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LT1533C Datasheet(PDF) 9 Page - Linear Technology |
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LT1533C Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LT1533 slower slewing waveforms will dissipate more power so that efficiency will drop. You can also monitor this as you make your slew adjustment by measuring input and out- put voltage and current. It is possible to use a single slew setting resistor. In this case the RVSL and RCSL pins are tied together. A resistor with a value of 2k to 34k (one half the individual resistors) can then be tied from these pins to ground. Emitter Inductance A small inductance in the power ground minimizes a potential dip in the output current falling edge that can occur under fast slewing, 25nH is usually sufficient. Greater than 50nH may produce unwanted oscillations in the voltage output. The inductance can be created by wire or board trace with the equivalent of one inch of straight length. A spiral board trace will require less length. Positive Output Voltage Setting Sensing of a positive output voltage is usually done using a resistor divider from the output to the FB pin. The positive input to the error amp is connected internally to a 1.25V bandgap reference. The FB pin will regulate to this voltage. internal regulator voltage (2.4V typ), output regulation may be disrupted. A series resistance with the feedback pin can eliminate this potential problem. Negative Output Voltage Setting Negative output voltage can be sensed using the NFB pin. In this case regulation will occur when the NFB pin is at – 2.5V. The input bias current for the NFB is –25 µA (INFB) which needs to be accounted for in setting up the divider. Referring to Figure 2, R1 is chosen such that: RR V RA OUT 12 25 25 2 25 = − + . .• µ APPLICATIONS INFORMATION FB PIN 1533 F01 VOUT R2 R1 Referring to Figure 1, R1 is determined by: RR VOUT 12 125 1 =− . The FB bias current represents a small error and can usually be ignored for values of R1 ||R2 up to 10k. One word of caution. Sometimes a feedback zero is added to the control loop by placing a capacitor across R1 above. If the feedback capacitively pulls the FB pin above the Figure 1 A suggested value for R2 is 2.5k. The NFB pin is normally left open if the FB pin is being used. Dual Polarity Output Voltage Sensing Certain applications may benefit from sensing both posi- tive and negative output voltages. When doing this each output voltage resistor divider is individually set as previ- ously described. When both FB and NFB pins are used, the LT1533 will act to prevent either output from going beyond its set output voltage. The highest output (lightest load) will dominate control of the regulator. This technique would prevent either output from going unregulated high at no load. However, this technique will also compromise output load regulation. Shutdown If the shutdown pin is pulled low, the regulator will turn off. The supply current will be reduced to less than 20 µA. NFB PIN INFB 1533 F02 –VOUT R2 R1 Figure 2 |
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