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LT1933HS6-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT1933HS6-PBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LT1933 11 1933fd APPLICATIONS INFORMATION BOOST Pin Considerations Capacitor C3 and diode D2 are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.1μF capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 2 shows two ways to arrange the boost circuit. The BOOST pin must be at least 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 2a) is best. For outputs between 2.5V and 3V, use a 0.47μF capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages the boost diode can be tied to the input (Figure 2b). The circuit in Figure 2a is more ef- ficient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. The minimum operating voltage of an LT1933 application is limited by the undervoltage lockout (~3.35V) and by the maximum duty cycle as outlined above. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT1933 is turned on with its SHDN pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 3 shows a plot of minimum load to start and to run as a function of input voltage. In many Figure 2. Two Circuits for Generating the Boost Voltage Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit VIN BOOST GND SW VIN LT1933 (2a) D2 1933 F02a VOUT C3 VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT VIN BOOST GND SW VIN LT1933 (2b) D2 1933 F02b VOUT C3 VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2VIN LOAD CURRENT (mA) 4.5 5.0 5.5 6.0 1933 F03a 3.0 3.5 4.0 VOUT = 3.3V TA = 25°C L = 22μH TO START TO RUN 1 10 100 1933 F03b LOAD CURRENT (mA) 4.5 5.0 5.5 6.0 3.0 3.5 4.0 VOUT = 5V TA = 25°C L = 22μH TO START TO RUN 1 10 100 Minimum Input Voltage VOUT = 3.3V Minimum Input Voltage VOUT = 5V |
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