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MAX1745AUB+ Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1745AUB+ Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 15 page High-Voltage, Step-Down DC-DC Controllers in µMAX _______________________________________________________________________________________ 9 lower frequencies, reducing the MOSFET gate drive and switching losses. In discontinuous mode, under most circumstances, the on-time will be the fixed mini- mum on-time of 1μs. If the inductor value is small, or the current-sense resistor large, the current limit will be tripped before the minimum on-time, terminating the on-time and thus setting the fixed on-time. If the inductance is too large, or the output capacitance high and equivalent series resistance (ESR) low, then the MOSFET remains on longer than the minimum on- time, until the output capacitor charges beyond the error comparator’s (VOUT / 1.25V) ✕ 8mV hysteresis, causing the part to operate in hysteretic mode. Operating in hysteretic mode results in lower frequency operation. The transition to hysteretic mode occurs at the critical output capacitor ESR: ESRCRITICAL = (VOUT / 1.25V) ✕ 8mV / IRIPPLE where IRIPPLE is the inductor ripple current, and can be determined by: IRIPPLE = (VIN - VOUT) ✕ tON(MIN) / L where tON(MIN) is the minimum on-time (1μs) for mini- mum on-time-control, or: IRIPPLE = (VOUT) ✕ tOFF(MIN) / L where tOFF(MIN) is the minimum off-time (2μs) for mini- mum off-time-control. REF MINIMUM OFF-TIME ONE SHOT Q TRIG VL LINEAR REGULATOR VH LINEAR REGULATOR 1.25 REFERENCE (FB) ERROR COMPARATOR OUT 3/5 SHDN CS 100mV + - Q R S VL SHDN VH EXT IN MINIMUM ON-TIME ONE SHOT Q TRIG ( ) MAX1745 ONLY - - - MAX1744 ONLY Figure 2. Simplified Functional Diagram |
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