Datasheet MAX4295 (Maxim) - 10

Mono, 2W, Switch-Mode (Class D) Audio Power Amplifier Soft-Start (Clickless Startup) Frequency Selection
The H-bridge is disabled under any of the following The MAX4295 has an internal logic-programmable conditions: oscillator controlled by FS1 and FS2 (Table 1). The • SHDN low oscillator can be programmed to frequencies of • H-bridge current exceeds the 1A current limit 125kHz, 250kHz, 500kHz, and 1MHz. The frequency should be chosen to best fit the application. As a rule of • Thermal overload thumb, choose fOSC to be 10 times the audio band- • Undervoltage lockout width. A lower switching frequency offers higher ampli- The circuit re-enters normal operation if none of the fier efficiency and lower THD but requires larger
MAX4295
above conditions are present. A soft-start function pre- external filter components. A higher switching frequen- vents an audible pop on restart. An external capacitor cy reduces the size and cost of the filter components at connected to SS is charged by an internal 1.2µA cur- the expense of THD and efficiency. In most applica- rent source and controls the soft-start rate. VSS is held tions, the optimal fOSC is 250kHz. low while the H-bridge is disabled and allowed to ramp up to begin a soft-start. Until VSS reaches 0.3 ✕ VCC, the H-bridge output is limited to a 50% duty cycle,
Table 1. Frequency Select Logic
independent of the input voltage. The H-bridge duty cycle is then gradually allowed to track the input signal
FS1 FS2 FREQUENCY (Hz)
at a rate determined by the ramp on SS. The soft-start 1 1 1M cycle is complete after VSS reaches 0.6 ✕ VCC. If the soft-start capacitor is omitted, the device starts up in 0 1 500k approximately 100µs. 1 0 250k 0 0 125k
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 R input resistor of the amplifier, creating a highpass filter F AOUT (Figure 3). Assuming the input node of the amplifier is a virtual ground, the -3dB point of the highpass filter is INPUT CIN RIN IN determined by: fLO = 1/(2π ✕ RIN ✕ CIN), where RIN is the input resistor, and CIN is the AC-coupling capaci- tor. Choose RIN as described in the Gain Setting sec- tion. Choose C VCM IN such that the corner frequency is below 20Hz. Figure 3. Input Amplifier Configuration
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