Understanding and troubleshooting the mini cp 3in1: Power supply circuit

[zzyzx]

The mini cp power supply circuit is powered by a 3.7v lithium polymer cell. `The analog portion of the circuit gets its power straight from the battery. This trace extends from the b+ and is piped through the motors and servos along the edge of the board and ends where it is coupled by a tantalum capacitor to the regulator providing power for the digital circuitry .

The regulated portiion of the circuit begins at the lt1615 regulator which is the 5 pin sot23 at the bottom of the board (labeled LTIZ). This is an adjustable switching regulator that is controlled by a feedback loop. It has several companion parts which are necessary for proper operation. There is a capacitor on both the input and output lines. The input bypass capacitor is between the b+ and regulator, it is a ceramic cap, value is about 5 microfarads. The output cap is above the coil and diode on the right front of the board. The control circuit branches from the output line to a 1 megaohm resistor. The feedback pin gets its input from this point (after 1M before 250k). The circuit terminates at the board gnd after being pushed through another resistor, this one valued at 250 kOhms. The output circuit is along the bottom corner of the board and frequently falls victim to crash damage. The inductor is forward biased by the schottky diode. Output voltage is present across the output cap. Comparator circuit turns the ic on when voltage is below 1.23 volts @ the feedback pin. Output is sourced from the output cap or the inductor depending on the state of the ic. See datasheet for additional information.

Pinout-LT1615 (as installed on mcp)

3-----2-----1
LTIZ

4-------5

1 - Switch - Collector of internal npn power switch

2 - Ground

3 - Feedback - 2 resistors set output voltage - 250k located between b- and regulator ic / 1M directly above ic

4 - Shutdown - Shuts down ic when tied low (> 0.25v) - connected to b+ on mcp

5 - Input - Connected to b+ - bypass cap = 5uf between b+ and ic



The output of the lt1615 is then conditioned by the lp2981(labeled L04A-5 pin sot23 located above inductor), which is a fixed voltage regulator rated @ 3.3 volts. Output is coupled by a tantalum capacitor to battery voltage. See datasheet for additional information.

Pinout-LP2981 (as installed on mcp)

5---------------------4

L04A

1--------2---------3


1 - Input

2 - Ground

3 - On/off - Regulator on when this pin is tied high

4 - Not connected

5 - Output


Some notes on troubleshooting / repair:

If your mini cp will not power on
- check that battery voltage >3.7v && <=4.2v - if ok
- check for batt.voltage @ board - if ok
- check for 3.3v @ between pin 4 of lp2981 and ground- if ok - problem is probably not in this circuit / if voltage is less than 3.3, check output of lt1615 across its output capacitor

- if 3.3v is available and still won't power on, check for mcu activity @ led output. Not likely but I did see a crash damaged led.

- you can check for an open inductor with an ohmmeter connected to b+ / diode above inductor

- use low esr capacitors - it's important

- Most parts are package size 0402

- 8 in of 30awg magnet wire (or similar) can be subsituted if the inductor is damaged or lost. For a neater look try winding it around a shaft.

- The supply circuit is the most common cause of mcp electronics failure, the supply and motor driver circuit are responsible for an overwhelming majority of all mcp electronic failure. They're the ones that handle all the current.

- Work on the back side of the board when servicing the battery leads, it's really easy to disturb nearby components in front. Most of the nearby components will result in a dead board if dislodged and forget about finding them if they fall off.

- Anytime repairs are performed it is a good habit to check the resistance across the battery cables before connecting power, it might not tell you whether it will work but at least you'll know it wont burn (probably.. haha). Normal resistance is around 4-5k ohms.

Want moar??? Comments and contributions are welcomed. All information in this post independently gathered and is not guaranteed to be comprehensive or accurate. As always, you are responsible for verifying accuracy of information and safety of procedures before attempting service.

[ChitownMatt]

Awesome. Much thanks!

[sbstnp]

Many thanks!

[kohansey]

That was an excellent explanation of the MCPs power circuitry. I was wondering, do you know what Digikey part I could use to replace the inductor on the bottom corner of the board? Or do you at least know its inductance?

[RogerRabit]

You find the values and part numbers from several manufaturers in the pdf of the parts spec. You can use between 4,7 - 10 µH. It is not critical.
http://cds.linear.com/docs/Datasheet/16151fas.pdf
Page 4 table 1
You can order them from rs-components or any other big distributor.
They cost near nothing.

Rewinding it as aircoil 8" wire wound around 1.5mm works fine too.

[kohansey]

My place of employment had one of these available that I through onto the board.

445-6396-1-ND

I'm going to go home tonight and test it out. It looks to be a better replacement and less likely to be knocked off in a crash.

[kohansey]

Well with the new inductor, I am able to get the board to power up and bind. I am also able to fly with no noticeable effect in performance. However, it seems to be exhibiting a new behavior after a rather nasty crash early in my first battery. When the voltage drains to somewhere around 3.7V the heli dramatically loses power and I had to land almost immediately. This has happened to me on two batteries so far. On the last battery, the whole board shutdown and won't respond until i replugged the battery. Thankfully I was hovering at that point. The heli just dropped to the ground. I had the mod done to two of my receiver boards. I'll try my other board and see if it is having the same problem or if it was something else from the crash.

[RogerRabit]

Look into the spec sheet of the low drop regulator zzyzx posted.
There is only certain coil manufacturers that are ok.
Or use a airwound inductivity.

Your inductivity may have the right value but not the necessary current specs.

use the part number i gave you, it is a cent part available from your distributor also.

