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MAX1837EUT50 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1837EUT50 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 13 page 24V Internal Switch, 100% Duty Cycle, Step-Down Converters 10 ______________________________________________________________________________________ The inductor’s saturation current rating must be greater than the peak switching current, which is determined by the switch current limit plus the overshoot due to the 300ns current-sense comparator propagation delay: where the switch current-limit (ILIM) is typically 312mA (MAX1836) or 625mA (MAX1837). Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support, and the induc- tance starts to fall. Inductor series resistance affects both efficiency and dropout voltage (see the Input-Output Voltage section). High series resistance limits the maximum current avail- able at lower input voltages and increases the dropout voltage. For optimum performance, select an inductor with the lowest possible DC resistance that fits in the allotted dimensions. Typically, the inductor’s series resistance should be significantly less than that of the internal P-channel MOSFET’s on-resistance (1.1 Ω typ). Inductors with a ferrite core, or equivalent, are recom- mended. The maximum output current of the MAX1836/MAX1837 current-limited converter is limited by the peak inductor current. For the typical application, the maximum out- put current is approximately: Output Capacitor Choose the output capacitor to supply the maximum load current with acceptable voltage ripple. The output ripple has two components: variations in the charge stored in the output capacitor with each LX pulse, and the voltage drop across the capacitor’s equivalent series resistance (ESR) caused by the current into and out of the capacitor: The output voltage ripple as a consequence of the ESR and output capacitance is: where IPEAK is the peak inductor current (see the Inductor Selection section). These equations are suit- able for initial capacitor selection, but final values should be set by testing a prototype or evaluation cir- cuit. As a general rule, a smaller amount of charge delivered in each pulse results in less output ripple. Since the amount of charge delivered in each oscillator pulse is determined by the inductor value and input voltage, the voltage ripple increases with larger induc- tance but decreases with lower input voltages. With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the current into and out of the capacitor. Consequently, high-quality low-ESR aluminum-electrolytic, tantalum, polymer, or ceramic filter capacitors are required to minimize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capacitor in the 100µF range, in parallel with a 0.1µF ceramic capacitor. Input Capacitor The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor must meet the ripple-current requirement (IRMS) imposed by the switching currents defined by the following equation: For most applications, nontantalum chemistries (ceram- ic, aluminum, polymer, or OS-CON) are preferred due to their robustness with high inrush currents typical of systems with low-impedance battery inputs. Alternatively, two (or more) smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose an input capacitor that exhibits <+10°C tem- perature rise at the RMS input current for optimal circuit longevity. Diode Selection The current in the external diode (D1) changes abruptly from zero to its peak value each time the LX switch turns off. To avoid excessive losses, the diode must have a fast turn-on time and a low forward voltage. Use a diode with an RMS current rating of 0.5A or greater, and with a breakdown voltage >VIN. Schottky diodes are preferred. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-high-speed sili- con rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance. II VV - V V RMS LOAD OUT IN OUT IN = () V ESR V -I 2C V V V- V RIPPLE(ESR) PEAK RIPPLE(C) PEAK OUT OUT OUT IN IN OUT 2 = = () I LI VV V RIPPLE RIPPLE(ESR) RIPPLE(C) ≈+ II OUT(MAX) PEAK = 1 2 I (V - V ) PEAK LIM IN OUT =+ I ns L 300 |
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