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UC37131 Datasheet(PDF) 6 Page - Texas Instruments |
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UC37131 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 10 page 6 UC17131/2/3 UC27131/2/3 UC37131/2/3 Reference The UC37131/2/3 family of devices features a 6V bandgap reference that is used to bias on-chip logic. Al- though the 6V reference is not trimmed, this bandgap ref- erence provides less than 200ppm/°C. It is also used to generate the on-chip 3V input comparator threshold and is needed for the programmable hysteresis. The on-chip reference has 8mA maximum current sourcing capacity that is designed to power up external circuitry. Input Comparator The input comparator is a high gain comparator with hys- teresis that fully switches with either a small signal (30mV, minimum for 1% hysteresis) or a logic signal (0 to 6V max). Only a 5mV overdrive of the 3V threshold is needed to switch the driver. The hysteresis is set to 30% on the UC37131 and UC37133. (This is 30% of 3V equating to 0.9V of hyster- esis.) On the UC37132 it is programmable from 1% to 30%. Fault Logic The output of the comparator is logic ANDed with the output of the fault logic. If a fault, either a power interrupt or an overcurrent condition, persists longer than it takes for the CDEL to discharge from its VCDEL_MAX level of 5.8V to its VFAULT_L of 1.3V, the fault protection block will output a logic 0 to the NAND gate and turn off the output driver. If the fault goes away prior to CDEL being dis- charged to 1.3V, the chip will resume normal operation without going through a turn-on delay. The power interrupt normal operation consists of the chip turning the driver immediately back on if the interrupt goes away prior to CDEL reaching its lower threshold as described above. The CDEL capacitor is chosen based upon the maximum power interrupt time (tINT) allowed without the output experiencing a turn-on delay. This in- terrupt time must be less than tD(OFF) where tD(OFF) is equal to the time it takes the CDEL capacitor to dis- charge from VCDEL_MAX (5.8V) to VFAULT_L (1.3V) with a discharge current of approximately 94 µA. If the power stays off only as long as tD(OFF), the minimum power up delay will be equal to the time it takes to charge CDEL from VFAULT_L (1.3V) to VFAULT_H (4.9V) with a charge current of approximately 4 µA. If the power stays off lon- ger than this time, then a power up delay will be initial- ized once power is resumed. This delay is the time it takes for CDEL to charge from 0V to VFAULT_H of 4.9V. The overcurrent fault normal operation consists of the chip staying off until CDEL fully recharges to VFAULT_H of 4.9V. This is tR(OFF). Once CDEL reaches 4.9V, the driver will turn back on. If the overcurrent fault is still present, the chip will operate in a very low duty cycle (approxi- mately 0.7%) based on the discharge (driver on) and charge time (driver off) of the CDEL capacitor. This overcurrent timing makes the chip act "smart" by allowing very high currents needed to drive large capacitive loads without setting off an overcurrent fault. The overcurrent and current limit thresholds are pro- grammed with the resistor RCSH from CSH to VCC (high side) or RCSL from CSL to GND (low side). For example, a 150mV (ILOAD • RCSH) threshold will set the high side overcurrent fault threshold. An overall short circuit protec- tion threshold is set at 300mV. Therefore, the recom- mended RCSH of 0.5Ω will result in the 600mA short circuit. By changing the RCSH value the user can opti- mally set the overcurrent and short circuit current limits. Output Driver Once the turn-on signal is gated through from the input comparator, the output transistor is turned on. The output drive transistor is a composite PNP, NPN structure. This is a specially designed structure that keeps all the drive current needed for the load to be sourced through the LS pin. This keeps the overall power dissipation to less than 4mA independent of the load. The output driver also has a 72V zener diode wired be- tween its base and collector. This allows the output to swing and clamp to 72V above ground when discharging an inductive load in a low side application. The inductive zener clamp can discharge the 250mA to 400mA full load current. This consequently allows the LS pin to safely swing above VCC. Similarly, the 72V zener diode will allow the HS pin to safely swing and clamp 72V be- low LS/VCC when discharging an inductive load in a high side application. This 72V zener diode simplifies the user application by eliminating the need for external clamp di- odes. DESCRIPTION OF OPERATION |
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Descripción similar - UC37131 |
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