MAX8647 查看數據表（PDF） - Maxim Integrated

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MAX8647

Ultra-Efficient Charge Pumps for Six White/RGB LEDs in 3mm x 3mm Thin QFN

Maxim Integrated 

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 VIN 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 VIN. 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-

nal to obtain higher resolution at lower currents. See

Single-Wire Pulse Dimming in the Typical Operating

Characteristics section. As the ILED setting is below

6.4mA, the IC adjusts not only ILED 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.

I2C Interface (MAX8647)

An I2C 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 I2C write-byte commands are

used. Figure 2 shows a timing diagram for the I2C 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

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

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