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A2557ELB-T Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A2557ELB-T Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 12 page 2557 PROTECTED QUAD DRIVER WITH FAULT DETECTION & SLEEP MODE 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 6 The A2557 low-current quad power drivers provide protected output driver functions, combined with a fault diagnostic scheme, plus an automatic low-current Sleep-Mode function. These devices monitor their outputs for fault (open or shorted) conditions. For each channel the input and output levels are compared. If these are different from the expected levels then a fault condition is flagged by pulling the common FAULT output low. Status INN ENABLE OUTN FAULT Normal Load HH L H LH H H Sleep Mode XL H H All L X H H Over-Current or Short to Supply HH R L Open Load or Short to Ground LH L L Thermal Fault H H H L R = Linear drive, current limited. The FAULT output is operational only if ENABLE is high. The output state is detected by monitoring the OUTn terminal using a comparator whose threshold is typically 2.5 V. In order to detect open-circuit outputs, a 30 µA current sink pulls the output below the comparator threshold. To ensure correct fault operation, a minimum load of approximately 1 mA is required. The fault function is disabled when in ‘sleep’ mode, i.e., FAULT goes high and the 30 µA output sinks are turned off. The FAULT output is a switched current sink of typically 60 µA. Each channel consists of a TTL/CMOS-compatible logic input gated with a common ENABLE input. A logic high at the input will provide drive to turn on the output npn switch. Each output has a current-limit circuit that limits the output current by detecting the voltage drop across a low-value internal resistor in the emitter of the output switch. If this drop reaches a threshold, then the base drive to the output switch is reduced to maintain constant current in the output. To keep the device within its safe operating area (SOA) this output current limit is further reduced CIRCUIT DESCRIPTION AND APPLICATION • if the power dissipation in the output device increases the local junction temperature above 165°C (nominal), so as to limit the power dissipation (and hence the local junction tempera- ture). As each channel has its own thermal limit circuitry this provides some independence between the output channels, i.e., one channel can be operating in thermally reduced current limit, while the others can provide full drive capability. • as a function of the output voltage. Full current limit of 500 mA (nominal) is available up to approximately VO = 8 V; above this the limit is reduced linearly to about 350 mA at VO = 32 V. This helps to improve SOA by immediately reducing the peak power pulse into a shorted load at high VO. A logic low at the ENABLE input causes all outputs to be switched off regardless of the state of the IN terminals. In addition, the device is put into a low quiescent current ‘sleep’ mode, reducing ICC below 100 µA. If ENABLE is taken high and any of the inputs go high, the circuit will ‘auto-wake-up’. However, if the device is enabled, but all inputs stay low, then the circuit remains in ‘sleep’ mode. All outputs have internal flyback diodes, with a common- cathode connection at the K terminal. Incandescent lamp driver High incandescent lamp turn-on (in-rush currents) can contribute to poor lamp reliability and destroy semiconductor lamp drivers. When an incandescent lamp is initially turned on, the cold filament is at minimum resistance and would normally allow a 10x to 12x in-rush current. Warming (parallel) or current-limiting (series) resistors protect both driver and lamp but use significant power either when the lamp is off or when the lamp is on, respectively. Lamps with steady-state current ratings up to 300 mA can be driven without the need for warming or current-limiting resis- tors, if lamp turn-on time is not a concern (10s of ms). With these drivers, during turn-on, the high in-rush current is sensed by the internal sense resistor, drive current to the output stage is reduced, and the output operates in a linear mode with the load current limited to approximately 500 mA. During lamp warmup, the filament resistance increases to its maximum value, the output driver goes into saturation and applies maxi- mum rated voltage to the lamp. |
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