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LT1175IT-5 Datasheet(PDF) 6 Page - Linear Technology |
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LT1175IT-5 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LT1175 1175fd APPLICATIONS INFORMATION 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. Figure 1. Typical LT1175 Adjustable Connection + CIN COUT ≥ 0.1µF VOUT –12V R2 825k 1% R1 383k 1% SHUTDOWN LOGIC SHDN GND LT1175 SENSE OUT > 2V OR < –2V TO TURN REGULATOR ON 1175 F01 ILIM4 ILIM2 INPUT + Setting Current Limit The LT1175 uses two ILIM pins to set current limit (typical) at 200mA, 400mA, 600mA or 800mA. The corresponding minimum guaranteed currents are 130mA, 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 excessive power dissipation in a faulted load are pre- vented. Power limiting in the form of foldback current limit is built in and reduces current limit as a function of input- 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 Note to Reader: To avoid confusion when working with negative voltages (is – 6V more or less than – 5V?), I have decided to treat the LT1175 as if it were a positive regulator and express all voltages as positive values, both in text and in formulas. If you do the same and simply add a negative sign to the eventual answer, confusion should be avoided. 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. R V I R RV V V Simple formula RV V VR I I Desired DIV OUT OUT FB DIV 1 38 2 13 8 38 13 8 38 1 = = − () () = − () + () ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = . . . . . R2 Taking SENSE pin bias current into account divider current 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% maxi- mum. A formula is also shown for calculating the resistors for any output voltage. Table 1 OUTPUT R1 R2 R1 R2 VOLTAGE IDIV = 25µA NEAREST 1% IDIV = 10µA 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 |
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