Rainbow Electronics MAX13331 - Manual (Page 8)

MAX13330/MAX13331
Automotive DirectDrive Headphone Amplifiers
with Output Protection and Diagnostics
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Detailed Description
The MAX13330/MAX13331 headphone amplifiers fea-
ture Maxim’s patented DirectDrive architecture,
eliminating the large output-coupling capacitors
required by conventional single-supply headphone
amplifiers. The devices consists of two Class AB head-
phone amplifiers, undervoltage 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 8Ω, and deliv-
er up to 120mW per channel into 16Ω and 135mW into
32Ω. 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
(V
BAT up to +45V). An integrated short-circuit diagnos-
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 patented DirectDrive architecture uses a
charge pump to create an internal negative-supply volt-
age, 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 32Ω 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.
V DD
V SS
GND V OUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
V DD /2
V DD
GND
V OUT
Figure 1. Conventional Driver Output Waveform vs. MAX13330/
MAX13331 Output Waveform