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19-0786; Rev 0; 4/07
White LED Charge Pump with 1.1W Audio Amplifier
General Description
The MAX8678 integrates a charge pump for white lightemitting diodes (LEDs) with an audio loudspeaker amplifier. The high-efficiency, adaptive charge pump drives up to four LEDs with constant current for uniform brightness. 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 maintained throughout the adjustment range. Individual adaptive mode switching for each LED provides high efficiency with a wide range of LED forward voltages. The mono class AB audio amplifier directly drives an 8 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 singlewire serial-pulse interface. Clicks and pops are suppressed during on/off and all gain adjustments. Differential inputs improve common-mode noise rejection. The MAX8678 includes soft-start, thermal shutdown, open-circuit and short-circuit protection, and is available in the 16-pin, 3mm x 3mm Thin QFN package (0.8mm max height).
Features
High-Efficiency White LED Charge Pump Individual Adaptive Supply for Each LED 4 Low-Dropout LED Current Regulators 24mA to 0.1mA Dimming Range Single-Wire, Serial-Pulse Dimming Interface 1% (typ) Accuracy Low 140A Quiescent Current Mono Class AB Audio Amplifier 1.1W RMS Mono BTL Output (8, VIN = 5V) Low 0.004% THD+N at 1kHz High 90dB PSRR at 1kHz Fully Differential Inputs -9dB to +18dB Gain Settings in 3dB Steps Integrated Click/Pop Suppression Low Quiescent Current Soft-Start Limits Inrush Current Thermal Shutdown, Open- and Short-Circuit Protection 16-Pin, 3mm x 3mm Thin QFN Package
MAX8678
Ordering Information
PART MAX8678ETE+ PIN-PACKAGE 16 Thin QFN 3mm x 3mm TOP MARK AFF PKG CODE T1633-5
Applications
Cell Phones and Smartphones Camera Phones PDAs, Digital Cameras, Camcorders
+Denotes a lead-free package.
Typical Operating Circuit
INPUT 2.7V TO 5.5V 1F
Note: The device is specified over the -40C to +85C extended temperature range.
Pin Configuration
LED4 LED3 LED2 10 CLED1 9
TOP VIEW
C+ IN 4.7F GND LED1 LED2 LED3 LED4 LED ON/OFF AND DIMMING AMP ON/OFF AND GAIN DIFFERENTIAL AUDIO INPUT 0.1F ENLED MAX8678 ENAMP ENLED 16 A+ AEP NEG OUT+ OUTLOUDSPEAKER 8 BIAS BACKLIGHT OR FLASH 0.1mA TO 24mA NEG 13 C- 14 C+ 15
12
11
8 7
BIAS OUTOUT+ ENAMP
MAX8678
6 5
+
1 IN 2 GND 3 A+ 4 A-
1F
THIN QFN 3mm x 3mm
________________________________________________________________ Maxim Integrated Products
1
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
ABSOLUTE MAXIMUM RATINGS
IN to GND ..............................................................-0.3V to +6.0V IN to NEG ..............................................................-0.3V to +6.0V ENLED, ENAMP, LED_, C+, A+, A-, OUT+, OUT-, BIAS to GND...........................................-0.3V to (VIN + 0.3V) ENLED, ENAMP, LED_, C+, A+, A-, OUT+, OUT-, BIAS, C- to NEG ......................................-0.3V to (VIN + 0.3V) GND to NEG .........................................................-0.3V to +6.0V GND to C-..............................................................