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LT337A Datasheet(PDF) 8 Page - Linear Technology |
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LT337A Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT137A/LM137 LT337A/LM337 8 137afb becomes shorted, due to the output capacitor discharging into the regulator. This can be prevented by using the diode D1 (see Figure 2) between the input and the output. The input capacitor, C2, is only required if the regulator is more than 4 inches from the raw supply filter capacitor. Bypassing the Adjustment Pin The adjustment pin of the LT137A may be bypassed with a capacitor to ground, C1, to reduce output ripple, noise, and impedance. These parameters scale directly with output voltage if the adjustment pin is not bypassed. A bypass capacitor reduces ripple, noise, and impedance to that of a 1.25V regulator. In a 15V regulator, for example, these parameters are improved by 15V/1.25V = 12 to 1. This improvement holds only for those frequencies where the impedance of the bypass capacitor is less than R1. Ten microfarads is generally sufficient for 60Hz power line applications where the ripple frequency is 120Hz since XC = 130Ω. The capacitor should have a voltage rating at least as high as the output voltage of the regulator. Values larger than 10μF may be used, but if the output is larger than 25V, a diode, D2, should be added between the output and adjustment pins (see Figure 2). Proper Connection of Divider Resistors The LT137A has an excellent load regulation specification of 0.5% and is measured at a point 1/8" from the bottom of the package. To prevent degradation of load regulation, the resistors which set output voltage, R1 and R2, must be connected as shown in Figure 3. Note that the positive side of the load has a true force and sense (Kelvin) connection, but the negative side of the load does not. R1 should be connected directly to the output lead of the regulator, as close as possible to the specified point 1/8" from the case. R2 should be connected to the positive side of the load separately from the positive (ground) connection to the raw supply. With this arrangement, load regulation is degraded only by the resistance between the regulator output pin and the load. If R1 is connected to the load, regulation will be degraded. LT137A ADJ VIN –VIN R2 *D1 PROTECTS THE REGULATOR FROM INPUT SHORTS TO GROUND. IT IS REQUIRED ONLY WHEN C3 IS LARGER THAN 20μF AND VOUT IS LARGER THAN 6V. **D2 PROTECTS THE ADJUST PIN OF THE REGULATOR FROM OUTPUT SHORTS IF C2 IS LARGER THAN 10μF AND VOUT IS LARGER THAN –25V. D1* 1N4002 D2** 1N4002 VOUT –VOUT 137A F02 + R1 C3 + C1 + C2 Figure 2 LT137A ADJ VIN –VIN R2 CONNECT R1 DIRECTLY TO REGULATOR PIN LEAD RESISTANCE HERE DEGRADES LOAD REGULATION. MINIMIZE THE LENGTH OF THIS LEAD. LEAD RESISTANCE HERE DOES NOT AFFECT LOAD REGULATION VOUT 137A F03 R1 LOAD Figure 3 APPLICATIONS INFORMATION |
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