LP8556 datasheet(27/63 Pages) TI1

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LP8556

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SNVS871I – JULY 2012 – REVISED MARCH 2016

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8.3.3 Fault Detection

LP8556 has fault detection for LED open and short conditions, UVLO, overcurrent, and thermal shutdown. The

cause for the fault can be read from status register. Reading the fault register also resets the fault.

8.3.3.1 LED Fault Detection

With LED fault detection, the voltages across the LED drivers are constantly monitored. Shorted or open LED

strings are detected.

8.3.3.1.1

Open Detect

The logic uses the LOW comparators and the requested boost voltage to detect the OPEN condition. If the logic

is asking the boost for the maximum allowed voltage and a LOW comparator is asserted, then the OPEN bit is

set in the STATUS register (ADDR = 02h). In normal operation, the adaptive headroom control loop raises the

requested boost voltage when the LOW comparator is asserted. If it has raised it as high as it can and an LED

string still needs more voltage, then it is assumed to be disconnected from the boost voltage (open or grounded).

The actual boost voltage is not part of the OPEN condition decision; only the requested boost voltage and the

LOW comparators.

8.3.3.1.2

Short Detect

The logic uses all three comparators (HIGH, MID and LOW) to detect the SHORT condition. When the MID and

LOW comparators are de-asserted, the headroom control loop considers that string to be optimized - enough

headroom, but not excessive. If at least one LED string is optimized and at least one other LED string has its

HIGH comparator asserted, then the SHORT condition is detected. It is important to note that the SHORT

condition requires at least two strings for detection: one in the optimized headroom zone (LOW/MID/HIGH

comparators all de-asserted) and one in the excessive headroom zone (HIGH comparator asserted).

Fault is cleared by reading the fault register.

8.3.3.2 Undervoltage Detection

The LP8556 device has detection for too-low VIN voltage. Threshold level for the voltage is set with EPROM

Table 7. UVLO Truth Table

UVLO_EN

UVLO_TH

THRESHOLD (V)

don't care

OFF

2.5

5.2

When undervoltage is detected the LED outputs and the boost shuts down, and the corresponding fault bit is set

in the fault register. The LEDs and the boost start again when the voltage has increased above the threshold

level. Hysteresis is implemented to threshold level to avoid continuous triggering of fault when threshold is

reached.

Fault is cleared by setting the EN / VDDIO pin low or by reading the fault register.

8.3.3.3 Overcurrent Protection

LP8556 has detection for too-high loading on the boost converter. When overcurrent fault is detected, the boost

shuts down and the corresponding fault bit is set in the fault register. The boost starts again when the current

has dropped below the OCP threshold.

Fault is cleared by reading the fault register.

8.3.3.4 Thermal Shutdown

If the LP8556 reaches thermal shutdown temperature (150°C) the LED outputs and boost shut down to protect it

from damage. The device re-activates when temperature drops below 130°C.

Fault is cleared by reading the fault register.

LP8556 Datasheet(PDF) 27 Page - Texas Instruments

LP8556 Datasheet(HTML) 27 Page - Texas Instruments

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