-0.3V to +6.0V OUT+, OUT- Short Circuit to GND or IN ....................Continuous Continuous Power Dissipation (TA = +70C) 16-Pin Thin QFN 3mm x 3mm (derate 17.5mW/C above +70C) ............................1398mW Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = 3.6V, VGND = 0V, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER IN Operating Voltage Undervoltage-Lockout (UVLO) Threshold UVLO Hysteresis Shutdown Supply Current (All Outputs Off) ENLED = ENAMP = GND TA = +25C TA = +85C VIN rising CONDITIONS MIN 2.7 2.25 2.45 100 0.1 0.1 140 0.45 3 +160 20 VIN = 2.7V to 4.2V VIN = 4.2V to 5.5V VIN = 2.7V to 5.5V VIL = 0V or VIH = 5.5V See Figures 2 and 3 See Figures 2 and 3 See Figures 2 and 3 See Figures 2 and 3 0.5 0.5 1 0.1 5 10 LED VF = 3.9V, VIN = 3.2V All LEDs off, ENLED = GND 96 10 23 TA = +25C TA = +85C 0.01 0.1 3.2 3.2 500 500 1.4 1.5 0.4 1 C C 200 1.5 TYP MAX 5.5 2.65 UNITS V V mV A A
Charge pump inactive, LED_ = 1.6mA ENAMP = GND, TA = +25C No-Load Supply Current Charge pump active, 1MHz switching, LEDs at 24mA setting, ENAMP = GND ENAMP = IN, ENLED = GND Thermal Shutdown Thermal-Shutdown Hysteresis SERIAL-PULSE INTERFACE (EN_) Logic Input-High Voltage Logic Input-Low Voltage Logic Input Current tSHDN tHOLD tLO tHI CHARGE PUMP Switching Frequency Soft-Start Time Regulation Voltage (OVP) (VIN - VNEG) Open-Loop NEG Output Resistance ((VNEG -VIN)/INEG) Guaranteed Output Current NEG Discharge Resistance in Shutdown
mA
V V A ms ms s s MHz ms V mA k
2
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White LED Charge Pump with 1.1W Audio Amplifier
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, VGND = 0V, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER LED DRIVER Current-Setting Range 24mA setting, TA = +25C Current Accuracy 24mA setting, TA = -40C to +85C 1.6mA setting, TA = +25C LED_ Dropout LED_ Voltage-Mode Transition Threshold (1x to 2x) LED_ Voltage-Mode Transition Hysteresis LED_ Leakage in Shutdown AUDIO AMPLIFIER Common-Mode Bias Voltage Output Offset Voltage Common-Mode Input Voltage Input Impedance (RIN) Common-Mode Rejection Ratio Power-Supply Rejection Ratio Table 2 VIN = 3.2V f = 1kHz, VIN = 3.2V VA+ = VA- = VBIAS, 200mVP-P at IN THD+N = 1%, f = 1kHz (Note 3) f = 217Hz f = 1kHz RL = 8, VIN = 3.2V RL = 4, VIN = 3.2V RL = 8, VIN = 5V Output Current Limit Gain Accuracy Total Harmonic Distortion + Noise Signal-to-Noise Ratio Maximum Capacitive Drive Power-Up from Shutdown Time, tUP Shutdown Time, tPD See Figure 3 See Figure 3 -9dB Gain 15dB Gain = 18dB RL = 8, f = 1kHz, POUT = 0.25W, VIN = 3.2V RL = 8, f = 1kHz, POUT = 0.25W, VIN = 3.2V 0.004 104 500 10 3.5 0.8 0.6 0.36 VA+ = VA- = VBIAS, gain 12dB -5% -25 0.5 10 50 67 60 70 93 90 0.5 0.65 1.1 1 1.6 0.13 0.17 A dB % dB pF ms s W VIN / 2 1 +5% +25 (VIN 1.2) 140 mV V k dB dB All LEDs off TA = +25C TA = +85C 24mA setting (Note 2) VLED_ falling Charge pump inactive Charge pump active 125 0.1 -2 -5 5 72 132 150 100 0.01 0.1 1 120 360 175 mV mV mV A 1 24.0 +2 +5 % mA CONDITIONS MIN TYP MAX UNITS
MAX8678
Output Power
Note 1: Limits are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design. Note 2: Dropout voltage is defined as the LED_ to GND (charge pump inactive) or LED_ to NEG (charge pump active) voltage at which current into LED_ drops 10% from the value at VLED_ = 0.5V. Note 3: Output power is specified by a combination of a functional output current test and characterization analysis.
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
Typical Operating Characteristics
(VIN = 3.2V, RL = 8, TA = +25C, unless otherwise noted.)
