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MAX8715 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX8715 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 14 page Low-Noise Step-Up DC-DC Converters ______________________________________________________________________________________ 11 ratio of inductor resistance to other power path resis- tances, the best LIR can shift up or down. If the induc- tor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. If extremely thin high-resistance inductors are used, as is common for LCD-panel applications, the best LIR can increase to between 0.5 and 1.0. Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. Calculate the approximate inductor value using the typ- ical input voltage (VIN), the maximum output current (IMAIN(MAX)), the expected efficiency ( ηTYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion: Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input cur- rent at the minimum input voltage VIN(MIN) using con- servation of energy and the expected efficiency at that operating point ( ηMIN) taken from an appropriate curve in the Typical Operating Characteristics: Calculate the ripple current at that operating point and the peak current required for the inductor: The inductor’s saturation current rating and the MAX1790/MAX8715s’ LX current limit (ILIM) should exceed IPEAK and the inductor’s DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1 Ω series resistance. Considering the application circuit in Figure 4, the maxi- mum load current (IMAIN(MAX)) is 150mA with a 9V output and a typical input voltage of 3.3V. Choosing an LIR of 0.5 and estimating efficiency of 85% at this operating point: Using the circuit’s minimum input voltage (3V) and esti- mating efficiency of 80% at that operating point: The ripple current and the peak current are: 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 MAX1790/MAX8715 operate with an adjustable output from VIN to 13V. 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 0, RR V V OUT FB 12 1 =− ⎛ ⎝⎜ ⎞ ⎠⎟ C 0.5 L I V V PK 2 RIPPLE OUT ≥ ×× ⎛⎝ ⎞ ⎠ × IA A A PEAK =+ ≈ 06 025 2 0 725 . . . I VV V H V MHz A RIPPLE = ×− ×× ≈ 39 3 68 9 1 2 025 () .. . μ I AV V A IN DC MAX (, ) . . . = × × ≈ 015 9 30 8 06 L V V VV A MHz H = ⎛ ⎝⎜ ⎞ ⎠⎟ − × ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ ≈ 33 9 93 3 015 1 2 085 05 68 2 .. .. . . . μ II I PEAK IN DC MAX RIPPLE =+ (, ) 2 I VV V LV f RIPPLE IN MIN MAIN IN MIN MAIN OSC = ×− ×× () () () I IV V IN DC MAX MAIN MAX MAIN IN MIN MIN (, ) () () = × ×η L V V VV I f LIR IN MAIN MAIN IN MAIN MAX OSC TYP = ⎛ ⎝⎜ ⎞ ⎠⎟ − × ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ 2 () η |
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