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MAX856 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX856 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page 3.3V/5V or Adjustable-Output, Step-Up DC-DC Converters _______________________________________________________________________________________ 9 Low-Battery Detection The MAX856 series contains an on-chip comparator for low-battery detection. If the voltage at LBI falls below the regulator’s internal reference voltage (1.25V), LBO (an open-drain output) sinks current to GND. The low- battery monitor’s threshold is set by two resistors, R3 and R4 (Figure 2). Set the threshold voltage using the following equation: VLBI R3 = R4 ( ______ - 1) VREF where VLBI is the desired threshold of the low-battery detector and VREF is the internal 1.25V reference. Since the LBI current is less than 100nA, large resistor values (typically 10k Ω to 300kΩ) can be used for R3 and R4 to minimize loading of the input supply. When the voltage at LBI is below the internal threshold, LBO sinks current to GND. Connect a pull-up resistor of 10k Ω or more from LBO to OUT when driving CMOS circuits. When LBI is above the threshold, the LBO out- put is off. If the low-battery comparator is not used, connect LBI to VIN and leave LBO open. Inductor Selection An inductor value of 47µH performs well in most MAX856–MAX859 applications. However, the inductance value is not critical, and the MAX856–MAX859 will work with inductors in the 10µH to 100µH range. Smaller inductance values typically offer a smaller physical size for a given series resistance, allowing the smallest overall circuit dimensions. However, due to higher peak inductor currents, the output voltage ripple (IPEAK x output filter capacitor ESR) also tends to be higher. Circuits using larger inductance values exhibit higher output current capability and larger physical dimen- sions for a given series resistance. The inductor’s incremental saturation current rating should be greater than the peak switch-current limit, which is 500mA for the MAX856/MAX857, and 125mA for the MAX858/MAX859. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficiency. The inductor’s DC resistance significantly affects effi- ciency. See the Efficiency vs. Load Current for Various Inductors graph in the Typical Operating Characteristics. See Tables 1 and 2 for a list of suggested inductor suppliers. Capacitor Selection A 68µF, 10V, 0.85 Ω, surface-mount tantalum (SMT) output filter capacitor typically provides 50mV output ripple when stepping up from 2V to 5V at 100mA (MAX856/ MAX857). Smaller capacitors (down to 10µF with higher ESRs) are acceptable for light loads or in applications that can tolerate higher output ripple. Values in the 10µF to 47µF range are recommended for the MAX858/MAX859. The equivalent series resistance (ESR) of both bypass and filter capacitors affects efficiency and output rip- ple. The output voltage ripple is the product of the peak inductor current and the output capacitor’s ESR. Use low-ESR capacitors for best performance, or connect two or more filter capacitors in parallel. Low-ESR, SMT tantalum capacitors are currently available from Sprague (595D series) and AVX (TPS series). Sanyo OS-CON organic-semiconductor through-hole capaci- tors also exhibit very low ESR, and are especially useful for operation at cold temperatures. See Table 1 for a list of suggested capacitor suppliers. Rectifier Diode For optimum performance, a switching Schottky diode (such as the 1N5817) is recommended. Refer to Table 1 for a list of component suppliers. For low output power applications, a PN-junction switching diode (such as the 1N4148) will also work well, although its greater forward voltage drop will reduce efficiency. PC Layout and Grounding The MAX856 series’ high-frequency operation makes PC layout important for minimizing ground bounce and noise. Keep the IC’s GND pin and the ground leads of C1 and C2 (Figure 1) less than 0.2in (5mm) apart. Also keep all connections to the FB and LX pins as short as possible. To maximize output power and efficiency and minimize output ripple voltage, use a ground plane and solder the IC’s GND (pin 7) directly to the ground plane. |
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