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White LED Charge Pump with
1.1W Audio Amplifier
Table 2. LED Current Levels
ENLED ENLED
LED CURRENT
RISING RISING
(mA)
EDGES EDGES
LED CURRENT
(mA)
Audio Amplifier
The MAX8678 contains a 1.1W RMS DirectDrive mono
speaker amplifier. It features a low-power shutdown
mode and click-and-pop suppression.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
24.0
22.4
20.8
19.2
17.6
16.0
14.4
12.8
11.2
9.6
8.0
6.4
5.6
4.8
4.0
3.2
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
—
2.8
2.4
2.0
1.6
1.4
1.2
1.0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
—
Bias
An internally generated common-mode BIAS voltage of
V IN / 2 sets the DC bias level for the audio outputs. The
BIAS capacitor (C4 in Figure 1) improves PSRR and
THD+N by reducing power supply and other noise
sources at the common-mode bias node, and also
generates the clickless/popless startup DC bias wave-
form for the speaker amplifier. A 0.1μF BIAS capacitor
is recommended for most applications. Increasing C4
to 1μF slows turn-on and turn-off times by a factor of 10
and improves PSRR by 20dB (at 1kHz). Do not connect
external loads to BIAS.
Shutdown Mode
The MAX8678 audio amplifier features a low-power shut-
down mode for reduced current consumption. Hold
ENAMP low for 3.2ms to shut down the audio amplifier.
Entering shutdown disables the amplifier’s bias circuitry,
causes the audio outputs to go high impedance, and
drives BIAS to GND.
Open-Circuit and Short-Circuit Protection
The short-circuit protection detects when an LED is
shorted and disables the corresponding current regula-
tor to avoid wasting battery power. If any LED fails as an
open circuit, the charge pump is forced on and the cor-
responding LED_ pin is internally connected to NEG.
Click-and-Pop Suppression
The MAX8678 features Maxim’s industry-leading click-
and-pop suppression circuitry. During startup, the amplifi-
er common-mode bias voltage ramps to the DC bias
point. When entering shutdown, the amplifier outputs are
high impedance to 100k Ω between the outputs. This
scheme minimizes the energy present in the audio band.
ENAMP
1
2
3
4
1
2
3
7
8
t HI = 500ns TO 500 μ s
t LO = 500ns TO 500 μ s
t HI
t LO
t HOLD
3.2ms
t HOLD
3.2ms
t SHDN
3.2ms
INTERNAL
GAIN
SETTING
OFF
-9dB
-6dB
-3dB
0dB
t UP
-9dB
-6dB
-3dB
+9dB
+12dB
t UP
10ms (typ)
+12dB
OFF
t PD
1 μ s (typ)
GAIN
OFF
10ms typ
0dB
OFF
Figure 3. Serial-Pulse Gain Adjustment
_______________________________________________________________________________________
9
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