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MAX15004 Datasheet(PDF) 13 Page - Maxim Integrated Products |
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MAX15004 Datasheet(HTML) 13 Page - Maxim Integrated Products |
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13 / 27 page 
4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers ______________________________________________________________________________________ 13 (tDISCHARGE). See Figures 3a and 3b for a coarse selec- tion of capacitor values for a given switching frequency and maximum duty cycle and then use the following equations to calculate the resistor value to fine-tune the switching frequency and verify the worst-case maximum duty cycle. where fOSC is the oscillator frequency, RT is a resistor connected from RTCT to REG5, and CT is a capacitor connected from RTCT to SGND. Verify that the oscilla- tor frequency value meets the target. Above calcula- tions could be repeated to fine-tune the switching frequency. The MAX15004A/B is a 50% maximum duty-cycle part, while the MAX15005A/B is 100% maximum duty-cycle part. for the MAX15004A/B and for the MAX15005A/B. The MAX15004A/B/MAX15005A/B can be synchronized using an external clock at the SYNC input. For proper frequency synchronization, SYNC’s input frequency must be at least 102% of the programmed internal oscillator frequency. Connect SYNC to SGND when not using an external clock. A rising clock edge on SYNC is interpret- ed as a synchronization input. If the SYNC signal is lost, the internal oscillator takes control of the switching rate, returning the switching frequency to that set by RC net- work connected to RTCT. This maintains output regula- tion even with intermittent SYNC signals. ff OUT OSC = ff OUT OSC = 1 2 t D f RT t CT t CHARGE MAX OSC CHARGE DISCHARGE = = × = 07 . 2 225 1 33 10 3 375 3 .( ) (. ( ) ) . ( ) VRT CT ART V fOS ×× ×× − − C C CHARGE DISCHARGE tt Us = + 1 ................... e e This Equation If f kHz tt OSC CHARGE DISCHA ≤ + 500 1 R RGE OSC ns Use This Equation If f k + > 160 500 ....... H Hz ⎧ ⎨ ⎪⎪ ⎩ ⎪ ⎪ Figure 2. Timing Diagram for Internal Oscillator vs. External SYNC and DMAX Behavior RTCT CLKINT SYNC OUT RTCT CLKINT SYNC OUT D = 50% D = 50% D = 81.25% D = 80% WITH SYNC INPUT MAX15004A/B (DMAX = 50%) MAX15005A/B (DMAX = 81%) WITHOUT SYNC INPUT WITH SYNC INPUT WITHOUT SYNC INPUT |
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