L99LD21 datasheet(28/73 Pages) STMICROELECTRONICS

28 / 73 page

Functional description

L99LD21

28/73

DS11130 Rev 5

4.3.5

Buck output’s short circuit to GND

A shorted buck output to GND is detected when LED string voltage (VLED) is lower than a

specified threshold (VLED_SHT) and the corresponding failure counter counts Nton_min_fail

switching cycles (also nonconsecutive), during which ILx_PEAK is reached between

TBLANK_BUCK and TON_MIN_BUCK. In normal mode (Active mode), once a short circuit is

validated, flag SHTx is set and the corresponding Buckx converter is latched off, until the

microcontroller sends a frame and clears the corresponding status bit (SR#1<7> and

SR#1<6>).

In fail safe mode (Limp Home), once a short circuit is validated, the corresponding buck

converter is latched off until automatically cleared by an auto-restart procedure, with a

period equal to tAUTORESTART.

The failure counter is not incremented during the startup phase. The failure counter is reset

if Nton_min_fail_reset consecutive pulses are detected with TON longer than TON_MIN_BUCK.

4.3.6

Buck TON maximum operation

Buck maximum on time operation is detected when switching on time is equal to

tON_MAX_BUCK for two consecutive cycles.

Once maximum Ton operation is validated, flag TON_MAX_OPx is set and the

corresponding Buckx converter is temporarily switched off for a Ttonmax_off.

Then, Buckx is enabled to switch on again while TON_MAX_OPx bit will be latched until a

R&C command clears corresponding status bit (SR#2<20> or SR#2<19>).

In fail safe mode (Limp Home), once a maximum TON violation is validated, the

corresponding buck converter is latched off until automatically cleared by an auto-restart

procedure, with a period equal to tAUTORESTART.

4.3.7

Buck Open Load detection

If one of the LED strings is disconnected, the converter will charge the output capacitor of

the buck converter by regulating the peak current of the switch, until VLED is equal to the

buck input voltage. From this point, since the output capacitor is charged at the maximum

possible value, it cannot absorb any current despite the activation of the switch, and the

target ILx_PEAK cannot be reached.

Upon these conditions, Buckx starts switching at maximum Ton: maximum Ton operation

detection (described in Section 4.3.6) guarantees Open Load failure protection as well.

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