Datasheet LTC3822 (Analog Devices) - 10

LTC3822
APPLICATIONS INFORMATION
The typical LTC3822 application circuit is shown on the where IRIPPLE is the inductor peak-to-peak ripple current front page of this data sheet. External component selection (see Inductor Value Calculation). for the controller is driven by the load requirement and A reasonable starting point is setting ripple current I begins with the selection of the inductor and the power RIPPLE to be 40% of I MOSFETs. OUT(MAX). Rearranging the above equation yields:
Power MOSFET Selection
ΔV R SENSE(MAX) DS(ON)MAX = 5 • for Duty Cycle < 20% The LTC3822’s controller requires external N-channel 6 I OUT(MAX) power MOSFETs for the topside (main) and bottom (synchronous) switches. The main selection criteria for However, for operation above 20% duty cycle, slope the power MOSFETs are the breakdown voltage V compensation has to be taken into consideration to select BR(DSS), threshold voltage V the appropriate value of R GS(TH), on-resistance RDS(ON), reverse DS(ON) to provide the required transfer capacitance C amount of load current: RSS, turn-off delay tD(OFF) and the total gate charge QG. ΔV R SENSE(MAX) DS(ON)MAX = 5 • SF • The gate drive voltage is usually the input supply voltage. 6 I OUT(MAX) See Figure 7 for an application with a higher gate drive voltage. Since the LTC3822 is designed for operation at where SF is a scale factor whose value is obtained from low input voltages, a sublogic level MOSFET (RDS(ON) the curve in Figure 1. guaranteed at VGS = 2.5V) is required. These must be further derated to take into account the The topside MOSFET’s on-resistance is chosen based on signifi cant variation in on-resistance with temperature. the required load current. The maximum average load The following equation is a good guide for determining the current IOUT(MAX) is equal to the peak inductor current required RDS(ON)MAX at 25°C (manufacturer’s specifi ca- minus half the peak-to-peak ripple current IRIPPLE. The tion), allowing some margin for variations in the LTC3822 LTC3822’s current comparator monitors the drain-to- and external component values: source voltage VDS of the top MOSFET, which is sensed ΔVSENSE(MAX) between the VIN and SW pins. The peak inductor current RDS(ON)MAX = 5 • 0.9 • SF • is limited by the current threshold, set by the voltage on 6 I OUT(MAX) • ρT the ITH pin, of the current comparator. The voltage on the The ρ I T is a normalizing term accounting for the tempera- TH pin is internally clamped, which limits the maximum ture variation in on-resistance, which is typically about current sense threshold ΔVSENSE(MAX) to approximately 0.4%/°C, as shown in Figure 2. Junction-to-case tem- 120mV when IPRG is fl oating (82mV when IPRG is tied perature ΔT low; 200mV when IPRG is tied high). JC is about 10°C in most applications. For a maximum ambient temperature of 70°C, using ρ80°C ~ 1.3 The output current that the LTC3822 can provide is given in the above equation is a reasonable choice. by: ΔV I I SENSE(MAX) RIPPLE OUT(MAX) = – R 2 DS(ON) 3822fa 10