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NCP81243 Datasheet(PDF) 20 Page - ON Semiconductor |
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NCP81243 Datasheet(HTML) 20 Page - ON Semiconductor |
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20 / 26 page  NCP81243 www.onsemi.com 20 F Z + DCR@25C 2 @ PI @ L Phase Programming the Current Limit The current limit thresholds are programmed with a resistor between the ILIMIT and CSCOMP pins. The ILIMIT pin mirrors the voltage at the CSREF pin and mirrors the sink current internally to IOUT (reduced by the IOUT Current Gain) and the current limit comparators. The 100% current limit trips if the ILIMIT sink current exceeds 10 mA for 50 ms. The 150% current limit trips with minimal delay if the ILIMIT sink current exceeds 15 mA. Set the value of the current limit resistor based on the CSCOMP−CSREF voltage as shown below. R LIMIT + Rcs2 ) Rcs1@Rth Rcs1 )Rth Rph @ Iout LIMIT @ DCR 10 m or R LIMIT + V CSCOMP−CSREF @ ILIMIT 10 m Programming DAC Feed−Forward Filter The DAC feed−forward implementation is realized by having a filter on the VSN pin. Programming Rvsn sets the gain of the DAC feed−forward and Cvsn provides the time constant to cancel the time constant of the system per the following equations. Cout is the total output capacitance and Rout is the output impedance of the system. Rvsn + Cout @ Rout @ 453.6 @ 106 Cvsn + Rout @ Cout Rvsn Programming DROOP An output loadline is a power supply characteristic wherein the regulated (DC) output voltage decreases proportional to the load current. This characteristic can reduce the output capacitance required to maintain output voltage within limits during load transient faster than those to which the regulation loop can respond. With the NCP81243 and projects the loadline is produced by adding a signal proportional to output load current (Vdroop) to the output voltage feedback signal− thereby satisfying the voltage regulator at an output voltage reduced proportional to load current. The loadline is programmed by setting the gain of the Total Current Sense Amplifier such that the total current signal is equal to the desired output voltage droop.The signals CSCOMP and CSREF are differentially summed with the output voltage feedback to add precision voltage droop to the output voltage. Droop = DCR * (RCS / Rph) Programming IOUT The IOUT pin sources a current in proportion to the ILIMIT sink current. The voltage on the IOUT pin is monitored by the internal A/D converter and should be scaled with an external resistor to ground such that a load equal to ICCMAX generates a 2 V signal on IOUT. A pull−up resistor from 5 V VCC can be used to offset the IOUT signal positive if needed. R IOUT + 2.0 V @ R LIMIT 10 @ Rcs2 ) Rcs1@Rth Rcs1 )Rth Rph @ Iout ICC_MAX @ DCR Programming ICC_MAX A resistor to ground on the IMAX pin programs these registers at the time the part is enabled. 10 uA is sourced from these pins to generate a voltage on the program resistor. ICC_MAX 21h + R @ 10 mA @ 255 A 2V Programming TSENSE A temperature sense input per rail is provided. A precision current is sourced out the output of the TSENSE pin to generate a voltage on the temperature sense network. The voltage on the temperature sense inputs are sampled by the internal A/D converter. A 100 k NTC similar to the VISHAY ERT−J1VS104JA should be used. Rcomp1 is optional to the user, and can be used to slightly change the hysteresis. See the specification table for the thermal sensing voltage thresholds and source current. |
Similar Part No. - NCP81243_17 |
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Similar Description - NCP81243_17 |
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