AN3288

System architecture

Doc ID 18011 Rev 2

7/27

2.4

HB-LED array driving

After the signal elaboration, the MCU opportunely drives the two buck converters, based on

the A5970AD regulators, to obtain a proper light brightness proportional to the effort of the

measured deceleration. The A5970AD is a step-down monolithic power switching regulator

for automotive applications qualified to AEC-Q100 requirements, with a minimum switch

current limit of 1.35 A to the load depending on application conditions, and an operating

input voltage from 4 V to 36 V.

The MCU digitally controls the dimming of the two LED arrays, applying a square wave with

variable duty cycle (pulse-width modulation or PWM) to the LED driving circuit. In order to

drive its switching, the STM8AF5168 MCU is directly connected to the INHIBIT pins of the

two A5970AD regulators.

Using pulse-width modulation (PWM) to set the period and duty cycle is the easiest way to

accomplish the digital dimming, and the buck regulator topology provides the best driving

performance for this application.

Figure 3.

Buck converter schematic based on A5970AD

The LEDs switching frequency must be faster than the human eye can detect in order to

avoid a flickering effect (about 200 Hz or faster is usually acceptable). To obtain the best

light efficiency from the 2 HB-LED arrays and to optimize their dimming, the MCU generates

a PWM signal with a frequency fixed to 300 Hz, while the duty cycle changes its value with

the variation of the deceleration detected by the AIS326DQ. This configuration allows LEDs

to switch between ON and OFF as quick as possible, allowing the system to have a fast

response and a low power consumption.

In order to identify four different kinds of light actuations corresponding to the different

deceleration ranges, a system of four main functional thresholds has been set in the

firmware. So the four major ranges of braking effort have been correlated to four fixed values

of duty cycle:

●

for no braking, duty cycle = 100 %

●

for soft braking, duty cycle = 65 %

●

for medium braking, duty cycle = 50 %

●

for strong braking, duty cycle = 25 %