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LM2744 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2744 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 22 page Application Information (Continued) TRACKING A VOLTAGE LEVEL The LM2744 can track the output of a master power supply during soft-start by connecting a resistor divider to the SS/ TRACK pin. In this way, the output voltage slew rate of the LM2744 will be controlled by the master supply for loads that require precise sequencing. Because the output of the mas- ter supply is divided down, in order to track properly the output voltage of the LM2744 must be lower than the voltage of the master supply. When the tracking function is used no soft-start capacitor should be connected to the SS/TRACK pin. However in all other cases, a C SS value of at least 1nF between the soft-start pin and ground should be used. One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output voltage (V OUT1) and the LM2744’s output voltage (represented sym- bolically in Figure 1 as V OUT2, i.e. without explicitly showing the power components) both rise together and reach their 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 3-4 mA for precise tracking. The final voltage of the SS/TRACK pin should be set slightly higher than the reference voltage (say about 50mV higher as in the above equation). If the master supply voltage was 5V and the LM2744 output voltage was 1.8V, for example, then the value of R T1 needed to give the two supplies identical soft-start times would be 150 Ω if V REF was set to 0.6V. A timing diagram for the equal soft-start time case is shown in Figure 2. 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 LM2744 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 267 Ω. A timing diagram for the case of equal slew rates is shown in Figure 3. SEQUENCING The start up/soft-start of the LM2744 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 4. 20106007 FIGURE 1. Tracking Circuit 20106008 FIGURE 2. Tracking with Equal Soft-Start Time 20106010 FIGURE 3. Tracking with Equal Slew Rates www.national.com 10 |
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