LTC3772B UUWUAPPLICATIO S I FOR ATIO The basic LTC3772B application circuit is shown on the front However, for operation above 20% duty cycle, slope com- page of this data sheet. The load requirement drives the pensation has to be taken into consideration to select the selection of external components: the power MOSFET, appropriate value of RDS(ON) for the required amount of load inductor and output diode, as well as the input bypass current: capacitor CIN and output bypass capacitor COUT. 5 ∆V – SF SENSE MAX ( ) Power MOSFET Selection RDS ON ( ) MAX ( ) = • 6 IOUT MAX ( ) An external P-channel power MOSFET must be selected for use with the LTC3772B. The main selection criteria for the where SF is a factor whose value is obtained from the curve power MOSFET are the threshold voltage V in Figure 1. GS(TH), the “on” resistance RDS(ON), reverse transfer capacitance CRSS and These must be further derated to take into account the total gate charge. significant variation in on-resistance with temperature. The Since the LTC3772B is designed for operation down to low following equation is a good guide for determining the input voltages, a sublogic level threshold MOSFET (R required RDS(ON)MAX at 25°C (manufacturer’s specifica- DS(ON) guaranteed at V tion), allowing some margin for variations in the LTC3772B GS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, and external component values: make sure that the input supply to the LTC3772B is less than 5 ∆V – SF the absolute maximum V SENSE MAX ( ) GS rating. RDS ON ( ) MAX ( ) = • . 0 9 • 6 IOUT MAX ( ) • ρT The P-channel MOSFET’s on-resistance is chosen based on the required load current. The maximum average output The ρT is a normalizing term accounting for the tempera- load current IOUT(MAX) is equal to the peak inductor current ture variation in on-resistance, which is typically about minus half the peak-to-peak ripple current IRIPPLE. The 0.4%/°C, as shown in Figure 2. Junction to case tempera- LTC3772B’s current comparator monitors the drain-to- ture TJC is about 10°C in most applications. For a maximum source voltage VDS of the P-channel MOSFET, which is ambient temperature of 70°C, using ρ80°C ≅ 1.3 in the above sensed between the VIN and SW pins. The peak inductor equation is a reasonable choice. current is limited by the current threshold, set by the volt- The required minimum R age on the I DS(ON) of the MOSFET is also TH pin of the current comparator. The voltage governed by its allowable power dissipation. For applica- on the ITH pin is internally clamped, which limits the maxi- tions that may operate the LTC3772B in dropout–i.e., 100% mum current sense threshold ∆VSENSE(MAX) to approxi- mately 138mV when IPRG is floating (70mV when IPRG is tied low; 208mV when I 2.0 PRG is tied high). The output current that the LTC3772B can provide is given by: 1.5 ∆VSENSE MAX ( ) I I RIPPLE OUT MAX ( ) = – RDS ON ( ) 2 1.0 A reasonable starting point is setting ripple current IRIPPLE to be 40% of IOUT(MAX). Rearranging the above equation 0.5 NORMALIZED ON RESISTANCE yields: ρ T 5 ∆V 0 R SENSE MAX ( ) – 50 0 50 100 150 DS ON ( ) MAX ( ) = • 6 IOUT MAX ( ) JUNCTION TEMPERATURE (ϒC) 3772B F02 for Duty Cycle < 20%. Figure 2. RDS(ON) vs Temperature 3772bfa 10
