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Ultra-Efficient Charge Pumps for
Six White/RGB LEDs in 3mm x 3mm Thin QFN
Detailed Description
The MAX8647/MAX8648 have an inverting charge
pump and six current regulators capable of 24mA each
to drive six white LEDs or two sets of RGB LEDs. The
current regulators are matched to within ±0.4% (typ)
providing uniform white LED brightness for LCD back-
light applications. To maximize efficiency, the current
regulators operate with as little as 0.15V voltage drop.
Individual white LED current regulators conduct current
to GND or NEG to extend usable battery life. In the
case of mismatched forward voltage of white LEDs,
only the white LEDs requiring higher voltage are
switched to pull current to NEG instead of GND, further
raising efficiency and reducing battery current drain.
Current-Regulator Switchover
When V IN is higher than the forward voltage of the
white LED plus the 0.15V headroom of the current regu-
lator, the LED current returns through GND. If this con-
dition is satisfied for all six white LEDs, the charge
pump remains inactive. When the input voltage drops
so that the current-regulator headroom cannot be main-
tained for any of the individual white LEDs, the inverting
charge pump activates and generates a voltage on the
NEG pin that is no greater than 5V below V IN . Each cur-
rent regulator contains circuitry that detects when it is
in dropout and switches that current-regulator return
path from GND to NEG. Since this is done on an LED-
by-LED basis, the LED current is switched for only the
individual LED requiring higher voltage, thus minimizing
power consumption.
Low LED Current Levels
The MAX8647/MAX8648 internally generate a PWM sig-
used. Figure 2 shows a timing diagram for the I 2 C pro-
tocol. The MAX8647 is a slave-only device, relying
upon a master to generate a clock signal. The master
(typically a microprocessor) initiates data transfer on
the bus and generates SCL to permit data transfer. A
master device communicates with the MAX8647 by
transmitting the proper 8-bit address (0x9A) followed
by the 8-bit control byte. Each 8-bit control byte con-
sists of a 3-bit command code and 5 bits of data (Table
1). Each transmit sequence is framed by a START (A)
condition and a STOP (L) condition (Figure 2). Each
word transmitted over the bus is 8 bits long and is
always followed by an ACKNOWLEDGE CLOCK
PULSE (K). The power-on default settings for D4 to D0
are all 0, which indicates that all LED_ are off.
Serial-Pulse Dimming Control (MAX8648)
When the LEDs are enabled by driving EN_ high, the
MAX8648 ramps LED current to 24mA. Dim the LEDs
by pulsing EN_ low (1μs to 500μs pulse width). Each
pulse reduces the LED current based on the LED dim-
ming table, Table 3. After the current reaches 0.1mA,
the next pulse restores the current to 24mA. Figure 3
shows a timing diagram for EN_. ENA controls LED1,
LED2, and LED3. ENB controls LED4 and LED5. ENC
controls LED6.
If dimming control is not required, EN_ work as simple
100% brightness or off controls. Drive EN_ high to enable
the LEDs, or drive EN_ low to disable. The IC is shut-
down when all three EN_ are low for 4ms or longer.
Table 1. Internal PWM Duty Cycle vs. LED
Set Current
nal to obtain higher resolution at lower currents. See
Single-Wire Pulse Dimming in the Typical Operating
Characteristics section. As the I LED setting is below
6.4mA, the IC adjusts not only I LED DC current, but the
duty cycle is controlled by the PWM signal. The fre-
quency 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.
I 2 C Interface (MAX8647)
An I 2 C 2-wire serial interface is provided on the
MAX8647 to control the LEDs. The serial interface
consists of a serial-data line (SDA) and a serial-clock
line (SCL). Standard I 2 C write-byte commands are
I LED
(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
I LED
(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
8
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相关代理商/技术参数
MAX8648ETE 制造商:MAXIM 功能描述:Pb Free
MAX8648ETE+ 功能描述:LED照明驱动器 Charge Pump for 6 White/RGB LEDs RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX8648ETE+T 功能描述:LED照明驱动器 Charge Pump for 6 White/RGB LEDs RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX8648EVKIT+ 制造商:Maxim Integrated Products 功能描述:MAX8648 EVAL KIT - Rail/Tube
MAX8649AEWE+T 功能描述:直流/直流开关调节器 1.8A Step-Down Regulator RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5
MAX8649BEWE+T 功能描述:直流/直流开关调节器 1.8A Step-Down Regulator RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5
MAX8649EVKIT+ 制造商:Maxim Integrated Products 功能描述:1.8A STEP-DOWN REGULATOR W/DIFFERENTIAL REMOTE SENSE IN 2MM - Boxed Product (Development Kits)
MAX8649EWE+ 制造商:Maxim Integrated Products 功能描述:1.8A STEP-DOWN REGULATOR W/DIFFEREN - Rail/Tube