MAX8821 Datasheet, PDF (18/29 Pages) Maxim Integrated Products – White LED Charge Pump with Mono Class D Audio Amp and Dual LDO

White LED Charge Pump with Mono Class D
Audio Amp and Dual LDO
Audio Amplifier
The audio amplifier in the MAX8821 is a 2W Class D
loudspeaker amplifier. The amplifier features a low-
power shutdown mode and industry-leading click-and-
pop suppression. The amplifier also features a
programmable gain control through an I²C interface.
The amplifier operates from a single 2.7V to 5.5V sup-
ply (V_IN3 = V_IN2) and features an internally generated
common-mode bias voltage of V_IN3 / 2 referenced to
ground.
Click-and-Pop Suppression
The MAX8821 features Maxim’s industry-leading click-
and-pop suppression circuitry. During startup, the
amplifier’s common-mode bias voltage ramps to the
DC bias point. When entering shutdown, the amplifier
outputs are high impedance with 100kΩ between both
inputs. This scheme minimizes the energy present in
the audio band.
Class D Amplifier
The MAX8821 ultra-low-EMI, filterless, Class D audio
power amplifier features several improvements to
switch mode amplifier technology. The MAX8821 audio
amplifier features output driver AEL circuitry to reduce
EMI. Zero dead-time technology maintains state-of-the-
art efficiency and THD+N performance by allowing the
output MOSFETs to switch simultaneously without
cross-conduction.
A unique filterless modulation scheme and spread-
spectrum mode create a compact, flexible, low-noise,
efficient audio power amplifier while occupying minimal
board space. The differential input architecture reduces
common-mode noise pickup with or without the use of
input-coupling capacitors. The MAX8821 audio amplifier
can also be configured as a single-ended input amplifier
without performance degradation. The input capacitors
C_IN (Figure 7) are required for single-ended input appli-
cations and are typically 1µF.
The output of the MAX8821 shuts down if the output cur-
rent reaches approximately 2A. Each output MOSFET
has its own short-circuit protection. This protection
scheme allows the amplifier to survive shorts to either
supply rail. After a thermal overload or short circuit, the
device remains disabled for a minimum of 110µs before
attempting to return to normal operation. The amplifier
shuts down immediately and waits another 110µs before
turning on if the fault condition remains. This operation
causes the output to pulse during a persistent fault.
Comparators monitor the MAX8821 inputs and compare
the complementary input voltages to the sawtooth wave-
form. The comparators trip when the input magnitude of
the sawtooth exceeds their corresponding input voltage.
ILED5[4:0]
ILED6[4:0]
0.512s = t_{OFF_BLINK} = 4.098s
64ms = t_{ON_BLINK} = 512ms
Figure 5. Blink Timer Behavior
Both comparators reset at a fixed time after the rising
edge of the second comparator trip point, generating a
minimum width pulse t_ON(MIN) at the output of the sec-
ond comparator. As the input voltage increases or
decreases, the duration of the pulse at one output
increases (the first comparator to trip), while the other
output pulse duration remains at t_ON(MIN). This causes
the net voltage across the speaker (SPK+ - SPK-)
to change.
Adjustable Differential Gain
The audio amplifier has an internal gain control. The
gain of the input amplifiers is controlled through the I²C
interface. The gain setting of the input amplifier can be
set from -3dB to +24dB (Table 15). This allows the
amplifier to be used for both hands-free and for receiv-
er mode without any external components.
Input Filter
The fully differential amplifier inputs can be biased at
voltages other than midsupply. The common-mode
feedback circuit adjusts for input bias, ensuring the
outputs are still biased at midsupply. Input capacitors
are not required as long as the common-mode input
voltage is within the specified range listed in the
Electrical Characteristics table. If input capacitors are
used, input capacitor C_IN, in conjunction with on-chip
R_INT, forms a highpass filter that removes the DC bias
from an incoming signal. The AC-coupling capacitor
allows the amplifier to bias the signal to an optimum DC
level. Assuming zero-source impedance, the -3dB point
of the highpass filter is given by:
f
−3dB
=
2×
1
π × R_INT
× C_IN
Setting the -3dB corner too high affects the low-fre-
quency response of the amplifier. Use capacitors with
dielectrics that have low-voltage coefficients, such as
aluminum electrolytic. Capacitors with high voltage
coefficients, such as ceramics, can increase distortion
at low frequencies.
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