MAX4295 Maxim, MAX4295 Datasheet - Page 13

• Manufacturer Part Number: MAX4295
• Description: Mono/Stereo 2W Switch-Mode (Class-D) Audio Power Amplifiers
• Manufacturer: Maxim

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 5kΩ and 20kΩ. The effective impedance is
given by (RF
50kΩ, 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.
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
low while the H-bridge is disabled and allowed to ramp
up to begin a soft-start. Until V
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
Figure 2. PWM Waveforms
✕
RIN)/(RF + RIN). For values of RF >
______________________________________________________________________________________
Mono/Stereo 2W Switch-Mode (Class-D)
SS
Soft-Start (Clickless Startup)
reaches 0.6
SS
reaches 0.3
✕
V
CC
SS
.
+5V
V
0
OUT
✕
is held
V
V
IN
RAMP
V
CC
,
Audio Power Amplifiers
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
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.
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
thumb, choose f
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
Figure 3. Input Amplifier Configuration
Table 1. Frequency Select Logic
INPUT CIN
FS1
0
0
1
1
RIN
LO
OSC
OSC
= 1/(2π
to be 10 times the audio band-
is 250kHz.
RF
FS2
0
1
0
1
AOUT
IN
✕
Frequency Selection
RIN
VCM
✕
FREQUENCY (Hz)
125k
250k
500k
1M
CIN), where RIN is
Input Filter
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