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TL750L Low Dropout 12V Regulator
Oscillations
My Z93 RF/IF prototype PCB powers three AD8007 amplifiers with Texas
Instruments TL750L12 12 volt low dropout regulators. These devices are
similar to the familiar 78L12 TO-92 packaged regulators, but will maintain a
stabilized 12V output with an input only 0.6V greater than the output
voltage. The 78L12, has a 1.7 V dropout voltage, which makes the TL750L an
attractive choice as it permits stabilized 12 V with only 12.6 V input.
Working with the prototype board, however, shows that the TL750L is not a
direct "drop in" replacement for a 78L12 regulator, and unless the output is
properly bypassed, the TL750L12 will oscillate.
TI's TL750L12 datasheet can be found at
http://focus.ti.com/lit/ds/symlink/tl750l05.pdf.
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Bypassing typically used for 78Lxx series regulators will
not work for TL750L12 low dropout regulators. Do NOT use this circuit!
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My prototype design uses three TL750L12 regulators, and here's how the +13 V
input voltage looked when first built:
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+13 V input bus with three TL750L12 regulators, improperly
bypassed.
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There's a 2 V peak-to-peak oscillation at around 50 KHz superimposed on the 13 V
bus. And, this is with a 4700 μF capacitor across the +13 V input. The +13V
input is supplied by an HP 6205C laboratory-grade variable DC power supply.
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Oscillation on +13 V input is at about 50 KHz.
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The TL750L12's output also has a high frequency oscillation, at about 1 V
peak-to-peak.
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TL750L12 Output voltage, with oscillation.
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To say the least, I was not expecting this, as I've never seen similar
instability with the 78Lxx regulators I've used in many other projects.
As usual, it pays to read the data sheet before
designing in the part. If I had, I would have seen the following caution:
The TL750L, TL751L series are low-dropout regulators.
This means that capacitance loading is important to the performance of the
regulator because it is a vital part of the control loop. The capacitor value
and its equivalent series resistance (ESR) both affect the control loop and
must be defined for the load range and temperature range. Figure 1 shows the
recommended range of ESR for a given load with a 10-μF capacitor on the
output.
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The load current in my design is around 30 mA for each
TL750L12, so Figure 1 says that the output 10 μF capacitor should have an
equivalent series resistance (ESR or RS) below 0.4 ohms.
My junkbox has a few 10 μF /16v tantalum capacitors, but
do they have the requisite low ESR? If you have an RLC bridge that measures C
and D, you can easily compute the ESR. (If you don't have a bridge, or an ESR
meter, there are other methods of determining the ESR.)
We know that D or "dissipation factor" is defined, in
terms of series capacitance and series resistance, as:
D = ωRSCS
Solving for Rs:
RS = D/ωCS
ω = 2πf where f is the test frequency
Here's what I found when measuring three 10 μF capacitors,
with a General Radio 1658 Digibridge, test frequency 1000 Hz:
| Nominal Value |
Type |
Measured CS |
Measured D |
Computed RS (Ohms) |
| 10 μF / 16 V |
Tantalum radial |
9.8 μF |
0.0250 |
0.406 |
| 10 μF / 10 V |
Tantalum radial |
10.28 μF |
0.0364 |
0.564 |
| 10 μF / 50 V |
Alum. Electrolytic |
9.70 μF |
0.1177 |
1.931 |
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Leaving aside the 10 V rated capacitor—definitely not a good
choice for a 12 V power supply rail—the first capacitor tested will work,
although its Rs is up against the recommended maximum 0.4 ohms. The aluminum
electrolytic's RS grossly exceeds the recommended 0.4 ohm value, and
hence is not usable for this purpose.
Applying one 10 μF / 16 V tantalum capacitor to each
TL750L12 regulator's output completely removed the instability. The output
voltage shows essentially zero noise and the input voltage shows about 25 mV
high frequency hash coupled into the oscilloscope probe from nearby digital
circuitry.
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+13 V input after applying 10 μF / 16 V tantalum capacitors
to outputs of all three TL750L12 regulators.The
noise is mostly coupled into the oscilloscope probe from digital circuitry on
the board.
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Recommended bypassing for TL750L12. The 10 μF tantalum
capacitor must have Rs < 0.4 Ω and should be installed physically close to the
regulator.
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