# Tag Info

34

The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor. To meet the demand for lower dropout voltage, PNP ...

21

It's not that a DCDC (Buck Regulator) saves power, it's that an LDO wastes power. In effect a buck regulator converts the voltage difference to more available current. An LDO converts the voltage difference to heat, and heat is a waste product you don't really want. An LDO regulating, say, 12V down to 5V has to drop 7V and dissipate that power as heat. ...

20

The data sheet for the regulator suggests ... How important is this? About as important as that your circuit work reliably. Trying to second-guess datasheets is a bad idea. Unless the datasheet explains exactly what is going on and gives you guidance on different choices, the specifications are requirements, not options. Only Microchip knows the limits ...

18

A voltage regulator is designed to take a variable voltage in (say, 2-5v), and output a constant voltage (say, 3.3v). Now, voltage regulators are typically used to power a circuit, which means they will have a current output of a few hundred mA or more, generally speaking. In order to keep cost, size, etc down, the output tolerance on voltage regulators ...

17

To be clear and to protect against changes to the question, here is your schematic: There are a number of problems here, both with the schematic and the circuit: The schematic is missing junction dots. It's a convention we expect everyone here to follow. Don't be so lazy. The logical flow in the schematic is right to left. It took me a little while to ...

17

Ferrite Beads and LDOs are typically solve two different kinds of noise problems, so very rarely will you be able to "replace" an LDO with only a Ferrite Bead. LDO's ability to filter noise is typically characterized as Power Supply Ripple Rejection (PSRR) and most LDOs are generally good attenuating Low Frequencies, but due to their bandwidth limitations ...

15

A LDO is a control loop. And like all control loops, there is always room for instability. So how do you make a control loop stable ? You provide sufficient phase margin (difference in phase from when the gain crosses the 0 dB axis and 180. The slope of open loop plot should be -20db/dec when crossing the 0dB axis Provide sufficient gain margin If you ...

14

The official stance on supplying power directly to the 5V pin on the Arduino Uno is thus: 5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the ...

14

How about using an alternative circuit like this: Q4 is a P-mosfet, kept off when there is a voltage in the main supply (USB in this case). When the main supply is disconnected the gate is pulled down and the mosfet conducts and provides output from the battery source. The diodes are Schottky type for low Vf and the mosfet should apparently be ...

13

Much depends what the frequency band of your noise is. For HF noise you'll often use ferrite beads. For instance the Murata BLM18PG221SN1 has a DC resistance of only 100 mΩ but a 220 Ω impedance at 100 MHz. If you can hear the noise it won't be 100 MHz, unless it's the result of mixing with a frequency in that range, so that difference ...

13

It's a two-diode package for convenience, but the diodes serve two different functions: The one on the left is providing reverse-voltage protection for the regulator. It keeps current from flowing if the connections to JP1 are accidentally swapped. The one on the right protects the regulator from excess positive VOUT to VIN differential voltage. Many ...

11

Here's a less formal way to look at it: Providing power directly to the +5V pin is (almost) exactly what happens when the Uno is powered over USB. As USB power is perfectly fine, by design, then your setup should be OK too. External regulated +5V can be supplied to the USBVCC net just as well, for example by using an USB-B cable. As per the second question,...

11

You certainly need a capacitor at the output. From dataheet. Stability The circuit design used in the AMS1117 series requires the use of an output capacitor as part of the device frequency compensation. The addition of 22 μF solid tantalum on the output will ensure stability for all operating conditions. When the adjustment ...

10

Not recommended officially Arduino (the company) does not recommend directly supplying 5 volts in, because: The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5 volts to the 5V line, and blowing things, causing customer service ...

10

"The older high-dropout regulators with N-type transistors didn't seem to have this problem. " The answer is as follows: The npn-type transistor used as a control element is operated in a common-collector configuration (collector potential must be higher than that of the emitter). In contrast - as shown in the figure (provided by efox29) - the pnp type has ...

