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LM2743 数据表(PDF) 11 Page - Texas Instruments |
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LM2743 数据表(HTML) 11 Page - Texas Instruments |
11 / 41 page 0.65 = VOUT1 RT1 RT1 + RT2 Master Power Supply VOUT1 = 5V SS/TRACK LM2743 RT2 1 k: RT1 150: RFB2 10 k: RFB1 5 k: FB VOUT2 = 1.8V VSS = 0.65V VFB RFADJ = -5.93 + 3.06 10 7 fSW + 0.24 10 12 (fSW) 2 RFB1 VOUT = RFB1 RFB2 + VFB (VFB = 0.6V) LM2743 www.ti.com SNVS276H – APRIL 2004 – REVISED OCTOBER 2015 Feature Description (continued) • CSS is in µF and tSS is in ms (1) During soft start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of 0.6V. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage monitoring starts. 7.3.2 Normal Operation While in normal operation mode, the LM2743 regulates the output voltage by controlling the duty cycle of the high side and low side MOSFETs (see Typical Application Diagram). The equation governing output voltage is: (2) The PWM frequency is adjustable between 50 kHz and 1 MHz and is set by an external resistor, RFADJ, between the FREQ pin and ground. The resistance needed for a desired frequency is approximately: (3) Where fSW is in Hz and RFADJ is in kΩ. 7.3.3 Tracking a Voltage Level The LM2743 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 LM2743 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. Otherwise, a CSS value of at least 1 nF between the soft-start pin and ground should be used. Figure 19. 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 LM2743’s output voltage (represented symbolically in Figure 19 as VOUT2, that is, 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) Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2743 |
类似零件编号 - LM2743_15 |
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类似说明 - LM2743_15 |
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