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ISL97675
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
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:
circuit failure of an LED will only result in the loss of one channel
of LEDs without affecting other channels. Similarly, a LED short
OVP = 1.21V × ( R UPPER + R LOWER ) ? R LOWER
(EQ. 12)
circuit condition which causes the FB voltage to rise to ~4V, will
result in that channel turning off. This does not affect any 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 ISL97675 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 3 for more details.
A fault condition that results in high input current due to a short
on V OUT with master fault protection switch will result in a
shutdown of all output channels. The control device logic will
remain functional.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on each
channel and disables faulty channels which are detected above
the programmed short circuit threshold. When an LED becomes
shorted, the action taken is described in Table 3. The short circuit
threshold is 4V.
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 ISL97675 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 ISL97675 reach the
OVP limit or should the lower over-temperature threshold be
reached, 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. This time-out is run 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 zener diode
structure in parallel with the LED for ESD enhancement, thus
enabling open circuit operation. When this type of LED goes open
circuit, the effect is as if the LED forward voltage has increased,
but no light is emitted. Any affected string will not be disabled,
unless the failure results in the boost OVP limit being reached,
allowing all other LEDs in the string to remain functional. Care
should be taken in this case that the boost OVP limit and SCP limit
are set properly, to make sure that multiple failures on one string
do not cause all other good channels to be faulted out. This is due
to the increased forward voltage of the faulty channel making all
other channels look as if they have LED shorts. See Table 3 for
details for responses to fault conditions.
15
These resistors should be large to minimize the power loss. For
example, a 1M Ω R UPPER and 30k Ω R LOWER sets OVP to 41.2V.
Large OVP resistors also allow C OUT discharges slowly during the
PWM Off time. Parallel capacitors should also be placed across
the OVP resistors such that R UPPER /R LOWER = C LOWER /C UPPER .
Using a C UPPER value of at least 30pF is recommended. These
capacitors reduce the AC impedance of the OVP node, which is
important when using high value resistors.
Undervoltage Lockout
If the input voltage falls below the UVLO level of 3.1V, the device
will stop switching and be reset. Operation will restart once the
input voltage is back in the normal operating range.
Master Fault Protection
During normal switching operation, the current through the
internal boost power FET is monitored. If the input current
exceeds the current limit due to output shorted to ground or
excessive loading, the internal switch will be turned off. This
monitoring happens on a cycle by cycle basis in a self protecting
way.
Additionally, the ISL97675 monitors the voltage at the LX and
OVP pins. At start-up, a fixed current is injected out of the LX pins
and into the output capacitor. The device will not start up 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 will be switched off.
Over-Temperature Protection (OTP)
The ISL97675 includes two over-temperature thresholds. The lower
threshold is set to +130°C. When this threshold is reached, any
channel which is outputting current at a level below the regulation
target will be treated as “open circuit” and disabled after a time-out
period. The intention of the lower threshold is to allow bad channels
to be isolated and disabled 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 is reached,
the boost will stop switching and the output current sources will
be switched off.
For the extensive fault protection conditions, please refer to
Figure 25 and Table 3 for details.
FN7630.1
September 14, 2011
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