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LM2744 Datasheet(PDF) 11 Page - Texas Instruments |
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LM2744 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 33 page VOUT2 = VOUT1 RT1 RT1 + RT2 5V 1.8V VOUT1 VOUT2 VREF + 0.05 = RT1 RT1 + RT2 Master Power Supply VOUT1 = 5V SS/TRACK LM2744 RT2 1 k: RT1 150: RFB2 10 k: RFB1 5 k: FB VOUT2 = 1.8V VSS = 0.65V VFB LM2744 www.ti.com SNVS292F – SEPTEMBER 2004 – REVISED MARCH 2013 Figure 21. Tracking Circuit One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output voltage (VOUT1) and the LM2744’s output voltage (represented symbolically in Figure 21 as VOUT2, 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 RT1 and RT2 is: (4) The current through RT1 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 RT1 needed to give the two supplies identical soft-start times would be 150 Ω if VREF was set to 0.6V. A timing diagram for the equal soft-start time case is shown in Figure 22. Figure 22. Tracking with Equal Soft-Start Time 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 always reaches regulation before the output voltage of the master supply. In this case, the tracking resistors can be determined based on the following equation: (5) For the example case of VOUT1 = 5V and VOUT2 = 1.8V, with RT1 set to 150Ω as before, RT2 is calculated from the above equation to be 267 Ω. A timing diagram for the case of equal slew rates is shown in Figure 23. Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2744 |
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