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LMQ61460 Arkusz danych(PDF) 31 Page - Texas Instruments |
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LMQ61460 Arkusz danych(HTML) 31 Page - Texas Instruments |
31 / 55 page fSW G VIN(MAX2) Â WON_MIN(MAX) VOUT (7) If foldback at low input voltage is a concern, use Equation 8: fSW ” VINeff(MIN2) Â WOFF_MIN(MAX) VINeff(MIN2) ± VOUT (8) where: • VINeff(MIN2) = VIN(MIN2) ± IOUT(MAX) Â (RDS(ON)_HS(MAX) + DCR(MAX)) • DCR(MAX) = maximum DCR of the inductor • tOFF_MIN(MAX) = see Section 6.5 • RDS(ON)_HS(MAX) = see Section 6.5 The fourth constraint is the rated frequency range of the IC. See fADJ in Section 6.5. All four constraints above, thermal, VIN(MAX2), VIN(MIN2), and device specified frequency range must be considered when selecting frequency. Many applications require that the AM band can be avoided. These applications tend to operate at either 400 kHz below the AM band or 2.1 MHz above the AM band. In this example, 400 kHz is chosen. 8.2.2.2 Setting the Output Voltage The output voltage of the device is externally adjustable using a resistor divider network. The range of recommended output voltage is found in Section 6.3. The divider network is comprised of RFBT and RFBB, and closes the loop between the output voltage and the converter. The converter regulates the output voltage by holding the voltage on the FB pin equal to the internal reference voltage, VREF. The resistance of the divider is a compromise between excessive noise pickup and excessive loading of the output. Smaller values of resistance reduce noise sensitivity but also reduce the light-load efficiency. The recommended value for RFBT is 100 kΩ with a maximum value of 1 MΩ. If 1 MΩ is selected for RFBT, then a feedforward capacitor must be used across this resistor to provide adequate loop phase margin (see Section 8.2.2.10). Once RFBT is selected, Equation 3 is used to select RFBB. VREF is nominally 1 V. For this 5-V example, RFBT = 100 kΩ and RFBB = 24.9 kΩ are chosen. 8.2.2.3 Inductor Selection The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current and is normally chosen to be in the range of 20% to 40% of the maximum output current. Experience shows that the best value for inductor ripple current is 30% of the maximum load current for systems with a fixed input voltage and 25% for systems with a variable input voltage such as the 12 volt battery in a car. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, the maximum device current must still be used. Equation 9 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example, K = 0.25 was chosen and an inductance of approximately 5.25 μH was found. The next standard value of 4.7 μH was selected. L= fSW Â . Â ,OUT(MAX) VIN Å 9OUT Â VOUT VIN (9) The saturation current rating of the inductor must be at least as large as the high-side switch current limit, IL-HS (see Section 6.5). This ensures that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, IL-LS, is designed to reduce the risk of current run-away, a www.ti.com LMQ61460 SNVSBR8A – JULY 2020 – REVISED AUGUST 2020 Copyright © 2020 Texas Instruments Incorporated Submit Document Feedback 31 Product Folder Links: LMQ61460 |
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