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MAX779L Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX779L Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Output Voltage Selection The output voltage of the MAX777L is fixed at 5V. The MAX778L output voltage can be set to 3V by leaving the SEL pin open. Connect SEL to AGND for 3.3V operation. The MAX779L’s output voltage is set by two resistors, R1 and R2 (Figure 3), which form a voltage divider between the output and the FB pin. The output voltage can be set from 2.5V to 6.0V by the equation: VOUT = (0.2025) [(R1 + R2)/R2] To simplify the resistor selection: R1 = (R2)[(VOUT/0.2025) - 1] Since the input current at FB is 40nA maximum, large val- ues (10k Ω to 50kΩ for R2) can be used with no significant loss of accuracy. For 1% error, the current through R2 should be at least 100 times FB’s bias current. When large values are used for the feedback resistors (R1 > 50k Ω), stray output impedance at FB can add “lag” to the feedback response, destabilizing the regula- tor and creating a larger ripple at the output. Lead lengths and circuit board traces at the FB node should be kept short. Reduce ripple by adding a “lead” compensa- tion capacitor (C3, 100pF to 50nF) in parallel with R1. __________Applications Information The Typical Operating Circuit shows a MAX777L step- up application circuit. This circuit starts up and oper- ates with inputs ranging from 1.0V to 4.5V. Start-up time is a function of the load, typically less than 5ms. Output current capability is a function of the input voltage. See Typical Operating Characteristics. Inductor Selection The 22µH inductor shown in the Typical Operating Circuit is sufficient for most MAX777L/MAX778L/ MAX779L designs. Other inductor values ranging from 10µH to 47µH are also suitable. The inductor should have a saturation rating equal to or greater than the peak switch-current limit, which is 1A without an exter- nal current limit (ILIM connected to IN). It is acceptable to operate the inductor at 120% of its saturation rating; however, this will reduce efficiency. For highest effi- ciency, use an inductor with a low DC resistance, preferably under 0.2 Ω. Table 1 lists suggested inductor suppliers. Capacitor Selection The 100µF, 10V surface-mount tantalum (SMT) output capacitor shown in the Typical Operating Circuit will provide a 20mV output ripple or less, stepping up from 2V to 3.3V at 200mA. Smaller capacitors, down to 10µF, are acceptable for light loads or in applications that tol- erate higher output ripple. The input capacitor may be omitted if the input lead length is less than 2 inches (5cm) or if the loads are small. The primary factor in selecting both the output and input filter capacitor is low ESR. The ESR of both bypass and filter capacitors affects efficiency. Optimize performance by increasing filter capacitors or using specialized low-ESR capacitors. The smallest low-ESR SMT tantalum capacitors currently available are Sprague 595D or 695D series. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit very low ESR, are rated for the wide temperature range, and are particularly useful for operation at cold temper- atures. Table 1 lists suggested capacitor suppliers. Layout The MAX777L/MAX778L/MAX779L’s high peak currents and high-frequency operation make PC layout impor- tant for minimum ground bounce and noise. Locate input bypass and output filter capacitors close to the device pins. All connections to the FB pin (MAX779L) should also be kept as short as possible. A ground plane is recommended. Solder AGND (pin 3) and PGND (pin 4), directly to the ground plane. Refer to the MAX777L/MAX778L/MAX779L evaluation kit (EV kit) manual for a suggested surface-mount layout. Low-Voltage Input, 3V/3.3V/5V/ Adjustable Output, Step-Up DC-DC Converters _______________________________________________________________________________________ 9 SHDN PGND MAX779L OUT AGND 7 1 3 4 8 6 LI 22 µH 5 LX FB 2 IN ILIM VIN C1 22µF R1 C3 C2 100µF VOUT R2 Figure 3. MAX779L Adjustable Voltage |
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