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LTC3709 数据表(PDF) 12 Page - Linear Technology |
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LTC3709 数据表(HTML) 12 Page - Linear Technology |
12 / 24 page 12 LTC3709 3709f APPLICATIO S I FOR ATIO The basic LTC3709 application circuit is shown on the first page of this data sheet. External component selec- tion is primarily determined by the maximum load cur- rent and begins with the selection of the power MOSFET switches and/or sense resistor. The inductor current is determined by the RDS(ON) of the synchronous MOSFET while the user has the option to use a sense resistor for a more accurate current limiting. The desired amount of ripple current and operating frequency largely deter- mines 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 specification. Maximum Sense Voltage and VRNG Pin Inductor current is determined by measuring the voltage across the RDS(ON) of the synchronous MOSFET or through a sense resistance that appears between the SENSE– and the SENSE+ pins. The maximum sense voltage is set by the voltage applied to the VRNG pin and is equal to approxi- mately VRNG/7.5. The current mode control loop will not allow the inductor current valleys to exceed VRNG/(7.5 • RSENSE). In practice, one should allow some margin for variations in the LTC3709 and external component values. A good guide for selecting the sense resistance for each channel is: R V I SENSE RNG OUT MAX = 2 10 • • () The voltage of the VRNG pin can be set using an external resistive divider from VCC between 0.5V and 2V resulting in nominal sense voltages of 50mV to 200mV. Addition- ally, the VRNG pin can be tied to ground or VCC, in which case the nominal sense voltage defaults to 70mV or 140mV, respectively. The maximum allowed sense volt- age is about 1.3 times this nominal value. Connecting the SENSE+ and SENSE– Pins The LTC3709 provides the user with an optional method to sense current through a sense resistor instead of using the RDS(ON) of the synchronous MOSFET. When using a sense resistor, it is placed between the source of the synchro- nous MOSFET and ground. To measure the voltage across this resistor, connect the SENSE+ pin to the source end of the resistor and the SENSE– pin to the other end of the resistor. The SENSE+ and SENSE– pin connections pro- vide the Kelvin connections, ensuring accurate voltage measurement across the resistor. Using a sense resistor provides a well-defined current limit, but adds cost and reduces efficiency. Alternatively, one can use the synchro- nous MOSFET as the current sense element by simply connecting the SENSE+ pin to the switch node SW and the SENSE– pin to the source of the synchronous MOSFET, eliminating the sense resistor. This improves efficiency, but one must carefully choose the MOSFET on-resistance as discussed in the Power MOSFET Selection section. Power MOSFET Selection The LTC3709 requires four external N-channel power MOSFETs, two for the top (main) switches and two for the bottom (synchronous) switches. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS,thresholdvoltageV(GS)TH,on-resistanceRDS(ON), reverse transfer capacitance CRSS and maximum current IDS(MAX). The gate drive voltage is set by the 5V DRVCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3709 applications. If the driver’s voltage is expected to drop below 5V, then sub-logic level threshold MOSFETs should be used. When the bottom MOSFETs are used as the current sense elements, particular attention must be paid to their on- resistance. 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) accounting for the significant variation in on-resistance with temperature, typically about 0.4%/°C. Junction-to- case temperature is about 20°C in most applications. For a maximum junction temperature of 100°C, using a value ρ100°C = 1.3 is reasonable (Figure 1). |
类似零件编号 - LTC3709 |
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类似说明 - LTC3709 |
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