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LT1175IST-5-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT1175IST-5-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 18 page  LT1175 1175fe applicaTions inForMaTion Setting Output Voltage The LT1175 adjustable version has a feedback sense voltage of 3.8V with a bias current of approximately 75nA flowing out of the SENSE pin. To avoid output voltage errors caused by this current, the output divider string (see Figure 1) should draw about 25µA. Table 1 shows suggested resistor values for a range of output voltages. The second part of the table shows resistor values which draw only 10µA of current. Output voltage error caused by bias current with the lower valued resistors is about 0.4% maximum and with the higher values, about 1% maximum. A formula is also shown for calculating the resistors for any output voltage. Table 1. Suggested Divider Resistors OUTPUT VOLTAGE R1 IDIV = 25µA R2 NEAREST 1% R1 IDIV = 10µA R2 NEAREST 1% 5V 150k 47.5k 383k 121k 6V 150k 86.6k 383k 221k 8V 150k 165k 383k 422k 10V 150k 243k 383k 619k 12V 150k 324k 383k 825k 15V 150k 442k 383k 1.13M R V I R R V V V Simple formula R V V V R I I Desired DIV OUT OUT FB DIV 1 3 8 2 1 3 8 3 8 1 3 8 3 8 1 = = − ( ) ( ) = − ( ) + ( ) = . . . . . R2 Taking SENSE pin bias current into account divider current Note to Reader: To avoid confusion when working with negative voltages (is –6V more or less than –5V?), I have decidedtotreattheLT1175asifitwereapositiveregulator andexpressallvoltagesaspositivevalues,bothintextand in formulas. If you do the same and simply add a negative signtotheeventualanswer,confusionshouldbeavoided. Please don’t give me a hard time about “preciseness” or “correctness.” I have to field phone calls from around the world and this is my way of dealing with a multitude of conventions. Thanks for your patience. The LT1175-5 is a fixed 5V design with the SENSE pin acting as a Kelvin connection to the output. Normally the SENSE pin and the OUTPUT pin are connected directly together, either close to the regulator or at the remote load point. Setting Current Limit The LT1175 uses two ILIM pins to set current limit (typical) at 200mA, 400mA, 600mA or 800mA. The corresponding minimumguaranteedcurrentsare130mA,260mA,390mA and 520mA. This allows the user to select a current limit tailored to his specific application and prevents the situa- tion where short-circuit current is many times higher than full-load current. Problems with input supply overload or excessivepowerdissipationinafaultedloadareprevented. Power limiting in the form of foldback current limit is built inandreducescurrentlimitasafunctionofinput-to-output voltage differential for differentials exceeding 14V. See the graph in Typical Performance Characteristics. The LT1175 is guaranteed to be blowout-proof regardless of current limit setting. The power limiting combined with thermal shutdown protects the device from destructive junction temperatures under all load conditions. Shutdown In shutdown, the LT1175 draws only about 10µA. Special circuitry is used to minimize increases in shutdown cur- rent at high temperatures, but a slight increase is seen above 125°C. One option not taken was to actively pull down on the output during shutdown. This means that the output will fall slowly after shutdown is initiated, at a rate determined by load current plus the 12µA internal load, and the size of the output capacitor. Active pull-down is Figure 1. Typical LT1175 Adjustable Connection + CIN COUT ≥ 0.1µF VOUT –12V R2 825k 1% R1 383k 1% SHUTDOWN LOGIC SHDN GND LT1175 SENSE OUTPUT > 2V OR < –2V TO TURN REGULATOR ON 1175 F01 ILIM4 ILIM2 VIN + |
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