L99LD01 datasheet(8/69 Pages) STMICROELECTRONICS

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Functional description

L99LD01

8/69

DocID025319 Rev 3

Functional description

2.1

Operating modes

The device is able to work both with a microcontroller and without it (stand alone

configuration).

2.1.1

Operation with an external microcontroller

This way allows parameters to be adjusted and checked by means of the SPI interface.

The adjusted device parameters, stored, i.e., inside the micro EEPROM, can be loaded into

device internal registers after the start up phase.

By means of a small 8 pins microcontroller it is possible to implement the following

functions:

•

Parameters setting:

–

LED current level and maximum input current limit can be adjusted according to

the application, the LED characteristic and spreads

–

Dither oscillator parameters as random, frequency modulation and deviation

percentage can be programmed

•

Flexible PWM operation with duty cycle and frequency managed by the microcontroller

•

Diagnostic feedback:

–

Fault condition is sent to the micro when the CSN pin is pulled down

•

Advanced LED monitoring:

–

LED voltage drop and temperature are multiplexed and sent to the microcontroller

through the dedicated MOUT pin in order to monitor the selected parameter with

the A/D of the microcontroller. The multiplexer is driven through a SPI command.

This function allows a sophisticated control of the LED status. For example, as an

alternative to the default overvoltage detection, it is possible to monitor the LED

drop voltage, reduced by the external R5/R6 resistor divider. So taking into

account the spread and temperature influence on the LED voltage drop, the

microcontroller is able to detect if there is one or more LED shorted.

Furthermore, it is possible to monitor the LED chain temperature, by means of the

voltage feedback through the dedicated NTC pin. The temperature limit control,

operated by the device by default, can be disabled via the SPI and the voltage

applied on the NTC pin can be sent back to the microcontroller via the multiplexed

output, MOUT, so allowing the microcontroller itself to control the LED chain either

acting on the internal current LED register or reducing the low frequency PWM

duty cycle.

•

In case of VR1 over temperature, its output will be switched off, the device enters in limp

home mode and a failsafe bit will be set in the internal status register (see details in the

following paragraph). In order to restart the normal operation, so clearing the

corresponding status register bit, the VS or ENABLE voltage has to be switched off and

then on. The mentioned bit can be cleared by the microcontroller only when it is

external supplied.

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