3.3V/5V or Adjustable-Output,Single-Cell DC-DC ConvertersMAX866/MAX867 MINIMUM VBATT OFF-TIME ONE-SHOT Q TRIG ONE-SHOT SHDN LX VOUT F/F S Q N R 3/5* GND MAXIMUM ON-TIME TRIG Q ONE-SHOT ONE-SHOT CURRENT-LIMIT COMPARATOR OUT MAX866/MAX867 ** * FB** ** * ERROR COMPARATOR LBO REF N LBI COMPARATOR REFERENCE LBI *MAX866 ONLY **MAX867 ONLY Figure 1. Block Diagram _______________Detailed Description drain (100µA typ). There is no oscillator; switching is accomplished by a pair of one shots that set a maxi- Operating Principle mum LX on-time (4.5µs typ) and a minimum LX off-time The MAX866/MAX867 combine a switch-mode regula- (1µs). LX on-time will be terminated early if the inductor tor, N-channel power MOSFET, precision voltage refer- current reaches 0.5A before 4.5µs elapses. With the ence, and power-fail detector in a single monolithic standard application circuit (Figure 2a), LX current is device. The MOSFET is a “sense-FET” type for best effi- typically less than 50mA, so LX on-time is normally not ciency, and has a very low gate threshold voltage to terminated by the 0.5A limit and lasts the complete ensure start-up with low battery voltages (0.8V typ). 4.5µs. The LX on-resistance is typically 1Ω to minimize switch losses. The MAX866/MAX867 switching frequen- PFM Control Scheme cy depends on load, input voltage, and inductor value, The MAX866/MAX867 control scheme (Figure 1) com- and it can range up to 250kHz with typical component bines low-voltage efficiency (80% typ) with low battery values. _______________________________________________________________________________________7
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