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LM337 データシート(PDF) 7 Page - Linear Technology |
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LM337 データシート(HTML) 7 Page - Linear Technology |
7 / 12 page 7 137afb LT137A/LM137 LT337A/LM337 Output Voltage The output voltage is determined by two external resis- tors, R1 and R2 (see Figure 1). The exact formula for the output voltage is: VV R R IR OUT REF ADJ =+ ⎛ ⎝⎜ ⎞ ⎠⎟ + () 1 2 1 2 Where: VREF = Reference Voltage, IADJ = Adjustment Pin Current. In most applications, the second term is small enough to be ignored, typically about 0.5% of VOUT. In more critical applications, the exact formula should be used, with IADJ equal to 65μA. Solving for R2 yields: R VV V R I OUT REF REF ADJ 2 1 = + – Smaller values of R1 and R2 will reduce the influence of IADJ on the output voltage, but the no-load current drain on the regulator will be increased. Typical values for R1 are between 100Ω and 300Ω, giving 12.5mA and 4.2mA no-load current respectively. There is an additional consideration in selecting R1, the minimum load current specification of the regulator. The operating current of the LT137A flows from input to output. If this current is not absorbed by the load, the output of the regulator will rise above the regulated value. The current drawn by R1 and R2 is normally high enough to absorb the current, but care must be taken in no-load situations where R1 and R2 have high values. The maximum value for the operating current, which must be absorbed, is 5mA for the LT137A. If input-output volt- age differential is less than 10V, the operating current that must be absorbed drops to 3mA. APPLICATIONS INFORMATION EXAMPLES: 1. A precision 10V regulator to supply up to 1A load current. a. Select R1 = 100Ω to minimize effect of IADJ b. Calculate R2 = VV V R I VV V μ OUT REF REF ADJ – –. . 1 10 1 25 125 100 65 + = + Ω A A = 696 4 . Ω Use R2 = 698Ω 2. A 15V regulator to run off batteries and supply 50mA. VIN MAX = 25V a. To minimize battery drain, select R1 as high as possible R V mA 1 125 3 417 == . ΩΩ ,use 402 ,1% b. The high value for R1 will exaggerate the error due to IADJ, so the exact formula to calculate R2 should be used. R VV V R I VV V OUT REF REF ADJ 2 1 15 1 25 125 402 = + = + – –. . Ω 6 65 4331 μA =Ω Use R2 = 4320Ω Capacitors and Protection Diodes An output capacitor, C3, is required to provide proper fre- quency compensation of the regulator feedback loop. A 1μF or larger solid tantalum capacitor is generally sufficient for this purpose if the 1MHz impedance of the capacitor is 2Ω or less. High Q capacitors, such as Mylar, are not recom- mended because they tend to reduce the phase margin at light load currents. Aluminum electrolytic capacitors may also be used, but the minimum value should be 10μF to ensure a low impedance at 1MHz. The output capacitor should be located within a few inches of the regulator to keep lead impedance to a minimum. The following caution should be noted: if the output voltage is greater than 6V and an output capacitor greater than 20μF has been used, it is possible to damage the regulator if the input voltage Figure 1 LT137A ADJ IADJ VIN –VIN R2 VOUT VREF –VOUT 137A F01 + R1 C3 1μF + C2 5μF + C1 10μF |
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同様の説明 - LM337 |
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