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LT1940L Datasheet(PDF) 11 Page - Linear Technology |
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LT1940L Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT1940/LT1940L 11 1940fa Table 2. Low-ESR Surface Mount Capacitors Vendor Type Series Taiyo Yuden Ceramic X5R, X7R AVX Ceramic X5R, X7R Tantalum TPS Kemet Tantalum T491,T494,T495 Ta Organic T520 Al Organic A700 Sanyo Ta or Al Organic POSCAP Panasonic Al Organic SP CAP TDK Ceramic X5R, X7R Catch Diode Use a 1A Schottky diode for the catch diode (D1 in Figure 2). The diode must have a reverse voltage rating greater than the maximum input voltage. The ON Semi- conductor MBRM120LT3 (20V) and MBRM130LT3 (30V) are good choices; they have a tiny package with good thermal properties. Many vendors have surface mount versions of the 1N5817 (20V) and 1N5818 (30V) 1A Schottky diodes such as the Microsemi UPS120 that are suitable. 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 0.1 µF capacitor and fast switching diode (such as the CMDSH-3 or FMMD914) will work 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 effi- ciency. 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 BAT-54). 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 any case, you must also be sure that the maximum voltage at the BOOST pin is less than the maximum specified in the Absolute Maximum Ratings section. Figure 3. Generating the Boost Voltage VIN BOOST GND SW VIN LT1940 (3a) D2 VOUT C3 VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT VIN BOOST GND SW VIN LT1940 (3b) D2 VOUT C3 VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2VIN VIN BOOST GND SW VIN LT1940 (3d) 1940 F03 VOUT MAX VBOOST – VSW ≅ VIN2 MAX VBOOST ≅ VIN2 MINIMUM VALUE FOR VIN2 = VIN + 3V VIN2 >VIN + 3V D2 VIN BOOST GND SW VIN LT1940 (3c) 1940 F03 VOUT VBOOST – VSW ≅ VIN2 MAX VBOOST ≅ VIN2 + VIN MINIMUM VALUE FOR VIN2 = 3V D2 VIN2 > 3V C3 APPLICATIO S I FOR ATIO |
Similar Part No. - LT1940L_15 |
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Similar Description - LT1940L_15 |
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