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MAX653CPA Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX653CPA Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page Step 3: L = 50 / IPEAK. L will be in µH. Do not use an inductor of less than 100µH. Step 4: Make sure that IPEAK does not exceed 0.6A or the inductor’s maximum current rating, whichever is lower. Inductor series resistance affects both efficiency and dropout voltage. A high series resistance severely limits the maximum current available at lower input voltages. Output currents up to 225mA are possible if the induc- tor has low series resistance. Inductor and series switch resistance form an LR circuit during tON. If the L/R time constant is less than the oscillator tON, the inductor’s peak current will fall short of the desired IPEAK. To maximize efficiency, choose the highest-value inductor that will provide the required output current over the whole range of your input voltage (see Typical Operating Characteristics). Inductors with peak cur- rents in the 600mA range do not need to be very large. They are about the size of a 1W resistor, with surface- mount versions less than 5mm in diameter. Table 1 lists suppliers of inductors suitable for use with the MAX639/MAX640/MAX653. Output Filter Capacitor The MAX639/MAX640/MAX653’s output ripple has two components. One component results from the variation in stored charge on the filter capacitor with each LX pulse. The other is the product of the current into the capacitor and the capacitor’s equivalent series resis- tance (ESR). The amount of charge delivered in each oscillator pulse is determined by the inductor value and input voltage. It decreases with larger inductance, but increases as the input voltage lessens. As a general rule, a smaller amount of charge delivered in each pulse results in less output ripple. With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the charge variation. Consequently, high-quality alu- minum-electrolytic or tantalum filter capacitors will mini- mize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capac- itor in the 100µF range, in parallel with a 0.1µF ceramic capacitor (Table 1). External Diode In most MAX639/MAX640/MAX653 circuits, the current in the external diode (D1, Figure 3) changes abruptly from zero to its peak value each time LX switches off. To avoid excessive losses, the diode must have a fast turn-on time. For low-power circuits with peak currents less than 100mA, signal diodes such as the 1N4148 perform well. The 1N5817 diode works well for high- power circuits, or for maximum efficiency at low power. 1N5817 equivalent diodes are also available in surface- mount packages (Table 1). Although the 1N4001 and other general-purpose rectifiers are rated for high cur- rents, they are unacceptable because their slow turn- off times result in excessive losses. Minimum Load Under no-load conditions, because of leakage from the PMOS power switch (see the LX Leakage Current vs. Temperature graph in the Typical Operating Characteristics) and from the internal resistor from V+ to VOUT, leakage current may be supplied to the output 5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters 10 ______________________________________________________________________________________ GND MAX639 MAX640 MAX653 SHDN 6 VOUT 4 LBI L = 100 µH 3 1 5 CIN 100 µF 1N5817 COUT 100 µF VFB 7 R3 R4 8 V+ LX OUTPUT INPUT +4.0V TO +11.5V Figure 4. Adjustable-Output Operation MAX639 MAX640 MAX653 Figure 5. Through-Hole PC Layout and Component Placement Diagram for Standard Step-Down Application (Top-Side View) |
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