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LM3101N Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM3101N Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 20 page Functional Description (Continued) Frequency-Shift Circuit The LM3101 has the ability to gradually reduce its operating frequency during an output short circuit The amount that the frequency shifts and the output voltage threshold deter- mining where the frequency starts to shift are both pro- grammed by two external resistors RSC1 and RSC2 con- nected to the pin RSC (pin 5) A simplified internal schematic of the Frequency Shift Circuit is shown in Figure 2 The oscillator operates at its nominal frequency as long as the voltage at the emitter of the tran- sistor Q2 is higher than the internal reference voltage VREF Q2 emitter voltage is the output voltage VOUT scaled down by the resistor divider VRSC e VQ2E e VOUT RSC2 (RSC1 a RSC2) V X where VQ2 l VREF (124V) for normal operation If VOUT drops due to an overload a current starts to flow through Q2 A cascoded current mirror causes one-tenth of this current to be subtracted from the timing capacitor charge current Reducing the timing capacitor charge cur- rent results in decreasing the oscillator frequency The breakpoint where the frequency-shift starts is programmed by the ratio of the two resistors VOUT(SC) e 124V 1 a (RSC1 RSC2) V X The typical short circuit frequency is set by the following equations FSC e IOSC b 01 ((124V b VOUT(SC))RSC1) a (124VRSC2) 1(20 pF 124V) kHz mA V X where IOSC e 025 (124VRT) For example say 140 kX and 100 kX were selected for RSC1 and RSC2 respectively with RT set to 25 kX Then the output voltage level where the frequency starts to decay is VOUT(CL) e 124V 1 a (140 kX 100 kX) j 30V and the short circuit frequency is (IOSC e 124 mA and as- suming VOUT is 0V during a short circuit) FSC e 124 mA b 01 (124V140 kX) a (124V100 kX) 1(20 pF 124V) e 4143 kHz j 415 kHz If RSC2 is omitted the frequency starts to shift when VOUT drops below VREF The short circuit frequency equation then becomes FSC e (120 pF) (0267RT) b (009RSC1) kHz X v RSC1 e 009 (0267RT) b (FSC 20 pF) X kHz Selecting a short-circuit frequency that is greater than one- third the operating frequency or 188 kHz leads to a resistor value of RSC1 e 009 (026725 kX) b (188 kHz 20 pF) e 13 kX Mode Control The LM3101 can operate in voltage mode current mode or charge mode control Two multi-function pins are involved in setting the operating mode the Mode Control pin (MC - pin 2) and the Current Mode Input pin (CMI - pin 6) Figure 3 shows the simplified schematic diagram of the mode control circuit To operate with voltage mode control the MC pin is pulled high with a resistor (typically 3 kX) and the CMI pin is connected to ground The mode comparator senses the MC pin voltage and sets the mode control multiplexer to voltage mode control Notice that there is a 56V zener diode clamp- ing the MC pin voltage TLH11436 – 8 FIGURE 3 Simplified Version of the Mode Control Circuit TLH11436 – 9 FIGURE 4 Current Mode Sense Circuit 9 |
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Similar Description - LM3101N |
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