AN4460 datasheet(25/54 Pages) STMICROELECTRONICS

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AN4460

Protection and regulation algorithm

This section describes the protection and regulation algorithms implemented in the

STLUX385A. Every channel is independent from the others and the same logic applies to all

channels.

The STLUX385A implements software and hardware protections aimed to preserve the

external hardware in case of abnormal events or variations. The STLUX385A doesn't need

external active analog circuitry on the board as the firmware replaces them. Once an error is

recognized, the FW stores the error code and keeps sensing the system to verify that the

problem has been fixed. The error code can be verified typing the “st” command (see

Section 7.4.6 on page 32). Protections can be summarized according to different priority

levels.

6.1

Overcurrent protection

The overcurrent protection (OCP), which preserves the external circuit from damage due to

a component fault, is the most important protection and requires a short detection and

reaction time. The STLUX385A implements OCP directly in the SMED hardware (through

states S1 and S2) and doesn't require any high priority firmware intervention.The internal

SMED clock guarantees a detection time of ~10 ns (Fck_smed = 96 MHz) however the

external circuit response time pushes the overall detection time to 200 ns. When an OCP is

detected, the SMED stops its PWM output and generates an interrupt. The firmware can

then execute the recovery procedure by periodically restarting the PWM line.

6.2

Initial estimation

During the startup phase, the input power voltage is monitored by the Set_check_V()

routine. This routine reads the power voltage (on ADC channel zero) and stores it into the

global variable Vpwr[]. The voltage read back is stored into all channel locations (0 to 3).

The routine Set_check_V() estimates the voltage drop generated by the LEDs by using the

number of LEDs as specified by the user and the maximum and minimum predefined LED

voltage drops. The following formula is used:

Equation 11

where x is the LED channel. The estimated voltage drop is used for the initial calculation of

the TOFF and TON values.

6.3

Adaptive voltage compensation

The adaptive compensation is a technique that monitors the system and changes the

operational parameters to guarantee that the configured average current always flows

through the LED. As the average current depends on the TON and TOFF values, the

STLUX385A recalculates the values at the beginning of every dimming ON cycles. The

adaptive compensation is activated with the serial command “au x 1".

wk_Volt x

.Min_vl wk_Volt x

.Max_vl

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