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LM1770TMF Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM1770TMF Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Application Information (Continued) on-time and expands the minimum off-time. In addition the part will only activate the PMOS allowing a discontinuous mode of operation enabling a pre-biased startup. The time spent in soft-start will depend on the load applied to the output, but is usually close to a set time that is dependent on the timing option. The approximate soft-start time can be seen below for each timing option. Product ID Timing T SS LM1770S 0.5µs 1ms LM1770T 1.0µs 1.2ms LM1770U 2.0µs 1.8ms It should be noted that as soon as soft-start terminates the short-circuit protection is enabled. This means that if the output voltage does not reach at least 68% of its final value the part will latch off. Therefore, if the input supply is ex- tremely slow rising such that at the end of soft-start the input voltage is still near the UVLO threshold, a timing option should be chosen to ensure that maximum duty-cycle per- mits the output to meet the minimum condition. As a general recommendation it is advisable to use the 2000ns option (LM1770U) in conditions where the output voltage is 2.5V or greater to avoid false latch offs when there is concern re- garding the input supply slew rate. JITTER The LM1770 utilizes a constant on-time control scheme that relies on the output voltage ripple to provide a consistent switching frequency. Under certain conditions, excessive noise can couple onto the feedback pin causing the switch node to appear to have a slight amount of jitter. This is not indicative of an unstable design. The output voltage will still regulate to the exact same value. Careful component selec- tion and layout should minimize any external influence. In addition to any external noise that can add to the jitter seen on the switch node, the LM1770 will always have a slight amount of switch jitter. This is because the LM1770 makes a small alteration in the reference voltage every 128 cycles to improve its accuracy and long term performance. This has the effect of causing a change in the switching frequency at that instant. When viewed on an oscilloscope this can be seen as a jitter in the switch node. The change in feedback voltage or output voltage, however, is almost indis- tinguishable. Design Guide The following section walks the designer through the steps necessary to select the external components to build a fully functional power supply. As with any DC-DC converter nu- merous trade-offs are possible to optimize the design for efficiency, size or performance. These will be taken into account and highlighted throughout this discussion. The first equation to calculate for any buck converter is duty-cycle. Ignoring conduction losses associated with the FETs and parasitic resistances it can be approximated by: A more accurate calculation for duty-cycle can be used that takes into account the voltage drops across the FETs. This equation can be used to determine the slight load depen- dency on switch frequency if needed. Otherwise the simpli- fied equation works well for component calculation. FREQUENCY SELECTION The LM1770 is available with three preset timing options that select the on-time and hence determine the switching fre- quency of the application. Increasing the switching fre- quency has the effect of reducing the inductor size needed for the application while requiring a slight trade-off in effi- ciency. The table below shows the same frequency table as shown earlier, with the exception that the recommended timing option for each V OUT is highlighted. It is not recom- mended to use a high switching frequency with V OUT equal to or greater than 2.5V due to the maximum duty-cycle limitations of the device coupled with the internal startup. V OUT Timing Options 500ns 1000ns 2000ns 0.8 485 242 - 1 606 303 - 1.2 727 364 - 1.5 909 455 227 1.8 - 545 273 2.5 - - 379 3.3 - - 500 Recommended switching frequency (kHz) based on output voltage and timing option. INDUCTOR SELECTION The inductor selection is an iterative process likely requiring several passes before settling on a final value. The reason for this is because it influences the amount of ripple seen at the output, a critical component to ensure general stability of an adaptive on-time circuit. For the first pass at inductor selection the value can be obtained by targeting a maximum peak-to-peak ripple current equal to 30% of the maximum load current. The inductor current ripple ( ∆I L) can be calcu- lated by: Therefore, L can be initially set to the following by applying the 30% rule: The other features of the inductor that can be selected besides inductance value are saturation current and core material. Because the LM1770 does not have a current limit, it is recommended to have a saturation current higher than the maximum output current to handle any ripple or momen- tary over-current events. The core material also influences the saturation characteristics as ferrite materials have a hard saturation curve and care should be taken such that they never saturate during normal use. A shielded inductor or low www.national.com 9 |
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