[kohansey]

I tried to find the datasheets for all the parts listed in table 1 on page 5. However, I haven't been very successful. It appears that all the parts listed have been deprecated and replace with new versions. I haven't been able to find any info about substitutes for the deprecated parts. Just searching through murata's product selection makes it hard to determine what is the best drop in replacement.

[kohansey]

I'm still not convinced its the inductor. I'm going to try my second rx board today and let you know.

[RogerRabit]

Everything is explained in the section "selecting a inductor".

Your inductor has to high resistance and too low saturation current.

you need min 350mA saturation current, read the operational principles in sec 1 of the pdf.

Use a airwind own wound coil. You need 4mm diameter, 2mm approx lenght, 24 windings fo 5µH.
Half of that works too.

michael

[zzyzx]

The stock inductor is about 5uH. It is matched to the output cap but there is some wiggle room. Without looking into your problem too closely, it indeed does sound like it could be related to this ic.

[zzyzx]

after taking a look, I agree with Roger rabbit. too high of resistance on that inductor. the board is shutting down when it is trying to source output from the. inductor. should have no resistance ideally, in practice should be as little as possible. The lt1615's purpose in this application is to prevent voltage from dropping too low. It ensures stable input for the 3.3v regulator because power is stored in both the output cap and inductor for use during adverse conditions (i.e. supply voltage drop due to high current demands).

I have tried using a core with the inductor, it is not necessary and there is no discernable difference in operation if you include one. Resistance should be less than 1 ohm across the coil.


edit: for those who don't own an LCR meter. http://daycounter.com/Articles/How-T...ductance.phtml

[kohansey]

Thanks for your help guys. Thanks to Roger, I decided to go with this inductor

http://www.digikey.com/product-detai...1-1-ND/1008236.

It should be coming in today, so I will slap it on and test it out tonight.

[RogerRabit]

Tell us the progress.

[kohansey]

It sucks living at the shore, its windy all the time. Hopefully AccuWeather is correct and the winds die down tomorrow evening. If that is the case, I'll have an answer for you then.

On a side note, the new inductor fits on there nice a little smaller than the stock inductor with the windings enclosed an nice SMT package.

[RogerRabit]

Wind does not matter for the mini cp as long as it is not a thunderstorm.
I flew yesterday my 700 size nitro and the mini cp in unpleasant winds.
The mini cp is dowing awsome.

[RogerRabit]

@zzyx

One of my boards had following failiure:

In the spool up process there was a reset of the cpu.
I checked and the voltage after booster is just around 4 V, on the other boards is near 4.8V.
I exchanged the booster and the coil also.

Now Voltage is around 4.1 V (but because of the measuring with the scope it might be not changed at all - did not measure that precise.
But the Voltage after the Low Drop is 1 V around?!?!?

I changed the low drop but that didn´t change a thing.

I do not see any shorts and a short would not allow 1V after the low drop regulator.

The PWM Signal from the booster is ok and if i solder out the low drop regulator, the voltage at the low drop´s regulator input is 4V arround.

Where is my error?

Michael

[kohansey]

You are right, I am able to find my mini in upto 10 mph winds (AccuWeather reading), but I am fighting the wind more when it is above 10 mph.

Needless to say, there is something terribly wrong with my receiver boards. I tried the new inductor on the first board (board A). When I would power up the board, I was getting glitches with telemetry. This board was brand new, but had the inductor ripped off in storage . I had tried the previous inductor on this same board, so I assumed I messed up something related to telemetry with the other inductor. The heli would fly normally with this board until I reached 3.7 - 3.6 volts on the battery. It would then lose altitude and didn't have enough power to get it back into the air. By the second or third battery the board would reset when it reached 3.6-3.7 volts causing the heli to crash unexpectedly. Because of this I decided to go to my second board. The second board (board B) had the inductor ripped off in a crash. The hot glue I used was too cheap and didn't offer any protection . Anyways, I tried the new inductor on this board as well. When I powered up the board there was no problem with the telemetry, everything seems copacetic. I starting practicing my figure 8s. At first, when I would go into a banked turn, the heli would lose power and I would have to bang the throttle just to keep it from hitting the concrete. By the third battery the heli started exhibiting a new behavior. When I would go into the bank turn, the heli would jump in mid-air then fall straight to the ground. I moved to a fresh battery and it did the same thing. On my last battery the heli crashed into the roof. When I finally got it back down and tried to power it up, it would no long stay powered. When I would give it any throttle the board would turn off. I noticed if I pressed on the inductor the board would power back up, but it still wasn't flyable.

I contacted Walkera and they no long offer free repairs and recommended that I just buy a new board instead. I have seen posts from other forums members that zzyzx can repair boards. zzyzx, do you think you would be able to recover these receiver boards? For a price I presume. I'd hate to throw them away, but with no one able to repair them, they are just paper weights.

[MiniCPflyer0110]

Hi everyone! This thread is great thank you so much for creating it. So I did an extremely quick solder job on the black power lead about 30 seconds before I jumped into the car. (Wanted to take it to NJ for thanksgiving...) anyway as it didn't work there I wasn't sure what the problem could be. Originally, the gyro didn't work (servos didn't react to movement as did tail). But on the second try in Jersey, it worked but with limited power. It would barely go up when I wanted it to. Longer story short, the black wire fell off, a tiny black thing was attached to it, now it's missing. My guess is that since I negligently soldered the black power lead onto that certain piece (of which I don't know what is called) it caused the helicopter to act funny. Before the power lead fell off, couple hours ago, the board and it's LED were acting strange and didn't spin the motor. If someone can explain this issue or let me know what the part is called and if it's definitely important (lol just hoping..) it would help a lot. I really don't know much about electronics cause I'm young so go easy on me