MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller 3.5 3.5 VOUT = 5V VOUT = 12V L = 22µH L = 22µH 3.0 3.0 RSENSE = 20mΩ RSENSE = 20mΩ 2.5 2.5 RSENSE = 25mΩ RSENSE = 25mΩ RSENSE = 35mΩ 2.0 RSENSE = 35mΩ 2.0 1.5 1.5 1.0 1.0 RSENSE = 50mΩ RSENSE = 50mΩ MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) 0.5 0.5 RSENSE = 100mΩ RSENSE = 100mΩ 0 0 2 3 4 5 2 4 6 8 10 12 INPUT VOLTAGE (V) INPUT VOLTAGE (V) Figure 4a. Maximum Output Current vs. Input Voltage Figure 4b. Maximum Output Current vs. Input Voltage (VOUT = 5V) (VOUT = 12V) 3.5 0.8 VOUT = 15V VOUT = 24V L = 22µH 3.0 L =150µH RSENSE = 20mΩ 0.6 2.5 RSENSE = 25mΩ R RSENSE = 50mΩ SENSE = 35mΩ R 2.0 SENSE = 100mΩ 0.4 1.5 1.0 R 0.2 SENSE = 50mΩ MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) 0.5 RSENSE = 200mΩ RSENSE = 100mΩ 0 0 2 4 6 8 10 12 14 16 2 6 10 14 INPUT VOLTAGE (V) INPUT VOLTAGE (V) Figure 4c. Maximum Output Current vs. Input Voltage Figure 4d. Maximum Output Current vs. Input Voltage (VOUT = 15V) (VOUT = 24V) approximately 2µs; select an inductor that allows the cur- Inductors with a ferrite core or equivalent are recom- rent to ramp up to ILIM. mended; powder iron cores are not recommended for use with high switching frequencies. Make sure the The standard operating circuits use a 22µH inductor. inductor’s saturation current rating (the current at which If a different inductance value is desired, select L such the core begins to saturate and the inductance starts to that: fall) exceeds the peak current rating set by RSENSE. VIN(max) x 2µs However, it is generally acceptable to bias the inductor L ≥ —————----—-- into saturation by approximately 20% (the point where ILIM the inductance is 20% below the nominal value). For Larger inductance values tend to increase the start-up highest efficiency, use a coil with low DC resistance, time slightly, while smaller inductance values allow the preferably under 20mΩ. To minimize radiated noise, coil current to ramp up to higher levels before the use a toroid, a pot core, or a shielded coil. switch turns off, increasing the ripple at light loads. Table 1 lists inductor suppliers and specific recom- mended inductors. 10 Maxim Integrated
