LP8556 datasheet(28/52 Pages) TI

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LP8556

SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012

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Fault Detection

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

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

LED FAULT DETECTION

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

strings are detected.

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.

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.

UNDER-VOLTAGE DETECTION

LP8556 has detection for too-low VIN voltage. Threshold level for the voltage is set with EPROM register bits as

shown in the following table:

Table 6. UVLO Truth Table

UVLO_EN

UVLO_TH

Threshold (V)

don't care

OFF

2.5V

5.2V

When under voltage is detected the LED outputs and the boost will shutdown and the corresponding fault bit is

set in the fault register. The LEDs and the boost will 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.

OVER-CURRENT PROTECTION

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

boost will shutdown and the corresponding fault bit is set in the fault register. The boost will start again when the

current has dropped below the OCP threshold.

Fault is cleared by reading the fault register.

THERMAL SHUTDOWN

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

protect it from damage. Device will re-activate again when temperature drops below 130 °C degrees.

Fault is cleared by reading the fault register.

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