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A2557KLB-T Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A2557KLB-T Datasheet(HTML) 8 Page - Allegro MicroSystems |
8 / 12 page 2557 PROTECTED QUAD DRIVER WITH FAULT DETECTION & SLEEP MODE www.allegromicro.com 8 CIRCUIT DESCRIPTION AND APPLICATION (continued) NORMAL LOAD TIME Dwg. WP-013-1 NOT TO SCALE CURRENT LIMIT (12 V SUPPLY) SHORT CIRCUIT CURRENT LIMIT (24 V SUPPLY) IN FAULT (SHORTED LOAD) FAULT (OPEN LOAD) WITH OUTPUT CAPACITOR Dwg. WP-035 Thermal considerations Device power dissipation can be calculated as: PD = (VO1 x IO1 x duty cycle1) + … + (VO4 x IO4 x duty cycle4) + (VCC x ICC) Note - ICC is also modulated by the duty cycle, but this is a reasonable approximation for most purposes. This can then be compared against the permitted package power dissipation, using: Permitted PD = (150 – TA)/RθJA where RθJA is given as: 28-lead PLCC (part number suffix ‘–EB’) = 36°C/W 16-pin PDIP (part number suffix ‘–B’) = 43°C/W 16-lead SOIC (part number suffix ‘–LB’) = 90°C/W RθJA is measured on typical two-sided PCB. Additional Under some conditions it is possible to get spurious glitches on the FAULT output at load turn-on and turn-off transitions: • Light load turn-off. Under light loading conditions the turn-off delay (see characteristics above) of the output stage increases and may result in a spurious fault output of a few µs (the duration being proportional to the turn-off delay). As it is difficult to define this over all operating conditions, if a particular application would be sensitive to this type of glitch, then it is generally recommended to include a small (about 0.01 µF) smoothing/storage capacitor at the FAULT output. • Incandescent lamp turn-on. As described above, driving an incandescent filament results in the driver operating in current limit for a period after turn-on. During this period a “fault” condition will be indicated (over current). As discussed above this period can be 10s of ms. To avoid this indication, the capacitor on the FAULT output would need to be increased to provide an appropriate time constant. Alternatively, in a microcontroller-based system, the code could be written to ignore the FAULT condition for an appropriate period after lamp turn on. Correct FAULT operation cannot be guaranteed with an unconnected output — unused outputs should not be turned on, or unused outputs should be pulled high to >2.5 V, and/or associated inputs tied low. information is available on the Allegro web site. |
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