features are included in the designthat increase its functionality and robustness. These func-tions comprise the remainder of the block diagram.A high side sense, current limit amplifier provides activecurrent limiting for the regulator. The current limit ampli-fier uses an external low value shunt resistor connected inseries with the external MOSFET s drain. This resistor canbe a discrete shunt resistor or can be manufactured froma Kelvin-sensed section of free PC board trace. All loadcurrent flows through the MOSFET drain and thus, throughthe sense resistor. The advantage of using high sidecurrent sensing in this topology is that the MOSFET s gainand the main feedback loop s gain remain unaffected. Thesense resistor develops a voltage equal to IOUT(RSENSE).The current limit amplifier s 50mV threshold voltage is agood compromise between power dissipation in the senseresistor, dropout voltage impact and noise immunity.Current limit activates when the sense resistor voltageequals the 50 specifications which apply over the full operatingtemperature range.Note 1: Absolute Maximum Ratings are those values beyond which the lifeof the device may be impaired.Note 2: TJ is calculated from the ambient temperature TA and powerdissipation PD according to the following formulas:LT1575CN8: TJ = TA + (PD 100 CW)LT1575CS8: TJ = TA + (PD 130 CW)LT1577CS: TJ = TA + (PD 100 CW)Because the LT1577 consists of two regulators in the package, the totalLT1577 power dissipation must be used for its junction temperaturecalculation. The total LT1577 PD = PD (Regulator 1) + PD (Regulator 2).Note 3: The VGS(th) of the external MOSFET must be greater than3V VOUT.TYPICAL PERFORMANCE CHARACTERISTICS UWQuiescent Current vs TemperatureFB Input Bias Currentvs TemperatureAdjustable LT1575 VREFvs TemperatureTEMPERATURE ( C) 755QUIESCENT CURRENT (mA)79111915 2525501501713681018141612 500751001251751575/77 G01IN = 8VVIN = 12V VIN = 20VTEMPERATURE ( C) 75 REFERENCE VOLTAGE (V)1.2101.2141.2181.2221251575/77 AUGWAWUWARBSO LUTEXITIS(Note 1)VIN, IPOS, INEG