ISL97672A

11

FN7710.2

May 2, 2011

For basic LEDs (which do not have built-in Zener diodes), an

open-circuit failure of an LED results only in the loss of one

channel of LEDs, without affecting other channels. Similarly, a

short-circuit condition on a channel that results in that channel

being turned off does not affect other channels unless a similar

fault is occurring.

Due to the lag in boost response to any load change at its output,

certain transient events (such as LED current steps or significant

step changes in LED duty cycle) can transiently look like LED

fault modes. The ISL97672A uses feedback from the LEDs to

determine when it is in a stable operating region and prevents

apparent faults during these transient events from allowing any

of the LED stacks to fault out. See Table 1 for details.

A fault condition that results in an input current that exceeds the

device’s electrical limits will result in a shutdown of all output

channels.

Short-Circuit Protection (SCP)

The short-circuit detection circuit monitors the voltage on each

channel and disables faulty channels that are above

approximately 7.5V (this action is described in Table 1 on

page 12).

Open-Circuit Protection (OCP)

When one of the LEDs becomes an open circuit, it can behave as

either an infinite resistance or as a gradually increasing finite

resistance. The ISL97672A monitors the current in each channel

such that any string that reaches the intended output current is

considered “good.” Should the current subsequently fall below the

target, the channel is considered an “open circuit.” Furthermore,

should the boost output of the ISL97672A reach the OVP limit, or

should the lower over-temperature threshold be reached, all

channels that are not good are immediately considered to be open

circuit. Detection of an open circuit channel results in a time-out

before the affected channel is disabled. This time-out is sped up

when the device is above the lower over-temperature threshold, in

an attempt to prevent the upper over-temperature trip point from

being reached.

Some users employ special types of LEDs that have a Zener

diode structure in parallel with the LED. This configuration

provides ESD enhancement and enables open-circuit operation.

When this type of LED is open circuited, the effect is as if the LED

forward voltage has increased but the lighting level has not

increased. Any affected string will not be disabled, unless the

failure results in the boost OVP limit being reached, which allows

all other LEDs in the string to remain functional. In this case, care

should be taken that the boost OVP limit and SCP limit are set

properly, to ensure that multiple failures on one string do not

cause all other good channels to fault out. This condition could

arise if the increased forward voltage of the faulty channel

makes all other channels look as if they have LED shorts. See

Table 1 for details of responses to fault conditions.

Overvoltage Protection (OVP)

The integrated OVP circuit monitors the output voltage and keeps

the voltage at a safe level. The OVP threshold is set as shown in

Equation 5:

The resistors should be large, to minimize power loss. For

example, a 1M

PWM Off time. Parallel capacitors should also be placed across

capacitors reduce the AC impedance of the OVP node, which is

important when using high-value resistors.

Undervoltage Lock-out

If the input voltage falls below the UVLO level of 2.45V, the device

returns to the normal operating range.

Input Overcurrent Protection

During a normal switching operation, the current through the

internal boost power FET is monitored. If the current exceeds the

current limit, the internal switch is turned off. Monitoring occurs

on a cycle-by-cycle basis in a self-protecting way.

Additionally, the ISL97672A monitors the voltage at the LX and

OVP pins. At start-up, the LX pins inject a fixed current into the

output capacitor. The device does not start unless the voltage at

LX exceeds 1.2V. The OVP pin is also monitored such that if it

rises above and subsequently falls below 20% of the target OVP

level, the input protection FET is also switched off.

Over-Temperature Protection (OTP)

The ISL97672A includes two over-temperature thresholds. The

lower threshold is set to +130°C. When this threshold is reached,

any channel that is outputting current at a level significantly below

the regulation target is treated as “open circuit” and is disabled after

a time-out period. This time-out period is reduced to 800µs when it

is above the lower threshold. The lower threshold isolates and

disables bad channels before they cause enough power dissipation

(as a result of other channels having large voltages across them) to

hit the upper temperature threshold.

The upper threshold is set to +150°C. Each time this threshold is

reached, the boost stops switching, and the output current

sources switch off. Once the device has cooled to approximately

+100°C, the device restarts, with the DC LED current level

reduced to 75% of the initial setting. If dissipation persists,

subsequent hitting of the limit causes identical behavior, with the

current reduced in steps to 50% and finally 25%. Unless disabled

via the EN pin, the device stays in an active state throughout.

For complete details of fault protection conditions, see Figure 22

and Table 1.

OVP

1.21V

+

() R

LOWER

⁄

×

=

(EQ. 5)