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LT3507EUHF-PBF 数据表(PDF) 13 Page - Linear Technology |
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LT3507EUHF-PBF 数据表(HTML) 13 Page - Linear Technology |
13 / 28 page LT3507 13 3507f Electrolytic capacitors are also an option. The ESRs of most aluminum electrolytic capacitors are too large to deliver low output ripple. Tantalum, as well as newer, lower-ESR organic electrolytic capacitors intended for power supply use are suitable. Chose a capacitor with a low enough ESR for the required output ripple. Because the volume of the capacitor determines its ESR, both the size and the value will be larger than a ceramic capacitor that would give similar ripple performance. One benefit is that the larger capacitance may give better transient response for large changes in load current. Table 2 lists several capacitor vendors. Table 2. Low ESR Surface Mount Capacitors VENDOR TYPE SERIES Taiyo-Yuden Ceramic AVX Ceramic Tantalum TPS Kemet Tantalum Tantalum Organic Aluminum Organic T491,T494,T495 T520 A700 Sanyo Tantalum or Aluminum Organic POSCAP Panasonic Aluminum Organic SP CAP TDK Ceramic Diode Selection The catch diode (D1 from Figure 2) conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT VIN –VOUT () VIN The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a diode with a reverse voltage rating greater than the input voltage. The programmable OVLO can protect the diode from excessive reverse voltage by shutting down the regulator if the input voltage exceeds the maximum rating of the diode. Table 3 lists several Schottky diodes and their manufacturers. Table 3. Schottky Diodes PART NUMBER VR (V) IAVE (A) VF AT 1A (mV) VF AT 2A (mV) On Semiconductor MBRM120E 20 1 530 595 MBRM140 40 1 550 Diodes Inc B120 20 1 500 B140 40 1 500 B220 20 2 500 B240 40 2 500 DFLS140L 40 1 550 DFLS240L 40 2 550 Boost Pin Considerations The capacitor and diode tied to the BOOST pin generate a voltage that is higher than the input voltage. In most cases, a small ceramic capacitor and fast switching diode (such as the CMDSH-3 or MMSD914LT1) will work well. The capacitor value is a function of the switching frequency, peak current, duty cycle and boost voltage; in general a value of (0.1μF • 1MHz/fSW) works well. Figure 3 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for full ef- ficiency. For outputs of 3.3V and higher, the standard circuit (Figure 3a) is best. For outputs between 2.8V and 3.3V, use a small Schottky diode (such as the BAT54). For lower output voltages, the boost diode can be tied to the input (Figure 3b). The circuit in Figure 3a is more efficient because the BOOST pin current comes from a lower voltage source. Finally, as shown in Figure 3c, the anode of the boost diode can be tied to another source that is at least 3V. For example, if you are generating 3.3V and 1.8V and the 3.3V is on whenever the 1.8V is on, the 1.8V boost diode can be connected to the 3.3V output. In this case, the 3.3V output cannot be set to track the 1.8V output (see Output Voltage Tracking). In any case, be sure that the maximum voltage at the BOOST pin is less than 55V and the voltage difference between the BOOST and SW pins is less than 25V. The minimum operating voltage of an LT3507 applica- tion is limited by the internal undervoltage lockout (4V for Channel 1, 3V for Channels 2 and 3) and by the APPLICATIONS INFORMATION |
类似零件编号 - LT3507EUHF-PBF |
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类似说明 - LT3507EUHF-PBF |
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