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A6265KLPTR-T Datasheet(PDF) 11 Page - Allegro MicroSystems |
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A6265KLPTR-T Datasheet(HTML) 11 Page - Allegro MicroSystems |
11 / 17 page Automotive High Current LED Controller A6265 11 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com These measurements are used to determine if there is an open circuit, if one or more LEDs are shorted, if the output is shorted, or if there is a short across the LED string. Each condition is described in turn in the following sections. Open LED–An open circuit is evaluated when: VSTR > VOCL (7) where VOCL is the LED open circuit voltage defined in the Elec- trical Characteristics table. Because the output is current-controlled it is possible for an open circuit on the output to cause extremely high voltages to be pres- ent. Therefore, to prevent any hazardous voltages or damage to the circuits, the gate drive output, SG, is immediately driven low when an open circuit is detected. After an open circuit fault has been detected, FF2 will become high impedance, and the open circuit fault state will remain until the open fault time-out period toto, expires. When the gate drive output is re-enabled at the end of the open fault time-out period, the output is again monitored for an open circuit. If the open circuit is still present, then the fault will again be flagged and the switch drive disabled. This cycle will continue, as long as the open circuit condition is pres- ent. Note that the Fault Blank timer is not used when an open LED fault occurs. This is to avoid potentially damaging voltages appearing in the power circuitry. Shorted LED – A short circuit on one or more LEDs is detected when: • for the first (reference) LED: VSTR > VLED + VSCOR (8) • for other than the first (reference) LED: VLED > VSTR + VSCO (9) where VSTR and VLED are as defined above, VSCO is the nonrefer- ence LED short offset voltage, and VSCOR is the reference LED short offset voltage. VSCO and VSCOR are defined in the Electrical Characteristics table. When a short is present, the fault flag FF2 is high impedance, but the regulator continues to operate and drives the remaining LEDs with the correct regulated current. FF2 will remain high imped- ance while the short circuit condition is present. A short circuit on one or more LEDs will not cause a hazard because the output is current-controlled. If one LED fails and becomes a short circuit, then the remaining LEDs will continue to be lit with the same current through, and voltage across, each LED. Note—Accuracy: The output status monitor relies on all the LEDs in the load having a similar forward voltage drop. Where possible all the LEDs forming the load for a single controller should be taken from the same voltage bin. With only two or three LEDs a wider variation in forward voltage is acceptable, but the selection of LEDs from the same bin is more critical when higher numbers of LEDs are used in a single string. Shorted LED String or Output Short – A short circuit across the LED string, is detected when: VSTR < VSCL (10) An output short can consist of the LP, LN, or LF terminals of the LED string being shorted, either to the battery terminal or to ground. Either a shorted LED string or output short will be latched and will only be cleared by pulling EN low for a period greater than the disable time, or by cycling the power. If either a shorted LED string, or an output short is detected, the A6265 will stop the switching action by pulling SG low. The fault flag FF1 will go high impedance and should be pulled up to the supply with suitable external pull-up resistors to indicate the fault. Either a shorted LED string or output short will be latched and will only be cleared by pulling EN low for a period greater than the disable time, or by cycling the power. The FF1 output can also be used with pull-up resistors and a P-channel MOSFET in the supply, to isolate the switching ele- ments and the load from the supply. This MOSFET should be connected, as shown in figure 3, with the source connected to the supply and the drain connected to the inductor of the converter. To FF1 VBAT To VIN Figure 3. Example of a supply isolation MOSFET |
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