TOTAL HARMONIC DISTORTION + NOISE vs. FREQUENCY
MAX8678 toc01
TOTAL HARMONIC DISTORTION + NOISE vs. OUTPUT POWER
MAX8678 toc02
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX8678 toc03
0.1
100
2000
10
OUTPUT POWER = 150mW THD+N (%) OUTPUT POWER = 50mW 0.01 THD+N (%) 1 fIN = 10kHz
OUTPUT POWER (mW)
fIN = 1kHz
THD+N = 10% 1000 THD+N = 1%
0.1
OUTPUT POWER = 300mW 0.001 10 GAIN = 3dB 100 1000 FREQUENCY (Hz) 10,000 100,000
0.01 GAIN = 3dB 0 0.2 0.4 fIN = 100kHz 0 0.6 0.8 1.0 2.5 3.5 4.5 5.5 OUTPUT POWER (W) SUPPLY VOLTAGE (V)
0.001
POWER DISSIPATION vs. OUTPUT POWER
MAX8678 toc04
OUTPUT POWER vs. LOAD RESISTANCE
MAX8678 toc05
AMPLIFIER GAIN AND PHASE vs. FREQUENCY
25dB 20dB AMPLIFIER GAIN AND PHASE 15dB 10dB 5dB 0dB 0 -30 -60 -90 GAIN = 18dB
MAX8678 toc06
500 POWER DISSIPATION POWER (mW)
1.2 1.0 OUTPUT POWER (W) 0.8 0.6 0.4 0.2 0 THD+N = 1% THD+N = 10%
400
300
200
100
0 0 100 200 300 400 500 600 700 OUTPUT POWER (mW)
4
6
8
10
12
14
16
10
100
1k FREQUENCY (Hz)
10k
100k
LOAD RESISTANCE ()
COMMON-MODE REJECTION RATIO vs. FREQUENCY
-25dB -35dB -45dB REJECTION RATIO REJECTION RATIO -55dB -65dB -75dB -85dB -95dB GAIN = 18dB VRIPPLE 100mVP-P 100 1k FREQUENCY (Hz) 10k
MAX8678 toc07
POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
-25dB -35dB -45dB -55dB -65dB -75dB -85dB -95dB -105dB -125dB 100 1k FREQUENCY (Hz) 10k VENAMP VOUTVOUT+ - VOUTGAIN = 18dB VRIPPLE 18mVP-P VOUT+
MAX8678 toc08
AMPLIFIER STARTUP AND SHUTDOWN
MAX8678 toc09
1V/div
1V/div
100mV/div
2V/div
4ms/div
4
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White LED Charge Pump with 1.1W Audio Amplifier
Typical Operating Characteristics (continued)
(VIN = 3.2V, RL = 8, TA = +25C, unless otherwise noted.)
LED EFFICIENCY vs. SUPPLY VOLTAGE (MISMATCHED LEDs)
MAX8678 toc10
MAX8678
SUPPLY CURRENT vs. SUPPLY VOLTAGE (MISMATCHED LEDs)
180 160 SUPPLY CURRENT (mA) 140 120 100 80 60 40 20 0 4 x 1.6mA 4 x 6.4mA 4 x 20.8mA 4 x 16mA
MAX8678 toc11
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 2.7 3.0 3.3 3.6 3.9 4 x 6.4mA 4 x 20.8mA 4 x 1.6mA
200
4.2
4.2
3.9
3.6
3.3
3.0
2.7
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
LED EFFICIENCY vs. SUPPLY VOLTAGE (MATCHED LEDs)
MAX8678 toc12
MAX8678 EFFICIENCY vs. SUPPLY VOLTAGE (MATCHED LEDs)
20.4mA/LED 1.6mA/LED
MAX8678 toc13
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 2.7 3.0 3.3 3.6 3.9 1.6mA/LED 16.4mA/LED 6.4mA/LED 20.4mA/LED
100 90 80 70
EFFICIENCY
6.4mA/LED 16.4mA/LED
60 50 40 4.2 4.20 3.90
SUPPLY VOLTAGE (V)
3.85 3.75 3.65 3.55 3.40 3.00 3.50 3.80 3.70 3.60
SUPPLY VOLTAGE, TIME WEIGHTED (V)
LED CURRENT MATCHING vs. SUPPLY VOLTAGE
16.8 16.6 LED CURRENT (mA) 16.4 16.2 16.0 15.8 15.6 15.4 15.2 15.0 2.7 3.0 3.3 3.6 3.9 4.2 4 OF 16mA/LED IIN VLED1
MAX8678 toc14
LED STARTUP/ SHUTDOWN WAVEFORMS
MAX8678 toc15
17.0
VENLED
2V/div
1V/div
500mA/div
1ms/div
SUPPLY VOLTAGE (V)
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
Typical Operating Characteristics (continued)
(VIN = 3.2V, RL = 8, TA = +25C, unless otherwise noted.)
