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LTC3822EDD Datasheet(PDF) 10 Page - Linear Technology |
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LTC3822EDD Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC3822 3822f APPLICATIO S I FOR ATIO The typical LTC3822 application circuit is shown on the front page of this data sheet. External component selection for the controller is driven by the load require- ment and begins with the selection of the inductor and the power MOSFETs. Power MOSFET Selection The LTC3822’s controller requires external N-channel power MOSFETs for the topside (main) and bottom (syn- chronous) switches. The main selection criteria for the power MOSFETs are the breakdown voltage VBR(DSS), threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS, turn-off delay tD(OFF) and the total gate charge QG. The gate drive voltage is usually the input supply voltage. See Figure 7 for an application with a higher gate drive voltage. Since the LTC3822 is designed for operation at low input voltages, a sublogic level MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required. The topside MOSFET’s on-resistance is chosen based on the required load current. The maximum average load current IOUT(MAX) is equal to the peak inductor current minus half the peak-to-peak ripple current IRIPPLE. The LTC3822’s current comparator monitors the drain-to- source voltage VDS of the top MOSFET, which is sensed between the VIN and SW pins. The peak inductor current is limited by the current threshold, set by the voltage on the ITH pin, of the current comparator. The voltage on the ITH pin is internally clamped, which limits the maximum current sense threshold ∆VSENSE(MAX) to approximately 120mV when IPRG is floating (82mV when IPRG is tied low; 200mV when IPRG is tied high). The output current that the LTC3822 can provide is given by: I V R I OUT MAX SENSE MAX DS ON RIPPLE () () () – = 2 ∆ where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation). A reasonable starting point is setting ripple current IRIPPLE to be 40% of IOUT(MAX). Rearranging the above equation yields: R V I for Duty Cycle DS ON MAX SENSE MAX OUT MAX () () () •% =< 5 6 20 ∆ However, for operation above 20% duty cycle, slope compensation has to be taken into consideration to select the appropriate value of RDS(ON) to provide the required amount of load current: RSF V I DS ON MAX SENSE MAX OUT MAX () () () •• = 5 6 ∆ where SF is a scale factor whose value is obtained from the curve in Figure 1. These must be further derated to take into account the significant variation in on-resistance with temperature. The following equation is a good guide for determining the required RDS(ON)MAX at 25°C (manufacturer’s specifica- tion), allowing some margin for variations in the LTC3822 and external component values: RSF V I DS ON MAX SENSE MAX OUT MAX T () () () •. • • • = 5 6 09 ρ ∆ The ρT is a normalizing term accounting for the tempera- ture variation in on-resistance, which is typically about 0.4%/ °C, as shown in Figure 2. Junction-to-case tempera- ture ∆TJC is about 10°C in most applications. For a maximum ambient temperature of 70 °C, using ρ80°C~ 1.3 in the above equation is a reasonable choice. |
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