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LM2747MTC bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM2747MTC bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
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10 / 23 page  Application Information (Continued) target values at the same time. For this case, the equation governing the values of the tracking divider resistors R T1 and R T2 is: The current through R T1 should be about 4 mA for precise tracking. The final voltage of the SS/TRACK pin should be set higher than the feedback voltage of 0.6V (say about 0.65V as in the above equation). If the master supply voltage was 5V and the LM2747 output voltage was 1.8V, for ex- ample, then the value of R T1 needed to give the two supplies identical soft-start times would be 150 Ω. A timing diagram for the equal soft-start time case is shown in Figure 4. TRACKING A VOLTAGE SLEW RATE The tracking feature can alternatively be used not to make both rails reach regulation at the same time but rather to have similar rise rates (in terms of output dV/dt). This method ensures that the output voltage of the LM2747 al- ways reaches regulation before the output voltage of the master supply. In this case, the tracking resistors can be determined based on the following equation: For the example case of V OUT1 = 5V and VOUT2 = 1.8V, with R T1 set to 150 Ω as before, R T2 is calculated from the above equation to be 265 Ω. A timing diagram for the case of equal slew rates is shown in Figure 5. SEQUENCING The start up/soft-start of the LM2747 can be delayed for the purpose of sequencing by connecting a resistor divider from the output of a master power supply to the SD pin, as shown in Figure 6. A desired delay time t DELAY between the startup of the master supply output voltage and the LM2747 output voltage can be set based on the SD pin low-to-high threshold V SD-IH and the slew rate of the voltage at the SD pin, SR SD: t DELAY =VSD-IH /SRSD Note again, that in Figure 6, the LM2747’s output voltage has been represented symbolically as V OUT2, i.e. without explicitly showing the power components. V SD-IH is typically 1.08V and SRSD is the slew rate of the SD pin voltage. The values of the sequencing divider resistors R S1 and RS2 set the SRSD based on the master supply output voltage slew rate, SR OUT1, using the following equa- tion: 20150908 FIGURE 4. Tracking with Equal Soft-Start Time 20150910 FIGURE 5. Tracking with Equal Slew Rates 20150914 FIGURE 6. Sequencing Circuit www.national.com 10 |
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