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LM350 Datasheet(PDF) 4 Page - STMicroelectronics |
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LM350 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 7 page TYPICAL APPLICATIONS + 1.2V to + 25V ADJUSTABLE REGULATOR APPLICATION HINTS In operation, the LM350 develops a nominal 1.25V reference voltage, V(ref), between the output and adjustment terminal. The reference voltage is im- pressed across program resistor R1 and, since the voltage is constant, a constant current I1 then flows through the output set resistor R2, giving an output voltage of VO =V(ref) (1+ R2 R1 )+ IadjR2 Figure 1. Since the 50 µA current from the adjustment termi- nal represents an error term, the LM350 was de- signed to minimize Iadj and make it very constant with line and load changes. To do this, all quiescent operating current is returned to the output establi- shing a minimum load current requirement. If there is insufficient load on the output, the output will rise. EXTERNAL CAPACITORS An input bypass capacitor is recommended. A 0.1 µF disc or 1µF solid tantalum on the input is suit- able input by passing for almost all applications. The device is more sensitive to the absence of input bypassing when adjustment or output capacitors are used byt the above values will eliminate the possibility of problems. The adjustment terminal can be bypassed to ground on the LM350 to improve ripple rejection. This bypass capacitor prevents ripple form being amplified as the output voltage is increased. With a10 µF bypass capacitor 75dB ripple rejection is obtainable at any output level. Increases over 20 µF do not appreciably improve the ripple rejection at frequencies above 120Hz. If the bypass capacitor is used, it is sometimes necessary to include pro- tection diodes to prevent the capacitor from dis- charging through internal low current paths and damaging the device. In general, the best type of capacitors to use are solid tantalum. Solid tantalum capacitors have low impedance even at high frequencies. Depending upon capacitor construction, it takes about 25 µFin aluminum electrolytic to equal 1 µF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies, but some types have a large decrease in capacitance at frequencies around 0.5MHz. For this reason, 0.01 µF disc may seem to work better than a 0.1 µF disc as a bypass. Although the LM350 is stable with no output capa- citors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500pF and 5000pF. A 1 µF solid tantalum (or 25µF aluminium electrolytic) on the output swamps this effect and insures stability. LOAD REGULATION The LM350 is capable of providing extremely good load regulation but a few precautions are needed to obtain maximum performance. The current set resistor connected between the adjustment termi- nal and the output terminal (usually 240 Ω) should be tied directly to the output of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regulation. For example, a 15V regulator with 0.05 Ω resistance between the regu- lator and load will have a load regulation due to line resistance of 0.05 Ω xIL. If the set resistor is con- nected near the load the effective line resistance ∆ Needed if device is far from filter capacitors. * Optional-improves transient response. Output capacitors in the range of 1 µF to 100µF of aluminium or tantalum electrolytic are commonly used to provide improved output impedance and rejec- tion of transients. R2 ** VO =1.25V ( 1 + ) R1 *** R1 =240 Ω for LM150 and LM250 LM350 LM350 LM150-LM250-LM350 4/7 |
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