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ISL97672IRZ-EVAL Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97672IRZ-EVAL Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 17 page  ISL97672 11 FN7632.0 June 24, 2010 Fault Protection and Monitoring The ISL97672 features extensive protection functions to cover all the perceivable failure conditions. The failure mode of a LED can be either open circuit or as 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 will only result 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 ISL97672 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. Short Circuit Protection (SCP) The short circuit detection circuit monitors the voltage on each channel and disables faulty channels which are above approximately 5V (the action taken is described in Table 1). 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 ISL97672 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 ISL97672 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 and enabling open circuit operation. When this type of LED is open circuited, 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 channels look as if they have LED shorts. See Table 1 for details regarding 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 Equation 5: These resistors should be large to minimize the power loss. For example, a 1MkΩ RUPPER and 30kΩ RLOWER sets OVP to 41.2V. Large OVP resistors also allow COUT discharges slowly during the PWM Off time. Parallel capacitors should also be placed across the OVP resistors such that RUPPER/RLOWER = CLOWER/CUPPER. Using a CUPPER 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 2.45V, the device will stop switching and be reset. Operation will restart only if the VIN is back in the normal operating range. 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 ISL97672 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 also be switched off. Over-Temperature Protection (OTP) The ISL97672 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 OVP 1.21V R UPPER R LOWER + () R LOWER ⁄ × = (EQ. 5) |
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