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MAX17067 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX17067 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 13 page Low-Noise Step-Up DC-DC Converter 10 ______________________________________________________________________________________ Diode Selection The output diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode’s peak current rating is at least IPK and that its breakdown voltage exceeds VOUT. Schottky diodes are recommended. Input and Output Capacitor Selection Low-ESR capacitors are recommended for input bypassing and output filtering. Low-ESR tantalum capacitors are a good compromise between cost and performance. Ceramic capacitors are also a good choice. Avoid standard aluminum electrolytic capaci- tors. A simple equation to estimate input and output- capacitor values for a given voltage ripple is as follows: where VRIPPLE is the peak-to-peak ripple voltage on the capacitor. Output Voltage The MAX17067 operates with an adjustable output from VIN to 20V. Connect a resistor voltage-divider to FB (see the Typical Operating Circuit) from the output to GND. Select the resistor values as follows: where VFB, the boost-regulator feedback set point, is 1.24V. Since the input bias current into FB is typically zero, R2 can have a value up to 100k Ω without sacrificing accuracy. Connect the resistor-divider as close to the IC as possible. Loop Compensation The voltage feedback loop needs proper compensation to prevent excessive output ripple and poor efficiency caused by instability. This is done by connecting a resis- tor (RCOMP) and capacitor (CCOMP) in series from COMP to GND, and another capacitor (CCOMP2) from COMP to GND. RCOMP is chosen to set the high-fre- quency integrator gain for fast-transient response, while CCOMP is chosen to set the integrator zero to maintain loop stability. The second capacitor, CCOMP2, is chosen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: RCOMP = (274Ω/A2 x VIN x VOUT x COUT/(L x IOUT) CCOMP ≅ (0.36 x 10-3 A/Ω) x L/VIN CCOMP2 ≅ (0.0036 A/Ω) x RESR x L x IOUT/(VIN x VOUT) For the ceramic output capacitor, where ESR is small, CCOMP2 is optional. Table 1 shows experimentally verified external component values for several applications. The best gauge of correct loop compensation is by inspecting the transient response of the MAX17067. Adjust RCOMP and CCOMP as necessary to obtain opti- mal transient performance. Soft-Start Capacitor The soft-start capacitor should be large enough that it does not reach final value before the output has reached regulation. Calculate CSS to be: where: COUT = total output capacitance including any bypass capacitor on the output bus VOUT = maximum output voltage IINRUSH = peak inrush current allowed IOUT = maximum output current during power-up stage VIN = minimum input voltage The load must wait for the soft-start cycle to finish before drawing a significant amount of load current. The duration after which the load can begin to draw maximum load current is: tMAX = 2.5 x 105 CSS C 21 10 C V V V V I I V SS 6 OUT IN OUT IN INRUSH OUT OUT OUT 2 >× × −× ×− × ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − RR V V OUT FB 12 1 =− ⎛ ⎝⎜ ⎞ ⎠⎟ C 0.5 L I V V PK 2 RIPPLE OUT ≥ ×× ⎛⎝ ⎞ ⎠ × |
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