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ZXSC420 Datasheet(PDF) 4 Page - Diodes Incorporated |
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ZXSC420 Datasheet(HTML) 4 Page - Diodes Incorporated |
4 / 12 page ![]() DEVICE DESCRIPTION Bandgap Reference All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of 1.22V nominal. Dynamic Drive Output Depending on the input signal, the output is either “LOW” or “HIGH”. In the high state a 2.5mA current source (max drive voltage = VCC-0.4V) drives the base or gate of the external transistor. In order to operate the external switching transistor at optimum efficiency, both output states are initiated with a short transient current in order to quickly discharge the base or the gate of the switching transistor. Switching Circuit The switching circuit consists of two comparators, Comp1 and Comp2, a gate U1, a monostable and the drive output. Normally the DRIVE output is “HIGH”; the external switching transistor is turned on. Current ramps up in the inductor, the switching transistor and external current sensing resistor. This voltage is sensed by comparator, Comp2, at input ISENSE. Once the current sense voltage across the sensing resistor exceeds 20mV, comparator Comp2 through gate U1 triggers a re-triggerable monostable and turns off the output drive stage for 2µs. The inductor discharges to the load of the application. After 2µs a new charge cycle begins, thus ramping the output voltage. When the output voltage reaches the nominal value and VFB gets an input voltage of more than 300mV, the monostable is forced “on” from Comp1 through gate U1, until the feedback voltage falls below 300mV. The above action continues to maintain regulation. EOR, End of Regulation Detector The EOR circuit is a retriggerable 120µs monostable, which is re-triggered by every down regulating action of comparator Comp1. As long as regulation takes place, output EOR is “HIGH“ (high impedance, 100K to VCC). Short dips of the output voltage of less than 120µs are ignored. If the output voltage falls below the nominal value for more than 120µs, output EOR goes ”LOW“. The reason for this to happen is usually a slowly progressing drop of input voltage from the discharging battery. Therefore the output voltage will also start to drop slowly. With the EOR detector, batteries can be used to the ultimate end of discharge, with enough time left for a safe shutdown. Block Diagrams ZXSC410 ZXSC420 SEMICO NDUC TORS ISSUE 2 - May 2003 4 + _ + _ R2 R3 R1 STDN VCC DRIVE SENSE GND VFB Shutdown Bandgap Reference Bias Generator Comp 1 Comp 2 U1 Dynamic Drive Monostable 2µs Fig. 1 ZXSC410 Fig. 1 ZXSC420 |
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Similar Description - ZXSC420 |
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