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ISL97671
occurring. All LED faults are reported via the SMBus/I 2 C interface
to Register 0x02 (Fault/Status register). The controller is able to
determine which channels have failed via Register 0x09 (Output
Masking register). The controller can also choose to use Register
0x09 to disable faulty channels at start-up, resulting in only
further faulty channels being reported by Register 0x02.
channel look as if they have LED shorts. See Table 1 for details for
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:
Due to the lag in boost response to any load change at its output,
certain transient events (such as LED current steps or significant
OVP = 1.21V × ( R UPPER + R LOWER ) ? R LOWER
(EQ. 15)
step changes in LED duty cycle) can transiently look like LED
fault modes. The ISL97671 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 more details.
A fault condition that results in an input current that exceeds the
devices electrical limits will result in a shutdown of all output
channels. The control device logic will remain functional such
that the Fault/Status Register can be interrogated by the system.
The root cause of the failure will be loaded to the volatile
Fault/Status Register so that the host processor can interrogate
the data for failure monitoring.
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. There are three
selectable levels of short circuit threshold (3.6V, 4.8V and 5.85V)
that can be programmed through the Configuration Register
0x08 bits 0 and 1. When an LED becomes shorted, the action
taken is described in Table 1. The default short circuit threshold
is 5V. The detection of this failure mode can be disabled via
Register 0x08; see Table 2B for additional information.
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 ISL97671 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 ISL97671 reaches
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 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 some 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 lighting. 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,
so as 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
14
These resistors should be large to minimize the power loss. For
example, a 1Mk 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 Lock-out
If the input voltage falls below the UVLO level of 2.45V, the device
will stop switching and be reset. Operation will restart only if the
device is re-enabled through SMBus/I 2 C interface once the input
voltage is back in the normal V IN and operating range. In non-
SMBus/I 2 C applications, the part will automatically restart once
the input voltage clears the UVLO threshold with the part already
enabled.
Input Overcurrent Protection
During normal switching operation, the current through the
internal boost power FET is monitored. If the current exceeds the
current limit, the internal switch will be turned off. This
monitoring happens on a cycle by cycle basis in a self protecting
way.
Additionally, the ISL97671 monitors the voltage at the LX and
OVP pins. At startup, 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 also be
switched off.
Over-Temperature Protection (OTP)
The ISL97671 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 significantly below the
regulation target will be treated as “open circuit” and disabled after
a time-out period. This time-out period is also reduced to 800μs
when it is above the lower threshold. 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. Once the device has cooled to approximately
+100°C, the device will restart with the DC LED current level
reduced to 75% of the initial setting. If the dissipation problem
persists, subsequent hitting of the limit will cause identical
behavior, with the current reduced in steps to 50% and finally
FN7631.3
August 14, 2012
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