NCV78514

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15

PROTECTIONS AND ERROR MANAGEMENT

Due to several embedded protection systems, the

NCV78514 is able to manage different errors and fault

types.

Depending on the error, the controller informs the system

through the DIM pin by changing current consumption.

The different errors types are described below.

Overvoltage

When BAT pin voltage is larger than P_OVLO_operating

threshold, the light is turned off or maintained off. An error

is reported to DIM pin in this case.

To retrieve normal operation, the BAT must be below

OVLO operating minus hysteresis. Error is not reported on

DIM pin, when BAT voltage falls below OVLO minus

hysteresis.

LED String Error

The device is able to detect several faults on the LED

string, through the SNSP and SNSN sense pins.

For the two error situations described below, the DC to DC

controller will instantaneously turn off the LED current, and

report error via DIM pin.

The first situation is when anode of LED string shorts to

cathode, or to GND. These events are detected, when SNSN

voltage drops below P_VOUT_L.

During the turn on sequence of the DC DC converter, this

protection

mechanism

is

deactivated

during

P_VOUT_L_IDLE_SU in order to avoid unexpected fault

detection. In case of an error is detected and the part goes to

error mode, a new start up sequence is started. The

brightness is recovered as soon as the error disappear

.

The second situation occurs, when LED string opens.

In this case the SNSN will cross the P_VOUT_H

threshold due to the boost mode. An auto−rearming is started

as soon as the output capacitor is fully discharged (below

UV out threshold).

A LED string error is reported via DIM pin.

The auto−rearming sequence is running as long as the led

string is opened

.

RSET Out of Range Value

An out of range RSET is considered as an error (See Error

Table) if RSET < 400

W, RSET > 15 kW for longer than

P_TIMEOUT_RSET.

This out of range detection acts after the P_SET_RSET

start up timer is elapsed or if the LEDs are already ON.

The out of range error is reported via DIM pin. The system

automatically rearms until the RSET recovers a valid value.

Feedback Loop Issue

SNSP and SNSN pins are used for the regulation loop.

A physical error making the pins shorted or opened, latch an

error and the LED current is stopped

.

DIM Pin Status Function

In case of some specific errors, the DIM pin is used as

a status pin.

The current consumption on the pin will switch from

P_DIM_IQ to P_DIM_IFAULT input current consumption,

in case of error.

Figure 12. DIM Pin, Diagnostic Current Sink

DIM

Dimming Generator

DIAG

Control

Light provided:

IDIM > P_DIM_IQ

No light provided:

IDIM < P_DIM_IFAULT

Depending on the kind of error, the output may be turned

off, requiring a BAT removal to reset the error mode

condition.

At power up, the P_DIM_IQ is activated when

P_VOUT_L is reached to make sure the current is present

into the LED string.

The P_DIM_IQ is de−activated on UVLO, falling edge.

The different errors are described in the below table

.