9

Three-terminal linear regulators usually can't sink current - the 5 volt regulator can't pull the ground terminal of the 10 volt regulator down. It can only pull its output up to 5 volts. If you put a small load on the 5 volt line, that should pull the 5 volt line back down to 5 volts (if the load draws more than the 10 volt regulator's ground current). ...

9

Generally (though there will be exceptions) references have better specifications than regulators. Included in those specifications are ... temperature stability input voltage stability output load stability (stability is a big thing with references!) output noise ... as well as initial accuracy. Though you will often find that some references have ...

9

It is never a good idea to power anything from a resistor divider. Any variation on the load will change the voltage, and your MCU will very likely vary its current consumption based on what it is doing at any time. This means your supply voltage will be affected, and as you are wanting to go to 1V8 (the lowest acceptable voltage) then this is an extremely ...

8

Using a switcher instead of a linear regulator is a no-brainer without even doing the math. Also a LDO specifically is silly, since your problem is that you have a very large drop range. Any linear regulator would do, not just low-dropout ones. In case some still need convincing, let's do the math. A linear regulator will drop 19 V, which times 100 mA is ...

8

You should follow the recommendations in the datasheet. If you don't, you risk oscillation. The regulator starts to oscillate - the output goes up and down. When I've had it happen, the regulator also got hot. The datasheet of the MCP1700 recommends a 1uF capacitor on the ouput. This is a minimum. It also recommends the capacitor be located as close as ...

8

The noise is internal to the LDO and is always on the output whether you power it directly from an SMPS, a battery, or the most perfect noiseless voltage source. The PSRR is how well the LDO can stop ripple injected into its input from passing straight through to its output. Really, it's how fast the control loop inside the LDO can react to adjust its ...

7

You are assuming the capacitor will be a true short, which it won't be, the voltage will never rise infinitely fast - remember there is inductance and resistance in real life to limit things. If we look at the formula for current through a capacitor: $I = C \cdot \dfrac{dV}{dt}$ We can see that I depends on the cap value and how fast the voltage source ...

7

At the bottom of Table 6.5, on page 6 of the datasheet, it says: (1) Minimum V$_{IN}$ = V$_{OUT}$ + V$_{DO}$ or 1.7V, whichever is greater. So V$_{IN}$ cannot be less than V$_{OUT}$. So it is a regular LDO. The datasheet is not written very well.

7

Do not ignore the power up sequences specified by chip manufacturers. They specify these because that is what it takes to ensure that the device powers up correctly and will run reliably. If you ignore sequencing you can cause internal chip latch-up if voltage rails and reset/power good signals come in the wrong order. If you are lucky the latch-up can be ...

7

Alternative design concept: simulate this circuit – Schematic created using CircuitLab You may be able to eliminate R2 if the minimum draw from other circuitry is similar (a few hundred uA). Or find a regulator that sinks current. Eg. LT1118

6

The best way to deal with a LDO that requires a minimum ESR output cap is to use a different LDO. These LDOs were designed when anything more than a few 100 nF was too big for ceramic or any other technology that has very little ESR. Tantalum was the usual choice for a few µF to a few 10s of µF. These had some inherent ESR, so LDOs were ...

6

It looks to me like a netlist/layout error: If you are placing your fixed resistor on the dig pot footprint, it would also be messed up. Looks like you accidentally wired the CO+ and NO2- together and the NO2+ and CO- together so the LDO(Vout-Vadj) is not across a resistance as you believe.

6

I can't find a picture of an SOT223, but here is a TO-220 which follows the same principle: You can see how the tab and the middle pin are one and the same piece of metal. As others have rightly mentioned, this is for heatsinking purposes, and is often required. It is also used in some packages for high current connections, where you may have the tab as ...

6

If you're asking about the table on page 2 (on which figure 2 ain't located), that's beacuse the datasheet covers multiple physical devices, which are actually called that way, i.e have that suffix on their part number. Only the one without any [voltage] suffix is the "officially" adjustable one. But as The Photon explained the difference is somewhat ...

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