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LTC3772ETS8 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3772ETS8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC3772 3772f The basic LTC3772 application circuit is shown on the front page of this data sheet. External component selection is driven by the load requirement and begins with the selec- tion of the power MOSFET inductor and the output diode. These are selected followed by the input bypass capacitor CIN and output bypass capacitor COUT. Power MOSFET Selection An external P-channel power MOSFET must be selected for use with the LTC3772. The main selection criteria for the power MOSFET are the threshold voltage VGS(TH) and the “on” resistance RDS(ON), reverse transfer capacitance CRSS and total gate charge. Since the LTC3772 is designed for operation down to low input voltages, a sublogic level threshold MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, make sure that the input supply to the LTC3772 is less than the absolute maximum VGS rating. The P-channel MOSFET’s on-resistance is chosen based on the required load current. The maximum average output load current IOUT(MAX) is equal to the peak inductor current minus half the peak-to-peak ripple current IRIPPLE. The LTC3772’s current comparator monitors the drain-to- source voltage VDS of the P-channel MOSFET, which is sensed between the VIN and SW pins. The peak inductor current is limited by the current threshold, set by the volt- age on the ITH pin of the current comparator. The voltage on the ITH pin is internally clamped, which limits the maxi- mum current sense threshold ∆VSENSE(MAX) to approxi- mately 175mV when IPRG is floating (100mV when IPRG is tied low; 250mV when IPRG is tied high). The output current that the LTC3772 can provide is given by: I V R I OUT MAX SENSE MAX DS ON RIPPLE () () () – = ∆ 2 A reasonable starting point is setting ripple current IRIPPLE to be 40% of IOUT(MAX). Rearranging the above equation yields: R V I DS ON MAX SENSE MAX OUT MAX ()( ) () () • = ∆ 5 6 for Duty Cycle < 20%. However, for operation above 20% duty cycle, slope com- pensation has to be taken into consideration to select the appropriate value of RDS(ON)toprovidetherequiredamount of load current: R VSF I DS ON MAX SENSE MAX OUT MAX ()( ) () () • – = ∆ 5 6 where SF is a 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 LTC3772 and external component values: R VSF 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, usingρ80°C~1.3intheabove equation is a reasonable choice. The required minimum RDS(ON) of the MOSFET is also governed by its allowable power dissipation. For applica- tions that may operate the LTC3772 in dropout–i.e., 100% APPLICATIO S I FOR ATIO Figure 2. RDS(ON) vs Temperature JUNCTION TEMPERATURE (ϒC) –50 1.0 1.5 150 0.5 0 0 50 100 2.0 3772 F02 |
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