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LM2704 Datasheet(PDF) 9 Page - Texas Instruments |
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LM2704 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 19 page LM2704 www.ti.com SNVS175D – FEBRUARY 2002 – REVISED MAY 2013 turns off the NMOS switch. The SW voltage will then rise to the output voltage plus a diode drop and the inductor current will begin to decrease as shown in Figure 3. During this time the energy stored in the inductor is transferred to COUT and the load. After the 400ns off-time the NMOS switch is turned on and energy is stored in the inductor again. This energy transfer from the inductor to the output causes a stepping effect in the output ripple as shown in Figure 3. This cycle is continued until the voltage at FB reaches 1.237V. When FB reaches this voltage, the enable comparator then disables the device turning off the NMOS switch and reducing the Iq of the device to 40uA. The load current is then supplied solely by COUT indicated by the gradually decreasing slope at the output as shown in Figure 3. When the FB pin drops slightly below 1.237V, the enable comparator enables the device and begins the cycle described previously. The SHDN pin can be used to turn off the LM2704 and reduce the Iq to 0.01µA. In shutdown mode the output voltage will be a diode drop lower than the input voltage. APPLICATION INFORMATION INDUCTOR SELECTION The appropriate inductor for a given application is calculated using the following equation: (2) where VD is the schottky diode voltage, ICL is the switch current limit found in the Typical Performance Characteristics section, and TOFF is the switch off time. When using this equation be sure to use the minimum input voltage for the application, such as for battery powered applications. For the LM2704 constant-off time control scheme, the NMOS power switch is turned off when the current limit is reached. There is approximately a 200ns delay from the time the current limit is reached in the NMOS power switch and when the internal logic actually turns off the switch. During this 200ns delay, the peak inductor current will increase. This increase in inductor current demands a larger saturation current rating for the inductor. This saturation current can be approximated by the following equation: (3) Choosing inductors with low ESR decrease power losses and increase efficiency. Care should be taken when choosing an inductor. For applications that require an input voltage that approaches the output voltage, such as when converting a Li-Ion battery voltage to 5V, the 400ns off time may not be enough time to discharge the energy in the inductor and transfer the energy to the output capacitor and load. This can cause a ramping effect in the inductor current waveform and an increased ripple on the output voltage. Using a smaller inductor will cause the IPK to increase and will increase the output voltage ripple further. This can be solved by adding a 4.7pF capacitor across the RF1 feedback resistor (Figure 2) and slightly increasing the output capacitor. A smaller inductor can then be used to ensure proper discharge in the 400ns off time. DIODE SELECTION To maintain high efficiency, the average current rating of the schottky diode should be larger than the peak inductor current, IPK. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown of the schottky diode larger than the output voltage. CAPACITOR SELECTION Choose low ESR capacitors for the output to minimize output voltage ripple. Multilayer ceramic capacitors are the best choice. For most applications, a 1µF ceramic capacitor is sufficient. For some applications a reduction in output voltage ripple can be achieved by increasing the output capacitor. Local bypassing for the input is needed on the LM2704. Multilayer ceramic capacitors are a good choice for this as well. A 4.7µF capacitor is sufficient for most applications. For additional bypassing, a 100nF ceramic capacitor can be used to shunt high frequency ripple on the input. Copyright © 2002–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2704 |
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