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LTC2801 数据表(PDF) 18 Page - Analog Devices |
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LTC2801 数据表(HTML) 18 Page - Analog Devices |
18 / 32 page LTC2873 18 REV B For more information www.analog.com APPLICATIONS INFORMATION Powering Multiple Devices Multiple LTC2873 devices can be powered using the boost regulator from only one of the devices, requiring only one inductor (L1) and charge pump cap (C1). Since the RS232 drivers provide the primary load to the circuit, the following guidelines apply: 1. No more than four RS232 drivers can be supplied from a single device. 2. If more than two RS232 drivers are being supplied from a single device, then the inductor, L1, must be increased to 22µH and the charge pump cap, C1, must be increased to 470nH, and VDD and VEE bypass caps must be increased to 2.2µF. 3. Ground the SW pin on devices with inactive boost converters. 4. Connect CAP pins together for all devices. 5. Connect VEE pins together for all devices. 6. Connect VDD pins together for all devices. Figure32showsanexampleofhowtoconnectfourdevices. Inductor Selection A 10µH or 22μH (±20%) inductor with a saturation current (ISAT) rating of at least 220mA and a DCR (copper wire resistance) of less than 1.3Ω is required. Some very small inductors meeting these requirements are listed in Table 1. Capacitor Selection The small size of ceramic capacitors makes them ideal for the LTC2873. Use X5R or X7R dielectric types; their ESR is low and they retain their capacitance over relatively wide voltage and temperature ranges. Use a voltage rating of at least 10V. Running with External VDD and VEE Supplies The inductor and charge pump cap, C1, can be omitted only if VDDandVEEareexternallysupplied.Bypasscapson VDD and VEE must remain in place. In this circumstance, ground the SW pin and float the CAP pin. External supplies must not exceed the absolute maximum levels of ±7.5V. Ideal supply levels are 7.2V and –6.5V as these are each just wider than the regulation points of 7.0V and –6.3V so the internal feedback is satisfied and the switching stops. Lower voltages can be used even at –6V and +6V but the internal boost regulator will be switching. This may cause some switching noise but will not harm the part. VDD and VEE supplies must be present for proper operation in RS232 mode and in RS485 mode when the termination is enabled. It is okay to run the LTC2873 in RS485 mode with internal termination disabled (TE485 high), when VDD and VEE are not present or fully settled. Inrush Current and Supply Overshoot Precaution Incertainapplications,fastsupplyslewratesaregenerated when power is connected. If the VCC voltage is greater than 4.5V and its rise time is faster than 10µs, the pins VDD and SW can exceed their absolute maximum values during start-up. When supply voltage is applied to VCC,the voltage difference between VCC and VDD generates inrush currentflowingthroughinductorL1andcapacitorsC1and C2. The peak inrush current must not exceed 2A. To avoid this condition, add a 1Ω resistor as shown in Figure 17. This precaution is not relevant for supply voltages below 4.5V or rise times longer than 10µs. Table 1. Recommended Inductors PART NUMBER L (μH) ISAT (mA) MAX DCR (Ω) SIZE (mm) MANUFACTURER 74479888310 10 250 0.5 2.5 × 2 × 1 Wurth Elektronik CBC2016T100K (or M) 10 380 1.07 2 × 1.6 × 1.6 Taiyo Yuden www.t-yuden.com CBC2518T220K (or M) 22 320 1.0 2.5 × 1.8 × 1.8 BRC2016T220K (or M) 22 310 1.3 2 × 1.6 × 1.6 LQH32CN220K53 22 250 0.92 3.2 × 2.5 × 1.6 Murata www.murata.com |
类似零件编号 - LTC2801 |
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类似说明 - LTC2801 |
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