LED SINGLE-WIRE PULSE DIMMING
MAX8678 toc16
LINE TRANSIENT WITH MODE TRANSITION
MAX8678 toc17
VENLED
5V/div
VIN
3.8V 3.4V
3.8V
1V/div
ILED 20mA/div
ILED (TOTAL) IIN 4 LEDs AT 24mA EACH
100mA/div
200mA/div
200ms/div
200s/div
Pin Description
PIN 1 2 3 4 5 6 7 8 9-12 NAME IN GND A+ AENAMP OUT+ OUTBIAS FUNCTION Analog Supply-Voltage Input. The input voltage range is 2.7V to 5.5V. Connect a 4.7F ceramic capacitor from IN to GND as close as possible to the IC. IN is high impedance during shutdown. Ground. Connect GND to the system ground plane. Do not connect GND to the exposed paddle. Noninverting Differential Audio Input Inverting Differential Audio Input Amplifier Enable and Gain Control. ENAMP is a serial-pulse interface logic input to control on/off and gain settings of the audio amplifier. Audio Amplifier Positive Output Audio Amplifier Negative Output DC Bias Bypass. Connect a 0.1F ceramic capacitor from BIAS to GND as close as possible to the IC.
LED Current Regulators. Current flowing into LED_ is regulated to the current programmed by the LED1-LED4 serial-pulse interface. Connect LED_ to the cathodes of the corresponding external LEDs. LED_ is high impedance during shutdown. If unused, connect LED_ to IN to disable the regulator. NEG CC+ ENLED EP Charge-Pump Negative Output. Connect a 1F ceramic capacitor from NEG to GND. In shutdown, an internal 10k resistor pulls NEG to GND. Transfer Capacitor Negative Connection. Connect a 1F ceramic capacitor from C+ to C-. Transfer Capacitor Positive Connection. Connect a 1F ceramic capacitor from C+ to C-. LED Enable and Dimming Control. ENLED is a serial-pulse interface logic input to control on/off and dimming of the LED regulators. Exposed Paddle. Connect the exposed paddle to NEG directly under the IC.
13 14 15 16 --
6
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
C3 1F C+ 2.7V TO 5.5V INPUT SUPPLY C1 4.7F IN INVERTING CHARGE PUMP CC2 1F NEG
MAX8678
GND
LED1
LED ON/OFF AND DIMMING
LED2 ENLED CONTROL CIRCUITRY LED CURRENT REGULATORS
LED3
LED4 AMP ON/OFF AND GAIN
ENAMP
CONTROL CIRCUITRY
HALF SUPPLY BIAS
BIAS C4 0.1F OUT+ OUTEARPIECE/ LOUDSPEAKER
A+ DIFFERENTIAL AUDIO INPUT AOPTIONAL FILTER CAPACITORS
MONO CLASS AB AUDIO AMP
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 constant current for uniform brightness for display backlighting 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 maintained throughout the adjustment range. Figure 1 is the MAX8678 functional diagram. The mono class AB audio amplifier directly drives an 8 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 singlewire serial-pulse interface. Clicks and pops are suppressed 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 0.1A.
