LTC3778 UUWUAPPLICATIO S I FOR ATIO The basic LTC3778 application circuit is shown in Connecting the SENSE+ and SENSE– Pins Figure 1. External component selection is primarily de- The LTC3778 can be used with or without a sense resistor. termined by the maximum load current and begins with When using a sense resistor, it is placed between the the selection of the sense resistance and power MOSFET source of the bottom MOSFET, M2, and PGND. Connect switches. The LTC3778 can use either a sense resistor or the SENSE+ and SENSE– pins to the top and bottom of the the on-resistance of the synchronous power MOSFET for sense resistor. Using a sense resistor provides a well determining the inductor current. The desired amount of defined current limit, but adds cost and reduces efficiency. ripple current and operating frequency largely deter- Alternatively, one can eliminate the sense resistor and use mines the inductor value. Finally, CIN is selected for its the bottom MOSFET as the current sense element by ability to handle the large RMS current into the converter simply connecting the SENSE+ pin to the SW pin and and COUT is chosen with low enough ESR to meet the SENSE– pin to PGND. This improves efficiency, but one output voltage ripple and transient specification. must carefully choose the MOSFET on-resistance as dis- Maximum Sense Voltage and VRNG Pin cussed below. Inductor current is determined by measuring the voltage Power MOSFET Selection across a sense resistance that appears between the SENSE– The LTC3778 requires two external N-channel power and SENSE+ pins. The maximum sense voltage is set by MOSFETs, one for the top (main) switch and one for the the voltage applied to the VRNG pin and is equal to bottom (synchronous) switch. Important parameters for approximately (0.133)VRNG. The current mode control the power MOSFETs are the breakdown voltage V loop will not allow the inductor current valleys to exceed (BR)DSS, threshold voltage V (0.133)V (GS)TH, on-resistance RDS(ON), reverse RNG/RSENSE. In practice, one should allow some transfer capacitance C margin for variations in the LTC3778 and external compo- RSS and maximum current IDS(MAX). nent values and a good guide for selecting the sense The gate drive voltage is set by the 5V INTVCC and DRVCC resistance is: supplies. Consequently, logic-level threshold MOSFETs must be used in LTC3778 applications. If the input voltage V R RNG or DRVCC voltage is expected to drop below 5V, then sub- SENSE = 10•IOUT MAX ( ) logic level threshold MOSFETs should be considered. An external resistive divider from INTV When the bottom MOSFET is used as the current sense CC can be used to set the voltage of the V element, particular attention must be paid to its on- RNG pin between 0.5V and 2V resulting in nominal sense voltages of 50mV to 200mV. resistance. MOSFET on-resistance is typically specified Additionally, the V with a maximum value RDS(ON)(MAX) at 25°C. In this case, RNG pin can be tied to SGND or INTVCC in which case the nominal sense voltage defaults to 70mV additional margin is required to accommodate the rise in or 140mV, respectively. The maximum allowed sense MOSFET on-resistance with temperature: voltage is about 1.33 times this nominal value. R R SENSE DS ON ( ) MAX ( ) = ρT 3778f 10
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