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MAX685EEE Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX685EEE Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Applications Information Figure 3 shows the standard application circuit for the MAX685. The values shown in Table 1 will work well for output currents up to 10mA. However, this circuit can be optimized to a particular application by using differ- ent capacitors and a different inductor. Higher Output Voltages If the application requires output voltages greater than -7.5V or +24V, use the circuit of Figure 4. This circuit uses a charge pump to increase the output voltage without increasing the voltage stress on the LX_ pin. The maximum output voltages of the circuit in Figure 4 are -15V and +48V. The voltage rating on D2, D5, and D6 must be 30V or greater. For a larger negative output voltage without a larger positive output (or vice versa), use one-half of the Figure 4 circuit with one-half of the Figure 3 circuit. Inductor Selection A 22µH inductor is suitable for most applications. Larger inductances will reduce inductor ripple current and output voltage ripple, but they also typically require larger physical size if increased resistance and losses are not also allowed. Small inductors are typically preferred because of compact design and low cost. Murata LHQ and TDK NLC types are examples of small surface-mount inductors that work for most applications. Because these small-size inductors use thinner wire, they exhibit higher resistance and have greater losses than larger ones. If the application demands higher efficiency, use larger, lower resistance coils such as the Sumida CD43 or CD54, Coilcraft DT1608 or DO1608, or Coiltronics UP1V series. Filter Capacitor Selection The output ripple voltage is a function of the peak in- ductor current, frequency, and type and value of the output capacitors. Capacitors with low equivalent- series resistance (ESR) and large capacitance reduce output ripple. Typically, tantalum or ceramic capacitors are optimal. Tantalum capacitors have higher ESR and higher capacitance than ceramic capacitors. Therefore the ESR of tantalum capacitors determines the output ripple, because at the frequencies used the ESR domi- nates the impedance of the capacitor. If ceramic capacitors are used, the capacitance determines the output ripple. Dual-Output (Positive and Negative), DC-DC Converter for CCD and LCD 8 _______________________________________________________________________________________ MAX685 REF VP VDD LXP 1 11 3 8 9 13, 14 16 FBN 10 POK 4 6 7 12 POK SHDN SYNC C5 47pF R1 1.0M R5 100k D2 NBR0520 L1 22 µH D1 NBR0520 R2 90.9k +15V VOUT+ C4 2.2 µF C3 2.2 µF C2 0.22 µF C1 10 µF VIN R3 750k R4 124k -7.5V VOUT- SHDN PGND LXN GND SYNC FBP Figure 3. Standard Application Circuit MAX685 REF VP VDD LXP C6 1 µF 1 11 3 8 9 13, 14 16 C8 1 µF FBN 10 POK 4 6 7 12 POK SHDN SYNC C5 47pF R1 R5 100k D2 D5 D6 L1 22 µH D3 D4 D1 R2 VOUT+ C9 2.2 µF C7 2.2 µF C3 2.2 µF C2 0.22 µF C1 10 µF VIN R3 R4 VOUT- SHDN PGND LXN GND SYNC FBP C4 2.2 µF Figure 4. Circuit for Output Voltages < -9V and > +24V |
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