For an AddOhms series, I created a DIY Arduino I am calling the “Pyramiduino.” It is an ATmega328p based board in the shape of a triangle. Other than being cute, the shape does not offer any other benefit. The design features a 3.3 volt LDO Regulator, which is also the subject of this post.
I forgot a fundamental aspect of design: read the freaking datasheet. The board’s LDO regulator was not turning on. Adding a passive scope probe to the circuit suddenly fixed the problem. The regulator turned on. When touching the enable pin, it measured about 1.25 volts. While I am sure Rohde & Schwarz would like me to ship scope probe with each board, that was not an option. With the impractical fix in place, I got to thinking about that voltage level. I remembered that the datasheet mentioned about 1.2 volts was needed for the “HIGH” threshold. Which meant, 1.25 volts applied to the pin enabled an active low input. Not only that, I remember the datasheet clearly said it had a pull-down resistor built-in. What was going on?
One day I am going to build a library of standard parts I use in all designs. That way, I do not confuse myself with bits of new knowledge. Until then, my current method is to go to Digi-Key, filter by in-stock, and sort by price. At least, for most components. That is how I found the Micrel (now Microchip) MIC5504. It is a 3.3 volt LDO Regulator with an output capability of 300 mA. It comes in a SOT-23-5 package, which includes an enable pin. I checked the datasheet to see how the enable works and saw the following description.
See the problem? If not, here are five things I learned you need to consider when picking an LDO Regulator. These characteristics go beyond the voltage ranges and current capability. The answer to my board’s problem is on this list.
1. LDO Regulator Packages: Small Transistor Outline (SOT)
When picking an LDO, it is widespread to see them in a SOT package variant. You might see package names like SOT-223-5, SOT23-3, and the SOT23.
SOT23 vs. SOT23-5 vs. SOT223
The SOT23 is a 3-pin package common for discrete transistors and diodes. A variant, the SOT23-5, is used for ICs, including voltage regulators. You might also see the industry name, set by JEDEC, which is TO-236AB.
A larger version is the SOT223 and SOT223-5. The SOT223 is a 4-pin package, which is why it is sometimes labeled “SOT223-4.” The SOT223-5 is a 7-pin package! Just kidding. The SOT223-5 has 5 pins.
Technically one pin of the SOT223 packages is a tab. These help with creating a heat sink from the internal silicon to the external PCB. Despite this feature, I chose a SOT23-5 package for my design. I wanted the smaller package, and I wanted an enable pin.
2. LDO Regulator Pinouts aren’t the same
I know, the obvious statement is obvious. However, if you only look at a few LDOs with similar packages, you might quickly assume that the pinout is the same for all of them.
In my comparison research, I found seven 3.3 volt LDO regulators with at least 300 mA current capability. Of those, most of th epins were in the same place. But that does not mean you can just glance at the pinout and say “Oh, the output is pin 5” and then assume everything else is where you expect it.
Even if the pin names or functions match, there could still be differences. In my case, those differences caused a headache in the finished Pyramiduino design.
3. NC, Bypass, and Adjust
Across the seven LDO regulators I surveyed, I found four functions for the “extra” pin. They were either a no-connect, reference bypass, or the adjustable input.
- No Connect (NC) means what it says. It is not necessary to connect the pin. Pay careful attention to the datasheet. Sometimes the chip’s designers say: “do not connect to anything, including ground.”
- Reference Bypass improves the ripple rejection for the output. An LDO regulator that breaks out the internal voltage reference to a pin, offers you the chance to add a tiny value capacitor. This filter capacitor can improve the regulator’s response. Regardless of what the data sheets suggest, I would only use a C0G type ceramic capacitor here.
- Adjust input is only available on regulators with an adjustable output. Instead of a fixed internal feedback look, this pin lets you feedback the output through a voltage divider. This divider sets the output voltage for the LDO regulator. Sometimes data sheets recommend a small value capacitor in this network to help with ripple rejection. Follow their guidelines.
