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ISL97636AIRZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97636AIRZ Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 18 page 11 FN6566.0 May 9, 2008 For example, if the maximum required LED current (ILED(max)) is 20mA, rearranging Equation 1 yields Equation 3: PWM CURRENT CONTROL The average LED current of each channel can be controlled by an external PWMI signal as shown in Equation 3: The PWM dimming frequency can be, for example, 20kHz, but there are a minimum on and off time requirements such that the dimming will be in the range of 10% to 99.5%. If the dimming frequency is below 5kHz, the dimming range can be 1% to 99.5%. The PWM dimming off time cannot be longer than 28ms or else the driver will enter shutdown. 5V Low Dropout Regulator A 5.2V LDO regulator is present at the VDC pin to develop the necessary low voltage supply which is used by the chips internal control circuitry. Because VDC is an LDO pin, it requires a bypass capacitor of 1µF or more for the regulation. For applications with an input voltage ≤ 5.5V, the VIN and VDC pins can be connected together. The VDC pin can be used as a coarse reference with few mA sourcing capability. Inrush Control and Soft-start The ISL97636A has separately built-in independent inrush control and soft-start functions. The inrush control function is built around the short circuit protection FET, and is only available in applications, which include this device. At start- up, the fault protection FET is turned on slowly due to a 30µA 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 weakly turned on FET before it becomes fully enhanced. This results in a low inrush 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 hard, it is assumed that inrush is complete. At this point, the boost regulator will begin to switch and the current in the inductor will ramp-up. The current in the boost power switch is monitored and the switching terminated in any cycle where the current exceeds the current limit. The ISL97636A includes a soft-start feature where this current limit starts at a low value (375mA). This is stepped up to the final 3A current limit in seven further steps of 375mA. These steps will happen over a 1ms total time, such that after 1ms the final limit will be reached. This allows the output capacitor to be charged 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 inrush current will flow towards COUT when VIN is applied and it is determined by the ramp rate of VIN and the values of COUT and L. Fault Protection and Monitoring The ISL97636A 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 ISL97636A 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 detected above 8V (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 ISL97636A monitors the current in each channel such that any string which reaches at least 75% of the intended output current is considered “good”. Should the current subsequently fall below 50% of the target the channel will be considered an “open circuit”. Furthermore, should the boost output of the ISL97636A 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 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. R SET 733 0.02 ⁄ 36.6k Ω == (EQ. 2) I LED ave () I LED PWMI × = (EQ. 3) ISL97636A |
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