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LM3485 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3485 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Functional Description (Continued) Start Up The current limit circuit is active during start-up. During start-up the PFET will stay on until either the current limit or the feedback comparator is tripped If the current limit comparator is tripped first then the fold back characteristic should be taken into account. Start-up into full load may require a higher current limit set point or the load must be applied after start-up. One problem with selecting a higher current limit is inrush current during start-up. Adding a capacitance (C ADJ) in par- allel with R ADJ results in soft-start. CADJ and RADJ create an RC time constant forcing current limit to activate at a lower current. The output voltage will ramp more slowly when using the soft-start functionality. The C ADJ also filters unwanted noise so that the ISENSE comparator will not be accidentally triggered. A value of 100pF to 1nF is recommended in most applications. These low values for C ADJ will have little to no effect on soft-start. There are example start-up plots for C ADJ equal to 1nF and 10nF in the Typical Performance Characteristics. External Sense Resistor The V DS of a PFET will tend to vary significantly over tem- perature. This will result an equivalent variation in current limit. To improve current limit accuracy an external sense resistor can be connected from V IN to the source of the PFET, as shown in Figure 5. 20034627 FIGURE 5. Current Sensing by External Resistor www.national.com 11 |
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