Automotive DirectDrive Headphone Amplifiers

with Output Protection and Diagnostics

______________________________________________________________________________________

11

Thermal-overload protection limits total power dissipa-

tion in the MAX13330/MAX13331. When the junction

temperature exceeds +145°C (typ), the thermal-protec-

tion circuitry disables the amplifier output stage. The

amplifiers are enabled once the junction temperature

cools by 5°C. This results in a pulsing output under

continuous thermal-overload conditions.

Output Power

The device has been specified for the worst-case sce-

nario, when both inputs are in-phase. Under this condi-

tion, the amplifiers simultaneously draw current from the

charge pump, leading to a proportional reduction in

left and right signals have differences in both magni-

tude and phase, subsequently leading to an increase in

the maximum attainable output power. Figure 4 shows

the two extreme cases for in- and out-of-phase. In reali-

ty, the available power lies between these extremes.

UVLO

The MAX13330/MAX13331 feature a UVLO function that

prevents the device from operating if the supply voltage

is less than 3.6V (typ). This feature ensures proper

operation during brownout conditions and prevents

deep battery discharge. Once the supply voltage

reaches the UVLO threshold, the charge-pump is

turned on and the amplifiers are powered.

Component Selection

Gain-Setting Resistors (MAX13331 Only)

The gain of the MAX13330 is internally set at -1.5V/V.

All gain-setting resistors are integrated into the device,

reducing external component count. The internally set

gain, in combination with DirectDrive, results in a head-

phone amplifier that requires only five tiny 1µF capaci-

tors to complete the amplifier circuit: two for the

charge-pump, two for audio input coupling, and one for

power-supply bypassing (see the

Typical Application

Circuits). The gain of the MAX13331 amplifier is set

externally as shown in the

Typical Application Circuits,

the gain is:

Choose feedback resistor values of 10k

Ω. Values other

than 10k

Ω increase output offset voltage due to the

input bias current, which in turn, increases the amount

of DC current flow to the load.

Input Filtering

DC bias from an incoming signal (see the

Typical

Application Circuits). The AC-coupling capacitor allows

the device to bias the signal to an optimum DC level.

Assuming zero source impedance, the -3dB point of

the highpass filter is given by:

given in the

Electrical Characteristics table. Setting

response. Use capacitors whose dielectrics have low-

voltage coefficients, such as tantalum or aluminum

electrolytic. Capacitors with high-voltage coefficients,

such as ceramics, can result in increased distortion at

low frequencies.

Charge-Pump Capacitor Selection

Use capacitors with an ESR less than 100m

Ω for opti-

mum performance. Low-ESR ceramic capacitors mini-

mize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric.

f

RC

Hz

dB

IN

IN

−

=

××

3

1

2

π

()

A

R

R

VV

V

F

IN

=−

(/ )

OUTPUT POWER vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

4.25

5.00

5.25

4.75

4.50

50

100

150

200

250

0

4.00

5.50

Ω

THD+N = 10%

INPUTS

IN PHASE

INPUTS 180

°

OUT OF PHASE

Figure 4. Output Power vs. Supply Voltage