Datasheet MAX1836, MAX1837 (Maxim) - 10

link to page 9 link to page 9 link to page 9 MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters The inductor’s saturation current rating must be greater capacitor selection, but final values should be set by test- than the peak switching current, which is determined ing a prototype or evaluation circuit. As a general rule, a by the switch current limit plus the overshoot due to the smaller amount of charge delivered in each pulse results 300ns current-sense comparator propagation delay: in less output ripple. Since the amount of charge deliv- ered in each oscillator pulse is determined by the inductor ( IVN − VOUT)300ns value and input voltage, the voltage ripple increases with IPEAK = ILIM + L larger inductance but decreases with lower input voltages. With low-cost aluminum electrolytic capacitors, the ESR- where the switch current-limit (ILIM) is typically 312mA induced ripple can be larger than that caused by the (MAX1836) or 625mA (MAX1837). Saturation occurs current into and out of the capacitor. Consequently, high- when the inductor’s magnetic flux density reaches the quality low-ESR aluminum-electrolytic, tantalum, polymer, maximum level the core can support, and the inductance or ceramic filter capacitors are required to minimize out- starts to fall. put ripple. Best results at reasonable cost are typically Inductor series resistance affects both efficiency and achieved with an aluminum-electrolytic capacitor in the dropout voltage. See the Input-Output (Dropout) Voltage 100μF range, in parallel with a 0.1μF ceramic capacitor. section. High series resistance limits the maximum current available at lower input voltages and increases the drop-
Input Capacitor
out voltage. For optimum performance, select an inductor The input filter capacitor reduces peak currents drawn with the lowest possible DC resistance that fits in the from the power source and reduces noise and voltage allotted dimensions. Typically, the inductor’s series resis- ripple on the input caused by the circuit’s switching. The tance should be significantly less than that of the internal input capacitor must meet the ripple-current requirement P-channel MOSFET’s on-resistance (1.1Ω typ). Inductors (IRMS) imposed by the switching currents defined by the with a ferrite core, or equivalent, are recommended. following equation: The maximum output current of the MAX1836/MAX1837 current-limited converter is limited by the peak inductor VOUT( I V N − VOUT) I = I current. For the typical application, the maximum output RMS LOAD I V N current is approximately: For most applications, nontantalum chemistries (ceramic, I aluminum, polymer, or OS-CON) are preferred due to OUT(MAX) IPEAK their robustness with high inrush currents typical of sys- tems with low-impedance battery inputs. Alternatively,
Output Capacitor
two (or more) smaller-value low-ESR capacitors can be Choose the output capacitor to supply the maximum load connected in parallel for lower cost. Choose an input current with acceptable voltage ripple. The output ripple capacitor that exhibits < +10°C temperature rise at the has two components: variations in the charge stored in RMS input current for optimal circuit longevity. the output capacitor with each LX pulse, and the voltage
Diode Selection
drop across the capacitor’s equivalent series resistance The current in the external diode (D1) changes abruptly (ESR) caused by the current into and out of the capacitor: from zero to its peak value each time the LX switch turns off. To avoid excessive losses, the diode must have a R V IPPLE ≈ R V IPPLE(ESR) + R V IPPLE(C) fast turn-on time and a low forward voltage. Use a diode The output voltage ripple as a consequence of the ESR with an RMS current rating of 0.5A or greater, and with a and output capacitance is: breakdown voltage > VIN. Schottky diodes are preferred. For high-temperature applications, Schottky diodes may R V IPPLE(ESR) = IPEAKESR be inadequate due to their high leakage currents. In such cases, ultra-high-speed silicon rectifiers are recom- L(I − I )2 PEAK OUT  I V N  mended, although a Schottky diode with a higher reverse R V IPPLE(C) =   2COUT VOUT I V N −  VOUT  voltage rating can often provide acceptable performance. where IPEAK is the peak inductor current. See the Inductor Selection section. These equations are suitable for initial www.maximintegrated.com Maxim Integrated │ 10