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White LED Charge Pump with
1.1W Audio Amplifier
C3
1 μ F
2.7V TO 5.5V
C+
C-
INPUT SUPPLY
C1
4.7 μ F
IN
GND
INVERTING
CHARGE
PUMP
MAX8678
NEG
C2
1 μ F
LED1
LED ON/OFF
LED2
AND DIMMING
ENLED
CONTROL
CIRCUITRY
LED CURRENT
REGULATORS
LED3
LED4
AMP ON/OFF
AND GAIN
ENAMP
CONTROL
CIRCUITRY
HALF
SUPPLY
BIAS
BIAS
C4
0.1 μ F
DIFFERENTIAL
AUDIO INPUT
A+
A-
MONO
CLASS AB
AUDIO AMP
OUT+
OUT-
EARPIECE/
LOUDSPEAKER
OPTIONAL FILTER
CAPACITORS
Figure 1. Functional Diagram
Detailed Description
The MAX8678 integrates a charge pump for white LEDs
with an audio loudspeaker amplifier. The high-efficiency,
adaptive charge pump drives up to four LEDs with con-
stant current for uniform brightness for display back-
lighting or camera flash. The LED current is adjustable
from 0.1mA/LED to 24mA/LED in 31 pseudo-logarithmic
steps through a single-wire serial-pulse interface. High
accuracy and LED-to-LED current matching are main-
tained throughout the adjustment range. Figure 1 is the
MAX8678 functional diagram.
The mono class AB audio amplifier directly drives an 8 Ω
loudspeaker with 1.1W RMS continuous power from a 5V
supply with less than 1% THD+N. The gain is adjustable
from -9dB to +18dB in ten 3dB steps through a single-
wire serial-pulse interface. Clicks and pops are sup-
pressed during on/off and all gain adjustments.
Differential inputs improve common-mode noise rejection.
Shutdown
To reduce power consumption when not in use, the
charge-pump LED driver and the audio amplifier have
separate shutdown controls. To disable the LED driver,
drive ENLED low for at least 3.2ms. To disable the
audio amplifier, drive ENAMP low for at least 3.2ms.
When both ENLED and ENAMP are held low, the IC
supply current is reduced to about 0.1μA.
Thermal-Overload Protection
The thermal-overload protection circuitry monitors the
temperature of the IC. When the die temperature reach-
es +160°C, the IC is shut down. The IC turns on after it
has cooled by approximately 20°C. During continuous
overload conditions, this results in a pulsed output.
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