AN4300 datasheet(9/40 Pages) STMICROELECTRONICS

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AN4300

Lighting application design challenge

1.4.1

Digital diagnosis

To read the digital diagnosis it is necessary to read an I/O pin of the driver for each load

driven. To read this type of diagnosis the microcontroller has to read the digital pin in polling

mode or by connecting this signal to an external interrupt line.

1.4.2

Analog diagnosis

To read the analog diagnosis a current sense pin is available for each load driven. This pin

is used to read a proportional current (voltage using a resistor) that flows in the load.

Comparing this current with certain thresholds can determine if problems on the load exist.

To read the voltage of the current sense pin, an analog-to-digital conversion must be

performed.

The analog diagnosis has to be read in synchronous mode to the input (PWM) signal which

is available after a delay from the rising edge of the input signals. The analog-to-digital

conversion has to be triggered with a delay from the rising edge of the input signals to allow

the load current to stabilize.

1.4.3

Serial diagnosis

When the power devices provide serial diagnosis, they usually save the type of fault in

internal registers. The microcontroller, using the SPI link, then performs a query on the

specified register to examine the fault status.

The application can perform the following types of diagnosis inspection:

•

During T_ON (time when the PWM input signal is switched on) of the input signal

–

Open-load

–

Overtemperature

–

Undervoltage

–

Short-circuit to GND

–

Short-circuit to VCC

•

During T_OFF (time when the PWM signal is switched off) of the input signal

–

Open-load

–

Short-circuit to VCC

The digital and serial diagnosis can usually be read asynchronously with respect to the

PWM input signals and the diagnosis action can be performed during a noncritical instant of

the system cycle. The CPU load used to collect this information is not an important aspect.

The analog diagnosis consists of checking the current that flows in the loads. The input

signal status must be known to collect diagnosis information in a specific instant of the

loads’ activity. The reading of the diagnosis and the input signal must be synchronized to

determine the status of the input signal.

The input signal is a PWM signal, making it necessary to select if the diagnosis is occurred

in the T_ON or T_OFF phase of the input signal. Furthermore, a delay exists between the

rising (falling) edge and the instant when the diagnosis read occurs because the current

sense signal is stabilized only after a transitory time (about 300 µs).

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