New voltage regulator

[jlamperez]

New voltage regulator

Since the voltage regulator we have now only gives 300 mA maximum current to the output load, this week I was searching for a new one with an output current between 1 and 1.5 A, in order to power the raspberry pi, the ADC and the temperature sensor.

The raspberry pi more less needs 600 mA to operate correctly and then 100 mA more for each USB. The raspberry pi one only has one mini USB but because we are working also with the one and two and using three USB connections for mouse, keyboard and Wifi I will do the operations for the worst case.

Maximum current needed in the worst case: Imax = 600 mA + 3x100 mA + 300mA(old regulator) = 1.2 A.

So based on this calculation I started the search.

LDO regulators

1. LM340-N/LM78XX Series 3-Terminal Positive Regulators: 5V output and If adequate heat sinking is provided, they can deliver over 1.0A output current.
2. µA78xx Fixed Positive Voltage Regulators: 5V output and Output Current up to 1.5 A.

Calculations: Power dissipation: Pdmax = (Vi - Vo) x Io , Iq of the linear regulator is ignores due to its relative small size to Io. Device's thermal resistance from junction to ambient: Θja max = (Tj – Ta)/Pdmax

So for and ambient temperature of ->Ta = [25 and 35], junction temperature in datasheet of 0-125ºC -> Tj = 125-15 = 110ºC.

LM340-N/LM78XX: Pdmax = (12 - 5) x 1 = 7W and Θja max = (110 - 35)/7 = 10.7 ºC/W. If we compare this values with the thermal resistance from junction to ambient for different package:

• TO-3 Θja = 39ºC/W
• TO-220 Θja = 54ºC/W
• SOT-223 Θja = 174ºC/W
• DDPAK/TO-263 Θja can be reduced by heatsink or increasing the PC board copper area thermally conneced to the package. Using 0.5 square inches copper area Θja is 50ºC/W with 1 square inch copper area Θja is 37ºC/W and with 1.6 more inches of copper area Θja = 32 ºC/W.

µA78xx: Pdmax = (12 - 5) x 1.5 = 10.5W and Θja max = (110 - 35)/10.5 = 7.14 ºC/W.

• Θja = 25.3ºC/W

Therefore in all cases we can see that the Θjamax is lower to the Θja for every package, this means that we will need a heatsink for our PCB or move to a switching power solution. I will choose the second one.

Switching power solution

I use Buck topology regulator: This ones reduce DC voltage to a lower DC voltage. This kind of regulators not need heatsink like LDO regulators.

There are tow different options here, one would be to embedded a module directly to the PCD, and the other is to find a IC and calculate the surrounding components needed for this Buck regulator or Step-Down Voltage Regulator.

1. Embedded module Like the APM25 Power Module we can use The PTH08080W is a highly integrated, low-cost switching regulator module that delivers up to 2.25 A of output current.
2. LM2575 and LM2576
   • LM2575: 1A Step-Down Voltage Regulator.
   • LM2576: 3A Step-Down Voltage Regulator.

Here I have done the calculations for the LM2576.

We will need the next components besides the LM2576:

• Cin
• Schottky diode D1
• Coil L1
• Cout.

For our application Vout = 5V, Vin(max)= 12V, I load(max) = 1.5A.

• Induction selection A. From figure 24 of the datasheet LM2576(HV)-5.0. B. Identify inductance region. -> 12V 1.5A -> L150. C. 150uH

Schott corporation	Pulse Engineering	Renco
67127010	PE-53113	RL1954

• Output capacitor A. Cout = 680uF to 2000uF B. Cout needs to be between 680 and 2000uF because of stability and ripple. -> Choose 680 or 1000 uF standard aluminium electrolytic capacitor. Voltage rating 8V appropriate 10V or 15V recommended. 680uF and 10V choose.
• Catch diode selection(D1) A. 1.2 x 1.5 = 1.8 at least 1.2 -> I choose 3A current rating adecuated. 1.25 x 12 = 15 at least 1.25 -> Choose 20 V

Schottky
1N5820
MBR320P
SR302

• Input capacitor(Cin) 100uF 25V aluminium electrolitic capacitor.

Calculations

1. Maximum ambient (25-35ºC)
2. Maximum regulator power dissipation
3. Maximum allowed junction temp. -40ºC <= Tj <= 125ºC -> 110ºC.
4. LM2576 package thermal resistances θJA= 45ºC/W and θJC= 2ºC/W -> worst case package.

Total power dissipated by the LM2576 can be estimated as follows: PD = (VIN)(IQ) + (VO/VIN)(ILOAD)(VSAT) = 12 x 10mA + (5/12) x 1.5 x 2 = 1.37W.

