Mono/Stereo 2W Switch-Mode (Class-D)

Audio Power Amplifiers

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13

The amplifier’s input bias current is low, ±50pA, and does

not affect the choice of feedback resistors. The noise in

the circuit increases as the value of RF increases.

The optimum impedance seen by the inverting input is

between 5k

Ω and 20kΩ. The effective impedance is

50k

Ω, a small capacitor (≈3pF) connected across RF

compensates for the pole formed by the input capaci-

tance and the effective resistance at the inverting input.

Soft-Start (Clickless Startup)

The H-bridge is disabled under any of the following

conditions:

• SHDN low

• H-bridge current exceeds the 1A current limit

• Thermal overload

• Undervoltage lockout

The circuit re-enters normal operation if none of the

above conditions are present. A soft-start function pre-

vents an audible pop on restart. An external capacitor

connected to SS is charged by an internal 1.2µA cur-

low while the H-bridge is disabled and allowed to ramp

the H-bridge output is limited to a 50% duty cycle,

independent of the input voltage. The H-bridge duty

cycle is then gradually allowed to track the input signal

at a rate determined by the ramp on SS. The soft-start

Input Filter

High-fidelity audio applications require gain flatness

between 20Hz to 20kHz. Set the low-frequency cutoff

point with an AC-coupling capacitor in series with the

input resistor of the amplifier, creating a highpass filter

(Figure 3). Assuming the input node of the amplifier is a

virtual ground, the -3dB point of the highpass filter is

the input resistor, and CIN is the AC-coupling capaci-

tor. Choose RIN as described in the Gain Setting sec-

tion. Choose CIN such that the corner frequency is

below 20Hz.

Frequency Selection

The MAX4295/MAX4297 have an internal logic-pro-

grammable oscillator controlled by FS1 and FS2 (Table

1). The oscillator can be programmed to frequencies of

125kHz, 250kHz, 500kHz, and 1MHz. The frequency

should be chosen to best fit the application. As a rule of

width. A lower switching frequency offers higher ampli-

fier efficiency and lower THD but requires larger

external filter components. A higher switching frequen-

cy reduces the size and cost of the filter components at

the expense of THD and efficiency. In most applica-

INPUT

CIN

RIN

RF

AOUT

IN

VCM

FS1

FS2

FREQUENCY (Hz)

0

0

125k

0

1

250k

1

0

500k

11

1M

Table 1. Frequency Select Logic

Figure 3. Input Amplifier Configuration

+5V

0

Figure 2. PWM Waveforms