L99LD01 datasheet(20/69 Pages) STMICROELECTRONICS

20 / 69 page

Functional description

L99LD01

20/69

DocID025319 Rev 3

In case VCC1 voltage drops below the internal threshold during the normal functioning, or

when the device is put in standby, the NRES pin is forced to low, but after a time interval TRR

has expired and kept low until the VCC1 has gone back again to the internal threshold (see

Figure 4 for more details).

2.2.7

Watchdog

In case the application uses a microcontroller, during the device power-up a reset pulse is

generated periodically every 200 ms (default) for 2.0 ms waiting for microcontroller

acknowledgment. Timeout window is selectable by SPI (100 ms or 200 ms) and the reset

time could be extended by the external capacitor C6.

•

Timeout WD is refreshed by bit toggling.

•

After the 1st WD timeout, a reset pulse is generated and the device enters in Limp

Home mode. After the second WD timeout, another reset pulse cycle is generated,

waiting for microcontroller response.

•

After 3 consecutive reset cycles without WD refresh, which means that microcontroller

is not responding, the voltage regulator, VCC1, is turned off and the device keeps

working in “Limp Home Mode” (see Figure 33). Safety critical functions like Low Beam

application require the LED Driver to be turned on if the microcontroller fails, while in

case of high beam application, it is required the driver to be switched off in case of

microcontroller failure. As a consequence, the device operates according to the state of

LHM pin which is enabled during the recognition of the microcontroller failure. In

particular, if LHM pin is kept low the device will be always OFF. If instead, LHM pin is

high or left open, the device will be switched permanently ON, regardless of the status

of PWM_L pin.If the application doesn’t use a microcontroller (stand alone operation),

the start-up WD control must to be deactivated. This can be done by connecting NRES

pin to the battery supply voltage VS. In such a case the driver will operate in normal

mode as above mentioned (see stand alone operation).

2.2.8

Standby and wake up by ENABLE pin

A low consumption mode is required in case of applications directly connected to the

battery.

The device enters in standby mode, that is the default operating modes because of an

internal pull down, in case of low level signal at the ENABLE pin and it wakes up in case of

high level signal.During standby mode, VCC1 and VCC2 are switched off. Figure 9 and

Figure 10 show two possible application schematics in case of direct connection to the

battery.

In case of Figure 9 the microcontroller of the application goes in standby when the

microcontroller sets the LIN transceiver in standby mode: NSLP = Low

→INH goes

Low

→the DRL driver goes in standby.

The application is waken up from the standby when a wake up source is detected by the LIN

transceiver. That means INH goes high and so ENABLE, then the DRL driver restarts and

consequently VCC1 is activated and supplies the microcontroller.

In case of Figure 10, a power management device is present, which supplies the

microcontroller. Normally the inverted FSO signal coming from the power management

device is high. This output is inverted by an external logic and applied to one of the two input

OR diodes and therefore, at the input of the OR the voltage is normally at logical zero.

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