Datenblatt-Suchmaschine für elektronische Bauteile |
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MAX877 Datenblatt(PDF) 9 Page - Maxim Integrated Products |
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MAX877 Datenblatt(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page __________Applications Information Figure 4 shows a MAX877 step-up application circuit. This circuit starts up and operates with inputs ranging from 1.0V to 6.2V. Start-up time is a function of the load, typically less than 5ms. Output current capability is a function of the input voltage (see Typical Operating Characteristics). The converters will regulate down to the output voltage and seamlessly switch into boost mode as the input drops below the output voltage. This is especially use- ful in battery-powered applications, where the battery voltage may initially exceed the output voltage. To gen- erate 5V from four alkaline cells in series, the input ranges from 6.2V to 3.6V. When the battery pack is fresh, the MAX877 will step down with the active rectifi- er acting as the switch. As the batteries approach 5V, or the desired output voltage, the converter’s control circuitry will ensure a smooth transition into step-up mode. The converter operates until the batteries are less than 3V; efficiency is typically 80% with fresh bat- teries, and is close to 85% at VIN = 4V. Inductor Selection The 22µH inductor shown in the Typical Operating Circuit is sufficient for most MAX877/MAX878/MAX879 designs. Other inductor values ranging from 10µH to 47µH are also suitable. The inductor should have a satu- ration rating equal to or greater than the peak switch- current limit, which is 1A without an external current limit (ILIM connected to IN). It is acceptable to operate the inductor at 120% of its saturation rating; however, this may slightly reduce efficiency. For highest efficiency, use an inductor with a low DC resistance, preferably under 0.2 Ω. Table 1 lists suggested inductor suppliers. Capacitor Selection The 100µF, 10V surface-mount tantalum (SMT) output capacitor shown in the Typical Operating Circuit will provide a 25mV output ripple or less, stepping up from 3V to 5V at 200mA. Smaller capacitors, down to 10µF, are acceptable for light loads or in applications that can tolerate higher output ripple. The input capacitor may be omitted if the supply has low output impedance and the input lead length is less than 2 inches (5cm) or the loads are small. The primary factor in selecting both the output and input filter capacitor is low ESR. The ESR of both bypass and filter capacitors affects efficiency. Optimize performance by increasing filter capacitors or using specialized low- ESR capacitors. The smallest low-ESR SMT tantalum capacitors currently available are Sprague 595D or 695D series. Sanyo OS-CON organic-semiconductor through- hole capacitors also exhibit very low ESR, are rated for the wide temperature range, and are especially suitable for operation at cold temperatures (below 0°C). Table 1 lists suggested capacitor suppliers. 5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters _______________________________________________________________________________________ 9 INPUT 1.0V TO 6.2V IN ILIM SHDN 100µF OUTPUT 5V AGND MAX877 LX OUT PGND 22µH ON/OFF 22 µF 1 2 7 4 3 6 5 SHDN PGND MAX879 OUT AGND 7 1 3 4 8 6 L1 22 µH 5 LX FB 2 IN ILIM VIN C1 22µF R1 C3 C2 100µF VOUT R2 Figure 3. MAX879 Adjustable Voltage Figure 4. MAX877 Standard Application Circuit |
Ähnliche Teilenummer - MAX877 |
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Ähnliche Beschreibung - MAX877 |
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