link to page 8 link to page 8 MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters INPUT OUTPUT 4.5V OR 24V L1 LX 3.3V OR 5V IN CIN SHDN D1 COUT VSENSE OUT R Q FB MAXIMUM OFF-TIME S DELAY Q TRIG 100mV VSET Q TRIG 1.25V MAXIMUM MAX1836 ON-TIME GND MAX1837 DELAY Figure 3. Functional Diagram When the output voltage is too low, an error compara- tor sets a flip-flop, which turns on the internal p-channel MOSFET and begins a switching cycle (Figure 3). As 10V shown in Figure 4, the inductor current ramps up linearly, A charging the output capacitor and servicing the load. The MOSFET turns off when the current limit is reached, or 0 when the maximum on-time is exceeded while the output B voltage is in regulation. Otherwise, the MOSFET remains 3.3V on, allowing a duty cycle up to 100% to ensure the lowest possible dropout voltage. Once the MOSFET turns off, the 500mA flip-flop resets, diode D1 turns on, and the current through C 0 the inductor ramps back down, transferring the stored energy to the output capacitor and load. The MOSFET 4µs/div remains off until the 0.5μs minimum off-time expires and CIRCUIT OF FIGURE 2, VIN = 12V the inductor current ramps down to zero, and the output A. VLX, 5V/div voltage drops back below the set point. B. VOUT = 3.3V, 20mV/div, 200mA LOAD C. INDUCTOR CURRENT, 500mA/div Figure 4. Discontinuous-Conduction Operation www.maximintegrated.com Maxim Integrated │ 8
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