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LM2853MH-1.2 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2853MH-1.2 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Applications Information (Continued) Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2853 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In this example, supposea3ms start time is desired. Three time constants are required for charging the soft-start capacitor to 95% of the final reference voltage. So in this case RC = 1 ms. The DAC resistor, R, is 450 k Ω so C can be calculated to be 2.2 nF. A 2.2 nF ceramic capacitor can be chosen to yield approximatelya3ms start-up time. SOFT-START CAPACITOR (C SS) AND FAULT CONDITIONS Various fault conditions such as short circuit and UVLO of the LM2853 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 LM2853 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 1nFto50nF. COMPENSATION The LM2853 provides a highly integrated solution to power supply design. The compensation of the LM2853, which is type-three, is included on-chip. The benefit of 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 O, C O and CESR values is restricted in order to ensure stability. OUTPUT FILTER VALUES Table 1 details the recommended inductor and capacitor ranges for the LM2853 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. For best performance when output voltage ripple is a concern, ESR values near the minimum of the recommended range should be paired with capacitance values near the maximum. If a minimum output voltage ripple solution from a 5V input voltage is desired, a 6.8 µH inductor can be paired with a 220 µF (50 m Ω) capacitor without degraded phase margin. TABLE 1. Recommended L O and CO Values V OUT (V) VIN (V) L O (µH) C O (µF) C ESR (m Ω) Min Max Min Max Min Max 0.8 5 4.7 6.8 120 220 70 100 0.8 3.3 4.7 4.7 150 220 50 100 1 5 4.7 6.8 120 220 70 100 1 3.3 4.7 4.7 150 220 50 100 1.2 5 4.7 6.8 120 220 70 100 1.2 3.3 4.7 4.7 120 220 60 100 1.5 5 4.7 6.8 120 220 70 100 1.5 3.3 4.7 4.7 120 220 60 100 1.8 5 4.7 6.8 120 220 70 120 1.8 3.3 4.7 4.7 100 220 70 120 2.5 5 4.7 6.8 120 220 70 150 2.5 3.3 4.7 4.7 100 220 80 150 3.0 5 4.7 6.8 120 220 70 150 3.0 3.3 4.7 4.7 100 220 80 150 3.3 5 4.7 6.8 120 220 70 150 www.national.com 8 |
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