Datasheet LTC3878 (Analog Devices) - 10

LTC3878 applicaTions inForMaTion The basic LTC3878 application circuit is shown on the first The gate drive voltages are set by the 5.3V INTVCC supply. page of this data sheet. External component selection is Consequently, logic-level threshold MOSFETs must be used largely determined by maximum load current and begins in LTC3878 applications. If the input voltage is expected with the selection of sense resistance and power MOSFET to drop below 5V, then sub-logic level threshold MOSFETs switches. The LTC3878 uses the on-resistance of the syn- should be considered. chronous power MOSFET to determine the inductor current. Using the bottom MOSFET as the current sense element The desired ripple current and operating frequency largely requires particular attention be paid to its on-resistance. determines the inductor value. Final y, CIN is selected for its MOSFET on-resistance is typically specified with a maxi- ability to handle the large RMS current into the converter, mum value R and C DS(ON)(MAX) at 25°C. In this case additional OUT is chosen with low enough ESR to meet output margin is required to accommodate the rise in MOSFET voltage ripple and transient specifications. on-resistance with temperature.
Maximum VDS Sense Voltage and VRNG Pin
Max V  Sense Voltage R DS = Inductor current is measured by sensing the bottom DS O ( N) M ( AX) IOUT • T ρ MOSFET VDS voltage that appears between the PGND and SW pins. The maximum allowed V The ρT term is a normalization factor (unity at 25°C) DS sense voltage is set by the voltage applied to the V accounting for the significant variation in on-resistance RNG pin and is approxi- mately equal to (0.133)V with temperature, typically about 0.4%/°C, as shown in RNG. The current mode control loop does not allow the inductor current valleys to exceed Figure 1. For a maximum junction temperature of 100°C (0.133)V using a value of ρT = 1.3 is reasonable. RNG. In practice, one should allow margin, to ac- count for variations in the LTC3878 and external component The power dissipated by the top and bottom MOSFETs values. A good guide for setting VRNG is: depends upon their respective duty cycles and the load V current. When the LTC3878 is operating in continuous RNG = 7.5 • (Maximum VDS Sense Voltage) mode, the duty cycles for the MOSFETs are: An external resistive divider from INTVCC can be used to set the voltage on the V V RNG pin between 0.2V and 2V, D OUT = resulting in peak sense voltages between 26.6mV and TOP VIN 266mV. The wide peak voltage sense range allows for a V – V IN OUT variety of applications and MOSFET choices. The V D = RNG pin BOT V can also be tied to either SGND or INTV IN CC to force internal defaults. When VRNG is tied to SGND, the device operates 2.0 at a valley current sense threshold of 93mV typical. If VRNG is tied to INTVCC, the device operates at a valley current sense threshold of 186mV typical. ANCE (Ω) 1.5
Power MOSFET Selection
1.0 The LTC3878 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the 0.5 bottom (synchronous) switch. Important parameters for NORMALIZED ON-RESIST the power MOSFETs are the breakdown voltage V R T BR(DSS), threshold voltage V 0 GS(TH), on-resistance RDS(ON), re- –50 0 50 100 150 verse transfer capacitance C JUNCTION TEMPERATURE (°C) RSS and maximum current I 3878 F01 DS(MAX).
Figure 1. RDS(ON) vs Temperature
3878fa 0 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts