Datasheet MAX856, MAX857, MAX858, MAX859 (Maxim) - 9

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