
ISL97692, ISL97693, ISL97694A
Fault Protection and Monitoring
The ISL97692/3/4A features extensive protection functions to
handle failure conditions automatically. Refer to Figure 35 and
Table 7 for details of the fault protections.
The LED failure mode is either open or short circuit. An open
circuit failure of an LED only results 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.
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 ISL97692/3/4A use 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 7 for more details.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on each
channel and disables faulty channels, which are above the short
circuit protection threshold, nominally 8V (the action taken is
described in Table 7).
Open Circuit Protection (OCP)
When one of the LEDs becomes open circuit, it can behave as
either an infinite resistance or a gradually increasing finite
resistance. The ISL97692/3/4A monitors the current in each
channel such that any string, which reaches the intended output
current is considered “good”. Should the current subsequently fall
below the target, the channel will be considered an “open circuit”.
Furthermore, should the boost output of the ISL97692/3/4A
reaches the V OVP limit, all channels which are not “good” will
immediately be considered as “open circuit”.
Detection of an “open circuit” channel will result in a time-out
before disabling of the affected channel.
24
Overvoltage Protection (OVP)
The integrated OVP circuit monitors the boost output voltage, V OUT ,
and keeps the voltage at a safe level. The OVP threshold is set as
Equation 18:
OVP = 1.22V × ( R UPPER + R LOWER ) ? R LOWER (EQ. 18)
Where:
- V OVP is the maximum boost output voltage, V OUT (V)
- R UPPER is resistor from OVP pin to the boost output ( Ω)
- R LOWER is resistor from OVP pin to GND ( Ω)
Undervoltage Lockout
If the input voltage falls below the V UVLO level of ~2V, the
ISL97692/3/4A will stop switching and be reset. Operation will
restart only if the V IN is back in the normal operating range.
Over-Temperature Protection (OTP)
The ISL97692/3/4A have an over-temperature protection
threshold set to +150°C. If this threshold is reached, the boost
stops switching and the ISL97692/3/4A output current sinks are
switched off. The ISL97692/3/4A can be restarted by toggling V IN
to below the V UVLO level of ~2V, then back up to the normal input
voltage level, or by power recycling VIN.
FN7839.4
December 20, 2012