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ISL97672AIRZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL97672AIRZ Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 16 page ISL97672A 10 FN7710.3 November 22, 2013 IC Protection Features and Fault Management ISL97672A has several protection and fault management features that improve system reliability. The following sections describe them in more detail. In-Rush Control and Soft-Start The ISL97672A has separate, built-in, independent in-rush control and soft-start functions. The in-rush 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-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 in-rush 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 in-rush 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 ISL97672A 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 in-rush current flows towards COUT when VIN is applied. The in-rush current is determined by the ramp rate of VIN and the values of COUT and L. Fault Protection and Monitoring The ISL97672A features extensive protection functions to cover all perceivable failure conditions. The /FLAG pin is a latched open-drain output that monitors string open, LED short, VOUT 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 ISL97672A 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 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 on page 12). 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 ISL97672A 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 ISL97672A 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. OVP and VOUT The Overvoltage Protection (OVP) pin has a function of setting the overvoltage trip level as well as limiting the VOUT regulation range. The ISL97672A OVP threshold is set by RUPPER and RLOWER such that: and output voltage VOUT can regulate between 64% and 100% of the VOUT_OVP such that:‘ Allowable VOUT = 64% to 100% of VOUT_OVP if, for example, 10 LEDs are used with the worst-case VOUT of 35V. V OUT_OVP 1.22Vx RUPPER RLOWER + () RLOWER -------------------------------------------------------- = (EQ. 5) |
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