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LTC3823IGNTRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3823IGNTRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 26 page  LTC3823 11 3823fd APPLICATIONS INFORMATION The basic LTC3823 application circuit is shown in Figure 12. External component selection is primarily de- termined by the maximum load current and begins with the selection of the sense resistance and power MOSFET switches. The LTC3823 uses either a sense resistor or the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency largely determines the inductor value. Finally, CIN is selected for its ability to handle the large RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple and transient specification. Maximum Sense Voltage and VRNG Pin Inductor current is determined by measuring the volt- age across a sense resistance that appears between the SENSE– and SENSE+ pins. The maximum sense voltage is set by the voltage applied to the VRNG pin and is equal to approximately (0.133)VRNG. The current mode control loop will not allow the inductor current valleys to exceed (0.133)VRNG/RSENSE. In practice, one should allow some margin for variations in the LTC3823 and external com- ponent values and a good guide for selecting the sense resistance is: R V I SENSE RNG OUT MAX = 10 • () An external resistive divider from INTVCC can be used to set the voltage of the VRNG pin between 0.5V and 2V resulting in nominal sense voltages of 50mV to 200mV. Additionally, the VRNG pin can be tied to SGND or INTVCC in which case the nominal sense voltage defaults to 50mV or 200mV, respectively. The maximum allowed sense voltage is about 1.33 times this nominal value. Connecting the SENSE+ and SENSE– Pins The IC can be used with or without a sense resistor. When using a sense resistor, place it between the source of the bottom MOSFET, M2, and PGND. Connect the SENSE+ and SENSE– pins to the top and bottom of the sense resistor. Using a sense resistor provides a well defined current limit, but adds cost and reduces efficiency. Alternatively, one can eliminate the sense resistor and use the bottom MOSFET as the current sense element by simply connecting the SENSE+ pin to the SW pin and SENSE– pin to PGND. This improves efficiency, but one must carefully choose the MOSFET on-resistance as discussed below. Power MOSFET Selection The LTC3823 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS, threshold voltage V(GS)TH, on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current IDS(MAX). The gate drive voltage is set by the 5V INTVCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3823 applications. If the input voltage is expected to drop below 5V, then sub-logic level threshold MOSFETs should be considered. When the bottom MOSFET is used as the current sense element, particular attention must be paid to its on-resis- tance. MOSFET on-resistance is typically specified with a maximum value RDS(ON)(MAX) at 25°C. In this case, additional margin is required to accommodate the rise in MOSFET on-resistance with temperature: R R DS ON MAX SENSE T ()( ) = ρ The ρT term is a normalization factor (unity at 25°C) ac- counting for the significant variation in on-resistance with temperature, typically about 0.4%/°C as shown in Figure 1. For a maximum junction temperature of 100°C, using a value ρT = 1.3 is reasonable. The power dissipated by the top and bottom MOSFETs strongly depends upon their respective duty cycles and the load current. When the LTC3823 is operating in continuous mode, the duty cycles for the MOSFETs are: D V V D VV V TOP OUT IN BOT IN OUT IN = = – |
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