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LM2747 датащи(PDF) 11 Page - Texas Instruments |
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LM2747 датащи(HTML) 11 Page - Texas Instruments |
11 / 38 page 0.65 = VOUT1 RT1 RT1 + RT2 Master Power Supply VOUT1 = 5V SS/TRACK LM2747 RT2 1 k: RT1 150: RFB2 10 k: RFB1 5 k: FB VOUT2 = 1.8V VSS = 0.65V VFB LM2747 www.ti.com SNVS370B – MARCH 2006 – REVISED MARCH 2013 OUTPUT PRE-BIAS STARTUP If there is a pre-biased load on the output of the LM2747 during startup, the IC will disable switching of the low- side MOSFET and monitor the SW node voltage during the off-time of the high-side MOSFET. There is no load current sensing while in pre-bias mode because the low-side MOSFET never turns on. The IC will remain in this pre-bias mode until it sees the SW node stays below 0V during the entire high-side MOSFET's off-time. Once it is determined that the SW node remained below 0V during the high-side off-time, the low-side MOSFET begins switching during the next switching cycle. Figure 19 shows the SW node, HG, and LG signals during pre-bias startup. The pre-biased output voltage should not exceed VCC + VGS of the external High-Side MOSFET to ensure that the High-Side MOSFET will be able to switch during startup. Figure 19. Output Pre-Bias Mode Waveforms TRACKING A VOLTAGE LEVEL The LM2747 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 LM2747 will be controlled by the master supply for loads that require precise sequencing. When the tracking function is used no soft-start capacitor should be connected to the SS/TRACK pin. However in all other cases, a CSS value of at least 1 nF between the soft-start pin and ground should be used. Figure 20. 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 LM2747’s output voltage (represented symbolically in Figure 20 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: (3) Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2747 |
Аналогичный номер детали - LM2747_15 |
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Аналогичное описание - LM2747_15 |
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