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Thermal-overload protection limits total power dissipation

in the MAX13330/MAX13331. When the junction tem-

perature exceeds +145°C (typ), the thermal-protection cir-

cuitry 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

VSS headroom. In typical stereo audio applications, the

left and right signals have differences in both magnitude

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 reality, the

available power lies between these extremes.

UVLO

The MAX13330/MAX13331 feature a UVLO function that

prevents the device from operating if the supply volt-

age 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 capacitors

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:

(V / V)

= −

Choose feedback resistor values of 10kΩ. Values other

than 10kΩ 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

The input capacitor (CIN), in conjunction with the input

resistor (RIN), forms a highpass filter that removes the 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:

3dB

(Hz)

−

π×

Choose CIN so f-3dB is well below the lowest frequency

of interest. For the MAX13330, use the value of RIN as

given in the Electrical Characteristics table. Setting f-3dB

too high affects the device’s low-frequency response. Use

capacitors whose dielectrics have lowvoltage 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 100mΩ for optimum

performance. Low-ESR ceramic capacitors minimize

the output resistance of the charge pump. For best per-

formance over the extended temperature range, select

capacitors with an X7R dielectric.

Figure 4. Output Power vs. Supply Voltage

OUTPUT POWER vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

4.25

5.00 5.25

4.75

4.50

100

150

200

250

4.00

5.50

fIN = 1kHz

RL = 32Ω

THD+N = 10%

INPUTS

IN PHASE

INPUTS 180°

OUT OF PHASE

MAX13330/MAX13331

Automotive DirectDrive Headphone Amplifiers

with Output Protection and Diagnostics

www.maximintegrated.com

Maxim Integrated │ 11

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