Thermal-Overload Protection
The thermal-overload protection circuitry monitors the temperature of the IC. When the die temperature reaches +160C, the IC is shut down. The IC turns on after it has cooled by approximately 20C. During continuous overload conditions, this results in a pulsed output.
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
LED Charge Pump
The charge pump drives up to four white LEDs with regulated constant current for displaying backlight applications. By utilizing an adaptive charge pump and extremely low-dropout current regulators, it achieves high efficiency over the full 1-cell lithium ion (Li+) battery input-voltage range. Fixed frequency of 1MHz switching allows for tiny external components. The regulation scheme is optimized to ensure low EMI and low input ripple.
Table 1. Internal PWM Duty Cycle vs. LED Set Current
ILED (mA) 6.4 5.6 4.8 4.0 3.2 2.8 2.4 2.0 1.6 1.4 DUTY CYCLE (n/16) 16 14 12 10 16 14 12 10 16 14 ILED (mA) 1.2 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 DUTY CYCLE (n/16) 12 10 8 7 6 5 4 3 2 1
Adaptive Charge-Pump Modes When the supply voltage is sufficient to drive the LEDs, the charge pump is turned off to minimize the input supply current and the LED currents are linearly regulated. When the supply voltage is insufficient to drive the LEDs at the set current, the charge pump is enabled, creating a negative voltage at NEG and allowing the LED_ outputs to pull below ground to maintain the set LED current. Low LED Current Levels The MAX8678 internally generates a PWM signal to obtain higher resolution at lower currents. As the ILED setting is below 6.4mA, the IC adjusts ILED DC current, and the duty cycle is controlled by the PWM signal. The frequency of the PWM dimming signal is set at 1kHz with a minimum duty cycle of 1/16 to avoid the LED flicking effect to human eyes. Table 1 shows the current level and the corresponding duty cycle. Soft-Start and Shutdown The MAX8678 LED driver features a low-power shutdown mode for reduced current consumption. Hold ENLED low for 3.2ms to enter shutdown mode. This disables the charge pump and the LED current drivers.
When initially powering the MAX8678, or when starting up from shutdown mode, a soft-start feature prevents input current overshoot. See the startup waveforms in the Typical Operating Characteristics section.
1 ENLED tHI INTERNAL CURRENT SETTING ILED_ OFF tLO tHOLD 3.2ms 19.2mA 19.2mA 24.0mA 2 3 4 1 2
Serial-Pulse Dimming Control (ENLED) The MAX8678 includes a serial-pulse logic interface for on/off and dimming of the backlight. The dimming range is pseudo-logarithmic from 24mA to 0.1mA in 31 steps. Driving ENLED high turns on the IC and sets the internal register to 24.0mA. To dim the MAX8678, pulse ENLED low (500ns to 500s pulse width). Each rising edge sets the LED current setting as shown in Table 2. Once the desired setting is reached, hold ENLED high for at least 3.2ms to set the internal register and the LED current changes. To set a new current level, repeat the previous sequence from the beginning. To turn off the LEDs, hold ENLED low for at least 3.2ms. Figure 2 shows a timing diagram for ENLED. If dimming control is not required, ENLED works as a simple on/off logic control. Drive ENLED high for at least 3.2ms to enable the LEDs, or drive ENLED low for at least 3.2ms for shutdown. The LED driver operates at 100% brightness under or OFF under these conditions.
3
13
14
tHI = 500ns TO 500s tLO = 500ns TO 500s tHOLD 3.2ms tSHDN 3.2ms OFF
OFF
24.0mA 22.4mA 20.8mA
22.4mA 20.8mA
5.6mA
4.8mA
4.8mA
OFF
Figure 2. Serial-Pulse Dimming
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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 --
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.
MAX8678
Bias An internally generated common-mode BIAS voltage of VIN / 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 waveform for the speaker amplifier. A 0.1F BIAS capacitor is recommended for most applications. Increasing C4 to 1F 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 shutdown 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 clickand-pop suppression circuitry. During startup, the amplifier common-mode bias voltage ramps to the DC bias point. When entering shutdown, the amplifier outputs are high impedance to 100k between the outputs. This scheme minimizes the energy present in the audio band.
