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LM2852YMXA-2.5 データシート(PDF) 9 Page - National Semiconductor (TI) |
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LM2852YMXA-2.5 データシート(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Applications Information (Continued) SOFT-START CAPACITOR (C SS) AND FAULT CONDITIONS Various fault conditions such as short circuit and UVLO of the LM2852 activate internal circuitry designed to control the voltage on the soft-start capacitor. For example, during a short circuit current limit event, the output voltage typically falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that once the short is re- moved, the LM2852 can restart gracefully from whatever voltage the output reached during the short circuit event. The range of soft-start capacitors is therefore restricted to values 1nF to 50nF. COMPENSATION The LM2852 provides a highly integrated solution to power supply design. The compensation of the LM2852, which is type three, is included on-chip. The benefit to integrated compensation is straight-forward, simple power supply de- sign. Since the output filter capacitor and inductor values impact the compensation of the control loop, the range of L, C and C ESR values is restricted in order to ensure stability. OUTPUT FILTER VALUES Table 1 details the recommended inductor and capacitor ranges for the LM2852 that are suggested for various typical output voltages. Values slightly different than those recom- mended may be used, however the phase margin of the power supply may be degraded. TABLE 1. Output Filter Values Frequency Option V OUT (V) PVIN (V) L (µH) C (µF) C ESR (m Ω) Min Max Min Max Min Max LM2852Y (500kHz) 0.8 3.3 10 15 100 220 70 200 0.8 5 10 15 100 120 70 200 1 3.3 10 15 100 180 70 200 1 5 10 15 100 180 70 200 1.2 3.3 10 15 100 180 70 200 1.2 5 15 22 100 120 70 200 1.5 3.3 10 15 100 120 70 200 1.5 5 22 22 100 120 70 200 1.8 3.3 10 15 100 120 100 200 1.8 5 22 33 100 120 100 200 2.5 3.3 6.8 10 68 120 95 275 2.5 5 15 22 68 120 95 275 3.3 5 15 22 68 100 100 275 CHOOSING AN INDUCTANCE VALUE The current ripple present in the output filter inductor is determined by the input voltage, output voltage, switching frequency and inductance according to the following equa- tion: where ∆I L is the peak to peak current ripple, D is the duty cycle V OUT/VIN,VIN is the input voltage applied to the output stage, V OUT is the output voltage of the switcher, f is the switching frequency and L is the inductance of the output filter inductor. Knowing the current ripple is important for inductor selection since the peak current through the induc- tor is the load current plus one half the ripple current. Care must be taken to ensure the peak inductor current does not reach a level high enough to trip the current limit circuitry of the LM2852. As an example, consider a 5V to 1.2V conversion and a 500kHz switching frequency. According to Table 1, a 15µH inductor may be used. Calculating the expected peak-to- peak ripple, The maximum inductor current for a 2A load would therefore be 2A plus 60.8 mA, 2.0608A. As shown in the ripple equa- tion, the current ripple is inversely proportional to induc- tance. OUTPUT FILTER INDUCTORS Once the inductance value is chosen, the key parameter for selecting the output filter inductor is its saturation current (I sat) specification. Typically Isat is given by the manufacturer as the current at which the inductance of the coil falls to a certain percentage of the nominal inductance. The I sat of an inductor used in an application should be greater than the maximum expected inductor current to avoid saturation. Be- low is a table of inductors that may be suitable in LM2852 applications. www.national.com 9 |
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同様の説明 - LM2852YMXA-2.5 |
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