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LM20343 Datasheet(PDF) 17 Page - Texas Instruments |
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LM20343 Datasheet(HTML) 17 Page - Texas Instruments |
17 / 29 page SS/TRK VOUT1 R1 R2 EN LM20343 External Power Supply VOUT2 RA = - 1 VTO VIH_EN x RB EN VOUT1 RA RB LM20343 External Power Supply VOUT2 LM20343 www.ti.com SNVS559B – MAY 2008 – REVISED APRIL 2013 Figure 29. Sequencing LM20343 with Precision Enable The value for resistor RB can be selected by the user to control the current through the divider. Typically this resistor will be selected to be between 10 k Ω and 49.9 kΩ. Once the value for RB is chosen the resistor RA can be solved using the equation below to set the desired turn-on voltage. (10) When designing for a specific turn-on threshold (VTO) the tolerance on the input supply, enable threshold (VIH_EN), and external resistors need to be considered to ensure proper turn-on of the device. The LM20343 features an open drain power good (PGOOD) pin to sequence external supplies or loads and to provide fault detection. This pin requires an external resistor (RPG) to pull PGOOD high when the output is within the PGOOD tolerance window. Typical values for this resistor range from 10 k Ω to 100 kΩ. TRACKING AN EXTERNAL SUPPLY By using a properly chosen resistor divider network connected to the SS/TRK pin, as shown in Figure 30, the output of the LM20343 can be configured to track an external voltage source to obtain a simultaneous or ratiometric start up. Figure 30. Tracking an External Supply Since the soft-start charging current ISS is always present on the SS/TRK pin, the size of R2 should be less than 10 k Ω to minimize the errors in the tracking output. Once a value for R2 is selected the value for R1 can be calculated using appropriate equation in Figure 31, to give the desired start up. Figure 30 shows two common start up sequences; the top waveform shows a simultaneous start up while the waveform at the bottom illustrates a ratiometric start up. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 17 Product Folder Links: LM20343 |
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