- MCP1824 Only: Power Good provides a signal based on the enable pin. The datasheet goes into depth about how it behaves.
Even considering “power good” as an oddity, I do not think any of those functions are a surprise. FYI, in most cases, it is safe to leave a bypass-pin unconnected. The pin that surprised me is the enable pin!
4. Not all LDO Regulator Enable pins are the same
On a past LDO-based project, I noticed that LDO regulator data sheets would sometimes refer to a pin as either “enable” or “shutdown.” A Vinculum, or bar, above “shutdown”, indicates its active low status. I just assumed the terms were interchangeable.
After reviewing seven different LDOs, I did notice there could be a slight difference between how “enable” and “shutdown” work. Six of the seven LDOs specifically described how the regulator behaviors when disabled.
When disabled, most LDOs connect the output pin to a resistor connected to ground. This path discharges the output capacitor(s). Different datasheets call out the value of this resistor differently. Curiously in the TI TLV733P, it is called a “pull-down resistor.” Grammatically that name makes sense. I do think it could confuse some people because most of us associate pull-down with inputs, not outputs.
A datasheet will either give you a value for this resistor, which seems to range in the 25-150 ohm range, or it just provides graphs. The left graphic above is the most common I saw. If you know what capacitor charge and discharge graphs look like, it looks pretty familiar. However, RC curves are dependent on both the resistance value and the amount of capacitance. So your output voltage, the load connected, and the size of the capacitor affects the curve.
In this batch, the only LDO regulator to show different curves is the ON Semiconductor NCP4625. The right graph shows different curves for VIN and VOUT, along with different IOUT currents. (I am only showing one of the VIN-VOUT graphs there.) This behavior and resistance value is something to keep in mind in your design.
Come to think of it. This resistor could be why injecting power into a node connected to a regulator’s output is a bad idea. I’ll have to come back to that in the future.
5. Pull-down Vs. Floating
Pull-up (and pull-down) resistors are something I have covered quite a bit. Including, how to pick their value. When it comes to the LDO regulator’s enable, or shutdown, pin different manufacturers handle the pin’s unconnected state differently. Some chips include a pull-down resistor, like the MIC5504 I picked and some require you as the user to handle it.
This feature alone means you should take care when replacing a regulator in a design. If the old regulator had a pull-down built-in, you need to make sure the new one does not leave the signal floating. In the seven I looked at, only the MCP1824 never mentioned whether or not you should include a resistor. The clue is that the performance curves have a note about using an external 10 Kohm resistor.
In cases where you do not care about the enable function, you can tie the pin to the input voltage and forget about it.
Oh yeah, my mistake
Now that I have gone through those five LDO regulator considerations, I need to come back to what I screwed up. When I read the MIC5504’s datasheet, I only looked at the electrical characteristics. I did not even bother to read the first page. (Why would I? It is just marketing fluff anyway, right?) The pin description table tells a different story. However, it was too late.
When I read “enable pull-down resistor,” my brain said, “oh, the enable pin is active low.” At the time, it did not occur to me how stupid that statement sounds. Why would a regulator with an enable pin, permanently enable it? If you do NOT care about the enable, you might use a SOT223 or SOT23 package which does not include the pin. If the pin is there, connecting it to VIN is not an issue. Traces on a PCB are free.
An LDO regulator enable-, or shutdown-, pin only exists in situations where you want to disable it.
The genesis of this post was from me looking for SOT23-5 LDO Regulator with active-high enable connected with an internal pull-up resistor. After about an hour and a half on Digi-Key, I realized this unicorn does not exist. And it should not.
Lesson learned, read the entire datasheet. A secondary lesson, consider why pin functions exist across manufacturers.
LDO Regulator Links
Here are the seven regulators I used in my comparison. The MIC5504 is the one I used in the design.
- Diodes, Inc AP2127
- Exar Corp SPX3819
- Microchip (Micrel) MIC5504-3.3YM5
- Microchip MCP1824T-3302E
- ON Semiconductor NCP4625
- Richtek RT9193
- Texas Instruments TLV733P