LED7708TR datasheet(44/53 Pages) STMICROELECTRONICS

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Recommended operating conditions

LED7708

44/53

Doc ID 022698 Rev 1

Equation 16

Once the voltage at the VFB pin has crossed the related threshold, the LED7708 shuts

down, the CFP (critical fault protection) flag of the DEVCFG0 register is asserted, the XFLT

pin is set low and the condition is latched.

Normal operation is restored by toggling the DEN bit of the DEVCFG0 register or by

performing a POR.

8.7.2

Overtemperature alert and thermal protection

The LED7708 implements a two-level thermal protection. When the junction temperature

rises above 130 °C (typ.), the overtemperature alert flag (OTAF) is asserted and the XFLT

pin goes low. The OTAF flag is cleared as soon as the chip temperature drops below 125 °C

(typ.). This bit is used by the host controller to enable output voltage regulation if temporarily

disabled.

If the temperature of the chip crosses 150 °C, the device shuts down. Automatic restart

occurs as soon as the temperature drops to 130 °C.

8.7.3

Overcurrent protection

Overcurrent occurring in the power MOSFET is managed by skipping subsequent switching

cycles until the faulty condition is present. The XFLT pin is not affected. See also

Section 8.3.3 for details.

8.7.4

LED short-circuit and open channel faults

The faults that can occur to the current generators section are basically the following:

●

one or more open (floating) channels

●

excessive voltage drop (shorted LEDs).

The LED7708 manages these faulty conditions following a particular algorithm.

When a channel opens, its voltage drop falls to zero: the output voltage regulation circuitry

tags it as “probably faulty” and increases the output voltage. If the “open” condition is

temporary, the voltage drop across the generator is self-recovered and the output voltage is

corrected. In case of a real open channel, the regulation DAC reaches the maximum value

(DAC overflow) and the OVFW bit of the DEVCFG0 register is asserted.

The LED7708 can automatically disconnect the open channels if required. Therefore, if the

open channels auto-disconnection (OCAD) bit of the DEVCFG0 register is asserted, the

faulty channel is suddenly turned off, excluded from the regulation loop and the output

voltage is quickly adjusted to the optimal level. During the open channel detection (i.e. when

the regulation DAC is rolling up), the LED short-circuit detection circuitry is masked to avoid

false detection due to the high output voltage.

If the OCAD bit is cleared, the output voltage is kept to the maximum value and the fault

management is demanded to the host controller (XFLT pin low).

In case of LED short-circuit, the voltage across the related channel increases. The LED7708

compares the drop across each channel with a fixed threshold in order to detect this

OUT OVP

--------------------

FB OV

⋅

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