5962R9951705VXA Datasheet, PDF (17/36 Pages) Texas Instruments – LM117QML 3-Terminal Adjustable Regulator

LM117QML, LM117QML-SP
www.ti.com
APPLICATION HINTS
SNVS356D – MARCH 2006 – REVISED APRIL 2013
In operation, the LM117 develops a nominal 1.25V reference voltage, V_REF, between the output and adjustment
terminal. The reference voltage is impressed 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
(1)
Since the 100μA current from the adjustment terminal represents an error term, the LM117 was designed to
minimize I_ADJ and make it very constant with line and load changes. To do this, all quiescent operating current is
returned to the output establishing 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 0.1μF disc or 1μF solid tantalum on the input is suitable input
bypassing for almost all applications. The device is more sensitive to the absence of input bypassing when
adjustment or output capacitors are used but the above values will eliminate the possibility of problems.
The adjustment terminal can be bypassed to ground on the LM117 to improve ripple rejection. This bypass
capacitor prevents ripple from being amplified as the output voltage is increased. With a 10μF bypass capacitor
80dB ripple rejection is obtainable at any output level. Increases over 10μF do not appreciably improve the ripple
rejection at frequencies above 120Hz. If the bypass capacitor is used, it is sometimes necessary to include
protection diodes to prevent the capacitor from discharging through internal low current paths and damaging the
device.
In general, the best type of capacitors to use is solid tantalum. Solid tantalum capacitors have low impedance
even at high frequencies. Depending upon capacitor construction, it takes about 25μF in aluminum electrolytic to
equal 1μF 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, 0.01μF disc may
seem to work better than a 0.1μF disc as a bypass.
Although the LM117 is stable with no output capacitors, like any feedback circuit, certain values of external
capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1μF solid
tantalum (or 25μF aluminum electrolytic) on the output swamps this effect and insures stability. Any increase of
the load capacitance larger than 10μF will merely improve the loop stability and output impedance.
LOAD REGULATION
The LM117 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 terminal and the output
terminal (usually 240Ω) should be tied directly to the output (case) 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 0.05Ω resistance between the regulator and load will have a load regulation due to
line resistance of 0.05Ω × I_L. If the set resistor is connected near the load the effective line resistance will be
0.05Ω (1 + R2/R1) or in this case, 11.5 times worse.
Figure 17 shows the effect of resistance between the regulator and 240Ω set resistor.
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