ISL97672B

FN7995 Rev.3.00

Page 10 of 17

Sep 7, 2017

IC Protection Features and Fault

Management

The ISL97672B has several protection and fault management

features that improve system reliability. The following sections

describe them in more detail.

INRUSH CONTROL AND SOFT-START

The ISL97672B has separate, built-in, independent inrush control

and soft-start functions. The inrush control function is built

around an external short-circuit protection P-channel FET in

series with VIN. At start-up, the fault protection FET is turned on

slowly due to a 21µA pull-down current output from the FAULT

pin. This discharges the fault FET's gate-to-source capacitance,

turning on the FET in a controlled fashion. As this happens, the

output capacitor is charged slowly through the low-current FET

before it becomes fully enhanced. This results in a low inrush

current. This current can be further reduced by adding a

capacitor (in the 1nF to 5nF range) across the gate source

terminals of the FET.

Once the chip detects that the fault protection FET is turned on

fully, it assumes that inrush is complete. At this point, the boost

regulator begins to switch, and the current in the inductor ramps

up. The current in the boost power switch is monitored, and

switching is terminated in any cycle in which the current exceeds

the current limit. The ISL97672B includes a soft-start feature in

which this current limit starts at a low value (275mA). This value

is stepped up to the final 2.2A current limit in seven additional

steps of 275mA each. These steps happen over at least 8ms and

are extended at low LED PWM frequencies if the LED duty cycle is

low. This extension allows the output capacitor to charge to the

required value at a low current limit and prevents high input

current for systems that have only a low to medium output

current requirement.

For systems with no master fault protection FET, the inrush current

FAULT PROTECTION AND MONITORING

The ISL97672B features extensive protection functions to cover

all perceivable failure conditions. The /FLAG pin is a latched

short, and overvoltage and over-temperature conditions. This pin

resets only when input power is recycled or the part is re-enabled.

The failure mode of an LED can be either an open circuit or a

short. The behavior of an open-circuited LED can additionally

take the form of either infinite resistance or, for some LEDs, a

Zener diode, which is integrated into the device in parallel with

the now-opened LED.

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 ISL97672B 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 on page 12 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).

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 ISL97672B 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 ISL97672B 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.

The Overvoltage Protection (OVP) pin has a function of setting the

range.

that:

35V.

1.22Vx

+



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

=

(EQ. 5)