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## NCP1239 Datasheet(PDF) 24 Page - ON Semiconductor

 Part No. NCP1239 Description Low−Standby High Performance PWM Controller Download 38 Pages Scroll/Zoom 100% Maker ONSEMI [ON Semiconductor] Homepage http://www.onsemi.com Logo

## NCP1239 Datasheet(HTML) 24 Page - ON Semiconductor

 24 / 38 page NCP1239http://onsemi.com24If by design we have selected a 47mF VCC capacitor, itbecomes easy to evaluate the burst period and its duty−cycle.This can be done by properly identifying all time events onFigure 42 and applying the classical formula: t = C *DV / i.To simplify, let’s consider t1 starts while VCC = VCCOFF.Then:• t1: I = ICC3 = 400 mA, DV= 11.2 – 6.9 = 3 V !t1 = 505 ms• t2: I = 3.6 mA, DV= 16.4 – 6.9 = 9.5 V ! t2 = 124 ms• t3: I = 400 mA, DV= 16.4 – 11.2 = 5.2 V ! t3 = 611 ms• t’1 = t1= 505 ms• t’2 = t2 = 124 msThe total period duration is thus the sum of all these eventswhich leads to Tfault = 1793 ms. If Tpulse = 100 ms, thenour burst duty−cycle equals 100/(1869 + 100)≈ 5%, whichis excellent.In fact, the calculation assumption, t1 starts whileVCC = VCCOFF, gives the worse case since the duty cycle iscalculated in the case where Tpulse exactly equals the activephase duration (switching period when VCC decreases fromVCCON to VCCOFF).In fact, Tpulse is generally:− shorter than the switching phase period. In thiscase, t1 is longer since the latched off phase startsearlier (at a VCC higher than VCCOFF). As aconsequence, the final duty cycle is lower thanpreviously estimated,− longer than the switching phase period. In this case,the circuit detects an overload condition simplybecause VCC drops below VCCOFF (11.2 V) beforethe fault timer has elapsed. Tpulse is lower than 100ms and as a result the duty cycle is also lower.(Major) Fault Detection and Latched Off ModeTheNCP1239featuresafastcomparatorthatpermanently monitors the “Fault Detect” pin level. If for anyreason this level exceeds 2.4 V (typical), the partimmediately stops oscillating and stays latched off until theuser cycles down the power supply. This enables the SMPSdesigner to externally shut down the part in particular whena major default occurs, e.g. an Overvoltage Protection(OVP). Figure 43 shows what happens when the part islatched:VCCOFFFigure 43.DrvLatch−off phase levelLogic reset levelPin 3Stop!2.4 VWhen Vpin3 exceeds 2.4 V, NCP1239 permanently latches−off the output pulses0until its VCC goes below 4 V. The figure canillustrate a case where a thermistor supplied by REF5V is connected to Pin 3 to detect excessive temperatures of the application(refer to application schematic).The user has unplugged, reset!VCCVCCON

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