Open-Circuit and Short-Circuit Protection The short-circuit protection detects when an LED is shorted and disables the corresponding current regulator to avoid wasting battery power. If any LED fails as an open circuit, the charge pump is forced on and the corresponding LED_ pin is internally connected to NEG.
1 ENAMP
2
3
4
1
2
3
7
8
tHI = 500ns TO 500s tLO = 500ns TO 500s tHOLD 3.2ms tSHDN 3.2ms OFF
tHI INTERNAL GAIN SETTING GAIN
tLO
tHOLD 3.2ms -3dB 0dB tUP 10ms typ -9dB -6dB -3dB +9dB
OFF
-9dB
-6dB
+12dB
tUP 10ms (typ) +12dB
tPD 1s (typ)
0dB OFF
OFF
Figure 3. Serial-Pulse Gain Adjustment
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White LED Charge Pump with 1.1W Audio Amplifier
Serial-Pulse Gain Adjustment (ENAMP) The audio amplifier has an internal gain adjustment, controlled by a serial-pulse interface. The gain setting of the input amplifier is adjustable from -9dB to +18dB in 3dB steps. This allows the amplifier to be used for both hands-free and for receiver mode, without any external components. Driving ENAMP high turns on the amplifier and sets the internal register to -9dB. Adjust the gain by pulsing ENAMP low (500ns to 500s pulse width). Each rising edge sets the gain as shown in Table 3. Once the desired setting is reached, hold ENAMP high for at least 3.2ms to set the internal register and change the gain. To set a new current level, repeat the previous sequence from the beginning. To shut down the amplifier, hold ENAMP low for at least 3.2ms. Figure 3 is a timing diagram for ENAMP.
MAX8678
Table 3. Audio Amplifier-Input Impedance
ENAMP RISING EDGES 1 2 3 4 5 6 7 8 9 10 GAIN SETTING (dB) -9 -6 -3 0 3 6 9 12 15 18 RIN (k) MIN 69.1 62.4 54.9 46.8 38.8 31.3 24.6 18.8 14.1 10.5 TYP 99.0 89.2 78.4 66.9 55.5 44.7 35.1 26.9 20.2 15.0 MAX 140.0 116.0 102.0 87.0 72.1 58.1 45.6 35.0 26.2 19.5
Applications Information
BTL Amplifier The MAX8678 is designed to drive a load differentially, a configuration referred to as bridge-tied load (BTL). The BTL configuration offers advantages over the single-ended configuration, where one side of the load is connected to ground. Driving the load differentially doubles the output voltage, thus quadrupling output power compared to a single-ended amplifier under similar configurations.
Since the differential outputs are biased at midsupply, there is no net DC voltage across the load. This eliminates the need for DC-blocking capacitors required for single-ended amplifiers. These capacitors can be large, expensive, consume board space, and degrade low-frequency performance.
A+ DIFFERENTIAL AUDIO INPUT
RIN
MAX8678
OUT+ MONO CLASS AB AUDIO AMP EARPIECE / LOUDSPEAKER
ARIN
OUT-
OPTIONAL FILTER CAPACITORS
Figure 4. Differential Audio Input with Optional Filter Capacitors
SINGLE-ENDED AUDIO INPUT
A+
RIN
MAX8678
MONO CLASS AB AUDIO AMP
OUT+ EARPIECE / LOUDSPEAKER
ARIN
OUT-
NOTE: FILTER CAPACITORS REQUIRED FOR SINGLE-ENDED AUDIO OUTPUT.
Figure 5. Single-Ended Audio Input
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White LED Charge Pump with 1.1W Audio Amplifier
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. If input capacitors are used (see Figure 4), the input capacitor in conjunction with the amplifier input impedance (RIN) form 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 = 1 2 x RIN x CIN Setting f -3dB too high affects the low-frequency response of the amplifier. Use capacitors with dielectrics that have low-voltage coefficients, such as tantalum or aluminum electrolytic. Capacitors with high voltage coefficients, such as ceramics, can increase distortion at low frequencies.
