NXP PCA9685 I2C总线控制LED控制方案

NXP公司的PCA9685是16路 I2C总线控制LED控制器,非常适合LCD红/绿/蓝/琥珀(RGBA)彩色背光应用.每路LED输出有它自己的12位分辨率(4096级)固定频率单独的PWM控制器,可编频率从40Hz到1000Hz,占空比可从0%到100%间调整,以设定LED亮度.LED输出驱动器可编成漏极开路(5V是沉电流25mA)或图腾极(5V时沉电流25mA,源电流10mA),电源电压从2.3V到5.5V.本文介绍了PCA9685主要特性,方框图,开关时间测试电路以及典型应用电路和LCD背光应用电路.

PCA9685: 16-channel, 12-bit PWM Fm+ I2C-bus LED controller

The PCA9685 is an I2C-bus controlled 16-channel LED controller optimized for LCD Red/Green/Blue/Amber (RGBA) color backlighting applications. Each LED output has its own 12-bit resolution (4096 steps) fixed frequency individual PWM controller that operates at a programmable frequency from a typical of 40 Hz to 1000 Hz with a duty cycle that is adjustable from 0 % to 100 % to allow the LED to be set to a specific brightness value. All outputs are set to the same PWM frequency.

Each LED output can be off or on (no PWM control), or set at its individual PWM controller value. The LED output driver is programmed to be either open-drain with a 25 mA current sink capability at 5 V or totem pole with a 25 mA sink, 10 mA source capability at 5 V. The PCA9685 operates with a supply voltage range of 2.3 V to 5.5 V and the inputs and outputs are 5.5 V tolerant. LEDs can be directly connected to the LED output (up to 25 mA, 5.5 V) or controlled with external drivers and a minimum amount of discrete components for larger current or higher voltage LEDs.

The PCA9685 is in the new Fast-mode Plus (Fm+) family. Fm+ devices offer higher frequency (up to 1 MHz) and more densely populated bus operation (up to 4000 pF). Although the PCA9635 and PCA9685 have many similar features, the PCA9685 has some unique features that make it more suitable for applications such as LCD backlighting and Ambilight:

• The PCA9685 allows staggered LED output on and off times to minimize current surges.The on and off time delay is independently programmable for each of the 16 channels. This feature is not available in PCA9635.

• The PCA9685 has 4096 steps (12-bit PWM) of individual LED brightness control. The PCA9635 has only 256 steps (8-bit PWM).

• When multiple LED controllers are incorporated in a system, the PWM pulse widths between multiple devices may differ if PCA9635s are used. The PCA9685 has a programmable prescaler to adjust the PWM pulse widths of multiple devices.

• The PCA9685 has an external clock input pin that will accept user-supplied clock (50 MHz max.) in place of the internal 25 MHz oscillator. This feature allows synchronization of multiple devices. The PCA9635 does not have external clock input feature.

• Like the PCA9635, PCA9685 also has a built-in oscillator for the PWM control. However, the frequency used for PWM control in the PCA9685 is adjustable from about 40 Hz to 1000 Hz as compared to the typical 97.6 kHz frequency of the PCA9635. This allows the use of PCA9685 with external power supply controllers. All bits are set at the same frequency.

• The Power-On Reset (POR) default state of LEDn output pins is LOW in the case of PCA9685. It is HIGH for PCA9635.

The active LOW Output Enable input pin (OE) allows asynchronous control of the LED outputs and can be used to set all the outputs to a defined I2C-bus programmable logic state. The OE can also be used to externally ‘pulse width modulate’ the outputs, which is useful when multiple devices need to be dimmed or blinked together using software control.

Software programmable LED All Call and three Sub Call I2C-bus addresses allow all or defined groups of PCA9685 devices to respond to a common I2C-bus address, allowing for example, all red LEDs to be turned on or off at the same time or marquee chasing effect, thus minimizing I2C-bus commands. Six hardware address pins allow up to 62 devices on the same bus.

The Software Reset (SWRST) General Call allows the master to perform a reset of the PCA9685 through the I2C-bus, identical to the Power-On Reset (POR) that initializes the registers to their default state causing the outputs to be set LOW. This allows an easy and quick way to reconfigure all device registers to the same condition via software.

PCA9685主要特性:

16 LED drivers. Each output programmable at:

Off

On

Programmable LED brightness

Programmable LED turn-on time to help reduce EMI

1 MHz Fast-mode Plus compatible I2C-bus interface with 30 mA high drive capability on SDA output for driving high capacitive buses

4096-step (12-bit) linear programmable brightness per LED output varying from fully off (default) to maximum brightness

LED output frequency (all LEDs) typically varies from 40 Hz to 1000 Hz (Default of 1Eh in PRE_SCALE register results in a 200 Hz refresh rate with oscillator clock of 25 MHz.)

Sixteen totem pole outputs (sink 25 mA and source 10 mA at 5 V) with software programmable open-drain LED outputs selection (default at totem pole). No input function.

Output state change programmable on the Acknowledge or the STOP Command to update outputs byte-by-byte or all at the same time (default to ‘Change on STOP’).

Active LOW Output Enable (OE) input pin. LEDn outputs programmable to logic 1, logic 0 (default at power-up) or ‘high-impedance’ when OE is HIGH.

6 hardware address pins allow 62 PCA9685 devices to be connected to the same I2C-bus

Toggling OE allows for hardware LED blinking

4 software programmable I2C-bus addresses (one LED All Call address and three LED Sub Call addresses) allow groups of devices to be addressed at the same time in any combination (for example, one register used for ‘All Call’ so that all the PCA9685s on the I2C-bus can be addressed at the same time and the second register used for three different addresses so that 1¤3 of all devices on the bus can be addressed at the same time in a group). Software enable and disable for these I2C-bus address.

Software Reset feature (SWRST General Call) allows the device to be reset through the I2C-bus.


图1.PCA9685方框图


图2.PCA9685典型应用电路图

图3.PCA9685开关时间测试电路

图4.PCA9685使能/禁止开关测试电路


图5.PCA9685 LCD背光应用电路