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MAX1651CSA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1651CSA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers 8 _______________________________________________________________________________________ Modes of Operation When delivering high output currents, the MAX1649/ MAX1651 operate in continuous-conduction mode. In this mode, current always flows in the inductor, and the control circuit adjusts the switch duty cycle to main- tain regulation without exceeding the switch current capability (Figure 3). This provides excellent load-tran- sient response and high efficiency. In discontinuous-conduction mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero. Although efficiency is still excel- lent, the output ripple increases slightly, and the switch waveform exhibits ringing (at the inductor's self-reso- nant frequency). This ringing is to be expected and poses no operational problems. Dropout The MAX1649/MAX1651 are in dropout when the input voltage (V+) is low enough that the output drops below the minimum output voltage specification (see Electrical Characteristics). The dropout voltage is the difference between the input and output voltage when dropout occurs. See the Typical Operating Characteristics for the Dropout Voltage vs. Load Current and Dropout Voltage vs. Temperature graphs. __________________Design Procedure Setting the Output Voltage The MAX1649/MAX1651 are preset for 5V and 3.3V out- put voltages, respectively; tie FB to GND for fixed-output operation. They may also be adjusted from 1.5V (the reference voltage) to the input voltage, using external resistors R2 and R3 configured as shown in Figure 4. For adjustable-output operation, 150k Ω is recommended for resistor R3—high enough to avoid wasting energy, yet low enough to avoid RC delays caused by parasitic capacitance at FB. R2 is given by: VOUT R2 = R3 x (——— -1) VREF where VREF = 1.5V. When using external resistors, it does no harm to con- nect OUT and the output together, or to leave OUT unconnected. Current-Sense Resistor Selection The current-sense resistor limits the peak switch cur- rent to 110mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 110mV is the current- limit trip level (see Electrical Characteristics). V+ = 10V, ILOAD = 1.3A CIRCUIT OF FIGURE 1, R1 = 75m Ω 1.5A 0A 1A 2 μs/div Figure 3. MAX1649 Continuous-Conduction Mode, Heavy Load-Current Waveform (500mA/div) ( ) MAX1649 MAX1651 V+ CS GND 5 6 2 8 3 VIN C2 330 μF 7 1 EXT OUT SHDN 4 C3 0.1 μF C4 0.1 μF C1 100 μF R1 0.05 Ω D1 1N5820 L1 47 μH P1 Si9430 OUTPUT @ 1.5A REF FB R2 R3 150k R2 = R3 VOUT VREF – 1 VREF = 1.5V Figure 4. Adjustable-Output Operation |
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