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LT3800 Datasheet(PDF) 11 Page - Linear Technology |
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LT3800 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3844 11 3844fb APPLICATIONS INFORMATION Therefore, the peak inductor current is 100mV/RSENSE. The maximum output load current, IOUT(MAX), is the peak inductor current minus half the peak-to-peak ripple cur- rent, ΔIL. Allowing adequate margin for ripple current and exter- nal component tolerances, RSENSE can be calculated as follows: R mV I SENSE OUT MAX = 70 () Typical values for RSENSE are in the range of 0.005Ω to 0.05Ω. Operating Frequency The choice of operating frequency and inductor value is a trade off between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses and gate charge losses. However, lower frequency operation requires more inductance for a given amount of ripple current, resulting in a larger induc- tor size and higher cost. If the ripple current is allowed to increase, larger output capacitors may be required to maintain the same output ripple. For converters with high step-down VIN-to-VOUT ratios, another consideration is the minimum on-time of the LT3844 (see the Minimum On-time Considerations section). A final consideration for operating frequency is that in noise-sensitive com- munications systems, it is often desirable to keep the switching noise out of a sensitive frequency band. The LT3844 uses a constant frequency architecture that can be programmed over a 100kHz to 500kHz range with a single resistor from the fSET pin to ground, as shown in Figure 1. The nominal voltage on the fSET pin is 1V and the current that flows from this pin is used to charge an internal oscillator capacitor. The value of RSET for a given operating frequency can be chosen from Figure 4 or from the following equation: Rk f SET SW () . • • (– . ) Ω= 84 1041 31 Table 1 lists typical resistor values for common operating frequencies: Table 1. Recommended 1% Standard Values RSET fSW 191kΩ 100kHz 118kΩ 150kHz 80.6kΩ 200kHz 63.4kΩ 250kHz 49.9kΩ 300kHz 40.2kΩ 350kHz 33.2kΩ 400kHz 27.4kΩ 450kHz 23.2kΩ 500kHz Step-Down Converter: Inductor Selection The critical parameters for selection of an inductor are minimum inductance value, volt-second product, satura- tion current and/or RMS current. For a given ΔI,Theminimuminductancevalueiscalculated as follows: LV VV fV I OUT IN MAX OUT SW IN MAX L ≥ Δ • – •• () () fSW is the switch frequency. FREQUENCY (kHz) 0 20 40 80 100 120 400 200 3844 G19 60 200 100 500 300 600 140 160 180 Figure 1. Timing Resistor (RSET) Value |
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