Datasheet

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-
rent source and controls the soft-start rate. V
SS
is held
low while the H-bridge is disabled and allowed to ramp
up to begin a soft-start. Until V
SS
reaches 0.3
✕
V
CC
,
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
cycle is complete after V
SS
reaches 0.6
✕
V
CC
. If the
soft-start capacitor is omitted, the device starts up in
approximately 100µs.
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
determined by: f
LO
= 1/(2π
✕
R
IN
✕
C
IN
), where R
IN
is
the input resistor, and C
IN
is the AC-coupling capaci-
tor. Choose R
IN
as described in the Gain Setting sec-
tion. Choose C
IN
such that the corner frequency is
below 20Hz.
Frequency Selection
The MAX4295 has an internal logic-programmable
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
thumb, choose f
OSC
to be 10 times the audio band-
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-
tions, the optimal f
OSC
is 250kHz.
MAX4295
Mono, 2W, Switch-Mode (Class D)
Audio Power Amplifier
10 ______________________________________________________________________________________
INPUT
C
IN
R
IN
R
F
AOUT
IN
VCM
FS1 FS2 FREQUENCY (Hz)
111M
01500k
10250k
00125k
Table 1. Frequency Select Logic
Figure 3. Input Amplifier Configuration