs. MAX13330/

MAX13331 Output Waveform