MAX8678 Datasheet, PDF (9/13 Pages) Maxim Integrated Products – White LED Charge Pump with 1.1W Audio Amplifier

White LED Charge Pump with
1.1W Audio Amplifier
Table 2. LED Current Levels
ENLED
RISING
EDGES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
LED CURRENT
(mA)
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
ENLED
RISING
EDGES
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
—
LED CURRENT
(mA)
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
—
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.
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.
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.
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 100kΩ between the outputs. This
scheme minimizes the energy present in the audio band.
ENAMP
1
2
3
4
1
2
3
INTERNAL
GAIN
SETTING
GAIN
tHI
tLO
tHOLD
3.2ms
OFF -9dB -6dB -3dB 0dB
OFF
tUP
10ms typ
-9dB -6dB -3dB
0dB
7
8
tHI = 500ns TO 500μs
tLO = 500ns TO 500μs
tHOLD
3.2ms
tSHDN
3.2ms
+9dB +12dB
tUP
10ms (typ) _+12dB
OFF
tPD
1μs (typ)
OFF
Figure 3. Serial-Pulse Gain Adjustment
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