AS3661 datasheet(15/87 Pages) AMSCO

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AS3661

Datasheet - D e t a i l e d D e s c r i p t i o n

8 Detailed Description

The AS3661 is a fully integrated lighting management unit for producing lighting effects for mobile devices. The

AS3661 includes all necessary power management, high-side current sources, temperature compensation, two wire

control interface and programmable pattern generators. The overall maximum current for each driver is set by an 8-bit

bits. The temperature compensation is also done by a PWM.

8.1 Programming

The AS3661 provides flexibility and programmability for dimming and sequencing control. Each LED can be controlled

directly and independently through the serial bus or LED drivers can be grouped together for pre-programmed flashing

patterns. The AS3661 has three independent program execution engines, so it is possible to form three independently

programmable LED banks. LED drivers can be grouped based on their function so that, for example, the first bank of

drivers can be assigned to the keypad illumination, the second bank to the “funlights” and the third group to the

indicator LED(s). Each bank can contain 1 to 9 LED driver outputs. Instructions for program execution engines are

stored in the program memory. The total amount of the program memory is 96 instructions and the user can allocate

the memory as required by the engines.

8.2 LED Error Detection

AS3661 has a built-in LED error detection. Error detection does not only detect open and short circuit, but provides an

opportunity to measure the VF’s of the LEDs. The test event is activated by a serial interface write and the result can be

read through the serial interface during the next cycle. This feature can also be addressed to measure the voltage on

VBAT, VCP and INT pins. Typical example usage includes monitoring battery voltage or using INT pin as a light sensor

interface.

8.3 Energy Efficiency

When charge pump automatic mode selection is enabled, the AS3661 monitors the voltage over the drivers of LED1 to

LED6 so that the device can select the best charge pump gain and maintain good efficiency over the whole operating

voltage range. The red LED element of an RGB LED typically has a forward voltage of about 2V. For that reason, the

outputs LED7, LED8 and LED9 are internally powered by VBAT, since battery voltage is high enough to drive red LEDs

over the whole operating voltage range. This allows to drive three RGB LEDs with good efficiency because the red

LEDs doesn't load the charge pump. AS3661 is able to automatically enter power-save mode, when LED outputs are

not active and thus lowering idle current consumption down to 10 µA (typ.). During the “downtime” of the PWM cycle

(constant current output status is low) additional power savings can be achieved when the PWM power save feature is

enabled.

8.4 Temperature Compensation

The luminance of an LED is typically a function of its temperature even though the current flowing through the LED

remains constant. Since luminance is temperature dependent, many LED applications require some form of

temperature compensation to decrease luminance and color purity variations due to temperature changes. The

AS3661 has a build in temperature sensing element and PWM duty cycle of the LED drivers changes linearly in

relationship to changes in temperature. User can select the slope of the graph (31 slopes) based on the LED

characteristics. This compensation can be done either constantly, or only right after when the device wakes up from

power save mode, to avoid error due to self-heating of the device. Linear compensation is considered to be practical

and accurate enough for most LED applications. Compensation is effective over the temperature range from -40°C to

+90°C.

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