LTC3772 UUWUAPPLICATIO S I FOR ATIO The basic LTC3772 application circuit is shown on the front However, for operation above 20% duty cycle, slope com- page of this data sheet. External component selection is pensation has to be taken into consideration to select the driven by the load requirement and begins with the selec- appropriate value of RDS(ON) to provide the required amount tion of the power MOSFET inductor and the output diode. of load current: These are selected followed by the input bypass capacitor CIN and output bypass capacitor COUT. 5 ∆V – SF SENSE MAX ( ) RDS ON ( ) MAX ( ) = • 6 IOUT MAX ( ) Power MOSFET Selection An external P-channel power MOSFET must be selected for where SF is a factor whose value is obtained from the curve use with the LTC3772. The main selection criteria for the in Figure 1. power MOSFET are the threshold voltage VGS(TH) and the These must be further derated to take into account the “on” resistance RDS(ON), reverse transfer capacitance CRSS significant variation in on-resistance with temperature. The and total gate charge. following equation is a good guide for determining the Since the LTC3772 is designed for operation down to low required RDS(ON)MAX at 25°C (manufacturer’s specifica- input voltages, a sublogic level threshold MOSFET (R tion), allowing some margin for variations in the LTC3772 DS(ON) guaranteed at V and external component values: GS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, 5 ∆V – SF make sure that the input supply to the LTC3772 is less than SENSE MAX ( ) RDS ON ( ) MAX ( ) = • . 0 9 • the absolute maximum V 6 I • ρ GS rating. OUT MAX ( ) T The P-channel MOSFET’s on-resistance is chosen based on The ρT is a normalizing term accounting for the tempera- the required load current. The maximum average output ture variation in on-resistance, which is typically about load current IOUT(MAX) is equal to the peak inductor current 0.4%/°C, as shown in Figure 2. Junction to case tempera- minus half the peak-to-peak ripple current IRIPPLE. The ture TJC is about 10°C in most applications. For a maximum LTC3772’s current comparator monitors the drain-to- ambient temperature of 70°C, using ρ80°C ~ 1.3 in the above source voltage VDS of the P-channel MOSFET, which is equation is a reasonable choice. sensed between the VIN and SW pins. The peak inductor The required minimum R current is limited by the current threshold, set by the volt- DS(ON) of the MOSFET is also governed by its allowable power dissipation. For applica- age on the ITH pin of the current comparator. The voltage tions that may operate the LTC3772 in dropout–i.e., 100% on the ITH pin is internally clamped, which limits the maxi- mum current sense threshold ∆VSENSE(MAX) to approxi- mately 175mV when I 2.0 PRG is floating (100mV when IPRG is tied low; 250mV when IPRG is tied high). 1.5 The output current that the LTC3772 can provide is given by: ∆VSENSE MAX ( ) I I RIPPLE 1.0 OUT MAX ( ) = – RDS ON ( ) 2 A reasonable starting point is setting ripple current IRIPPLE 0.5 NORMALIZED ON RESISTANCE to be 40% of I T OUT(MAX). Rearranging the above equation ρ yields: 0–50 0 50 100 150 5 ∆V R SENSE M ( AX) JUNCTION TEMPERATURE (ϒC) DS ON ( ) MAX ( ) = • 6 I 3772 F02 OUT MAX ( ) for Duty Cycle < 20%. Figure 2. RDS(ON) vs Temperature 3772f 10
