MAX8847Z 查看數據表（PDF） - Maxim Integrated

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MAX8847Z

High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN

Maxim Integrated 

MAX8847Y/MAX8847Z

High-Performance Negative Charge Pump

for 6 White LEDs in 3mm x 3mm Thin QFN

Pin Description (continued)

PIN

NAME

FUNCTION

15

CPWM

Filter Capacitor Connection for PWM Dimming. Connect a capacitor from CPWM to GND to form a filter

with the internal 360kI resistor. The recommended capacitor for a 2Hz corner frequency is 0.22FF.

Enable or PWM Dimming Control Input A. For the MAX8847Y, ENA functions as on/off control for

LED1–LED4 and PWM dimming control for LED1–LED4. For the MAX8847Z, ENA functions as on/

16

ENA

off control for LED1–LED4 and PWM dimming control for LED1–LED6. Drive ENA high to turn on the

LED1–LED4 current regulators at 24mA each. Drive ENA low for greater than 8ms to turn off the LED1–

LED4 current regulators or drive both ENA and ENB low to place the IC in shutdown. Drive ENA with a

PWM signal from 200Hz to 200kHz to dim the LEDs. See the PWM Dimming Control section.

—

EP

Exposed Paddle. Connect EP to NEG directly under the IC.

Detailed Description

The MAX8847Y/MAX8847Z have an inverting charge

pump and six current regulators capable of 24mA each

to drive up to 6 white LEDs. The current regulators are

matched to within 1% (typ) providing uniform white LED

brightness for LCD backlight 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 LED forward voltage plus the

150mV dropout voltage of the current regulator, the LED

current returns through GND. If this condition is satis-

fied for all active white LEDs, the charge pump remains

inactive. When the input voltage drops so that the cur-

rent regulator voltage (VLED_) cannot be maintained for

any of the individual white LEDs, the inverting charge

pump activates and generates a voltage on NEG that is

no greater than 5V below VIN. For any current regulator

that is detected at the switchover threshold voltage of

150mV (typ, VIN falling), internal circuitry switches that

current regulator’s return path from GND to NEG to pro-

vide enough voltage across that regulator to overcome

dropout. When VLED_ rises to 250mV (typ), the return

of that current regulator is switched back from NEG to

GND. Each current regulator is independently monitored

to detect when switchover is required. Since the LED

current is switched for only the individual LED current

regulators requiring higher voltage, power consumption

is minimized.

Enable and Dimming Control Input

(ENA, ENB)

ENA and ENB inputs have dual functions: LED on/off

control and PWM or serial-pulse dimming control. See

Table 1 for details. For the MAX8847Y, ENA functions as

an on/off control and PWM dimming control for LED1–

LED4. ENB functions as on/off control and serial-pulse

dimming control for LED5 and LED6. For the MAX8847Z,

ENA functions as an on/off control for LED1–LED4 as

well as PWM dimming control for LED1–LED6. ENB is

used for on/off control for LED5 and LED6.

Table 1. ENA and ENB Enable and Dimming Control

PART

ENA

MAX8847Y LED1–LED4 enable and PWM dimming control

MAX8847Z LED1–LED4 enable and LED1–LED6 PWM dimming control

ENB

LED5 and LED6 enable and serial-pulse

dimming control

LED5 and LED6 enable control

8  

Maxim Integrated

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