MAX8678
where RIN is the input impedance given in Table 2. Figure 5 shows a single-ended audio input.
PCB Layout Good PC board (PCB) layout is essential for optimizing performance. Use large traces for the power-supply inputs and amplifier outputs to minimize losses due to parasitic trace resistance and to route heat away from the device. Good grounding improves audio performance and prevents any digital switching noise from coupling into the audio signal. The exposed paddle lowers the thermal resistance of the package by providing a direct-heat conduction path from the die to the PCB. Connect the exposed paddle to NEG directly under the IC. Refer to the MAX8678 Evaluation Kit for a good PCB layout example.
Chip Information
PROCESS: BiCMOS
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White LED Charge Pump with 1.1W Audio Amplifier MAX8678
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
12x16L QFN THIN.EPS
L
MARKING
E E/2
(ND - 1) X e
(NE - 1) X e
D2/2
D/2 D
AAAA
C L
e D2
k
b E2/2
0.10 M C A B
C L
L
E2
0.10 C
0.08 C A A2 A1 L
C L
C L
e
e
PACKAGE OUTLINE 8, 12, 16L THIN QFN, 3x3x0.8mm
21-0136
I
1 2
12
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White LED Charge Pump with 1.1W Audio Amplifier
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
MAX8678
PKG REF. A b D E e L N ND NE A1 A2 k 0.25 0 0.35
8L 3x3 MIN. NOM. MAX. 0.70 0.25 2.90 2.90 0.75 0.30 3.00 3.00 0.55 8 2 2 0.02 0.20 REF 0.25 0.05 0 0.80 0.35 3.10 3.10 0.75
12L 3x3 MIN. NOM. MAX. 0.70 0.20 2.90 2.90 0.45 0.75 0.25 3.00 3.00 0.55 12 3 3 0.02 0.20 REF 0.25 0.05 0 0.80 0.30 3.10 3.10 0.65
16L 3x3 MIN. NOM. MAX. 0.70 0.20 2.90 2.90 0.30 0.75 0.25 3.00 3.00 0.40 16 4 4 0.02 0.20 REF 0.05 0.80 0.30 3.10 3.10 0.50 PKG. CODES TQ833-1 T1233-1 T1233-3 T1233-4 T1633-2 T1633F-3 T1633FH-3 T1633-4 T1633-5
EXPOSED PAD VARIATIONS
D2 MIN. 0.25 0.95 0.95 0.95 0.95 0.65 0.65 0.95 0.95 NOM. 0.70 1.10 1.10 1.10 1.10 0.80 0.80 1.10 1.10 MAX. 1.25 1.25 1.25 1.25 1.25 0.95 0.95 1.25 1.25 MIN. 0.25 0.95 0.95 0.95 0.95 0.65 0.65 0.95 0.95 E2 NOM. 0.70 1.10 1.10 1.10 1.10 0.80 0.80 1.10 1.10 MAX. 1.25 1.25 1.25 1.25 1.25 0.95 0.95 1.25 1.25 PIN ID 0.35 x 45 0.35 x 45 0.35 x 45 0.35 x 45 0.35 x 45 0.225 x 45 0.225 x 45 0.35 x 45 0.35 x 45 JEDEC WEEC WEED-1 WEED-1 WEED-1 WEED-2 WEED-2 WEED-2 WEED-2 WEED-2
0.65 BSC.
0.50 BSC.
0.50 BSC.
NOTES: 1. 2. 3. 4. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES. N IS THE TOTAL NUMBER OF TERMINALS. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL CONFORM TO JESD 95-1 SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE TERMINAL #1 IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.20 mm AND 0.25 mm FROM TERMINAL TIP. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS. DRAWING CONFORMS TO JEDEC MO220 REVISION C. MARKING IS FOR PACKAGE ORIENTATION REFERENCE ONLY. NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY. WARPAGE NOT TO EXCEED 0.10mm.
5. 6. 7. 8. 9. 10. 11. 12.
PACKAGE OUTLINE 8, 12, 16L THIN QFN, 3x3x0.8mm
21-0136
I
2 2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13
(c) 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

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