When no heat sink is used, the junction temperature rise can be determined by the following: ΔTJ = (PD) (θJA) = 1.37 x 45 = 61.65.

To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient temperature. TJ = ΔTJ + TA = 61.65 + 35 = 96.95 < 110ºC.

So this calculations validate that it is not needed a heatsink now.

[jlamperez]

Excel with the links for the necessary components to use LM2576 switched regulator:

RPI_SmartServo_BOM.xlsx

[vmayoral]

Thanks @jlamperez. I'm ok purchasing those parts but the xlsx provided has broken links and they point at digikey.com (instead of digikey.es).

Can you provide the links again (a simple list here would do) in the .es domain?

[jlamperez]

Sorry, here you have in .es domain. The quantity column matches with the number of components to buy to build one prototype.

Quantity	Vendor P/N	Item Description	Link
1	553-1562-5-ND	FIXED IND 150UH, 2A, 100 MOHM TH, THROUGH HOLE	http://www.digikey.es/product-search/en?keywords=PE-53113NL
1	493-1495-ND	CAP ALUM 680UF, 20%, 10V, RADIAL THROUGH HOLE	http://www.digikey.es/product-detail/en/nichicon/UHE1A681MPD6/493-1495-ND/589236
1	565-4405-ND	CAP ALUM 100UF 20% 25V THROUGH HOLE	http://www.digikey.es/product-detail/en/united-chemi-con/EFL-250ELL101MH07D/565-4405-ND/5824608
1	LM2576T-5.0/NOPB-ND	IC REG BUCK 5V 3A TO220-5	http://www.digikey.es/product-detail/en/texas-instruments/LM2576T-5.0%2FNOPB/LM2576T-5.0%2FNOPB-ND/212636
1	1N5820DICT-ND	DIODE SCHOTTKY 20V 3A DO201AD	http://www.digikey.es/product-detail/en/diodes-incorporated/1N5820-T/1N5820DICT-ND/190969
2	2N7008-G-ND	MOSFET N-CH 60V 0.23A TO92-3	http://www.digikey.es/product-search/en?keywords=%092N7008-G-ND

[vmayoral]

ordered.

2016-04-25 12:28 GMT+02:00 Jorge notifications@github.com:

Sorry, here you have in .es domain. The quantity column matches with the number of components to buy to build one prototype. Quantity Vendor P/N Item Description Link 1 553-1562-5-ND FIXED IND 150UH, 2A, 100 MOHM TH, THROUGH HOLE http://www.digikey.es/product-search/en?keywords=PE-53113NL 1 493-1495-ND CAP ALUM 680UF, 20%, 10V, RADIAL THROUGH HOLE http://www.digikey.es/product-detail/en/nichicon/UHE1A681MPD6/493-1495-ND/589236 1 565-4405-ND CAP ALUM 100UF 20% 25V THROUGH HOLE http://www.digikey.es/product-detail/en/united-chemi-con/EFL-250ELL101MH07D/565-4405-ND/5824608 1 LM2576T-5.0/NOPB-ND IC REG BUCK 5V 3A TO220-5 http://www.digikey.es/product-detail/en/texas-instruments/LM2576T-5.0%2FNOPB/LM2576T-5.0%2FNOPB-ND/212636 1 1N5820DICT-ND DIODE SCHOTTKY 20V 3A DO201AD http://www.digikey.es/product-detail/en/diodes-incorporated/1N5820-T/1N5820DICT-ND/190969 2 2N7008-G-ND MOSFET N-CH 60V 0.23A TO92-3 http://www.digikey.es/product-search/en?keywords=%092N7008-G-ND

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[vmayoral]

@jlamperez, parts arrived. I'm taking a flight tomorrow and coming back next Thrusday. If you'd like the parts before Thrusday, reach out @imuguruza.

[jlamperez]

Can I go today to take them? Also I have some questions about the PCB I have designed it will be a problem?

[vmayoral]

That's fine. Can you come before 5PM?

2016-04-29 15:05 GMT+02:00 Jorge notifications@github.com:

Can I go today to take them? Also I have some questions about the PCB I have designed it will be a problem?

— You are receiving this because you commented. Reply to this email directly or view it on GitHub https://github.com/erlerobot/smart_motor/issues/36#issuecomment-215705839

[jlamperez]

Yes, but I need one hour more or less.

[vmayoral]

No problem.

2016-04-29 15:07 GMT+02:00 Jorge notifications@github.com:

Yes, but I need one hour more or less.

— You are receiving this because you commented. Reply to this email directly or view it on GitHub https://github.com/erlerobot/smart_motor/issues/36#issuecomment-215706250