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Simple, cheap and efficient L78xx replacement

Status: Proposal
May 8, 2015

You might know this following problem: You want to build up your new project on a breadboard and you need a 3.3V power supply from a higher voltage. As usual you will pick a linear regulator for testing and plan to use a buck converter in the final design, but this could make a lot of trouble in the aftermath. Because of this reason I developed a simple buck converter to replace a linear regulator like the L78xx-series. The converter module is pin-compatible to the 78xx-series and has the same size as a TO220-package. This module is based on a MCP16301 and can deliver up to 600mA which is enough for most applications. Only one IC, one coil, three resistors, three capacitors, two diodes and one pinheader is needed to build up a DC-DC converter with a efficiency up to 96%. The total circuit is pretty simple: R1 and R2 are the voltage divider to set desired output voltage according to the equation 5-1 on page 17 in the MCP16301 datasheet. To choose the right inductor for your output voltage pick one according to the table 5-1 on page 17 in the datasheet. C1 and C2 are buffer capacitors to compensate the ripple current through L1. D1 and C3 is needed for the bootstrapping inside the MCP16301. D2 conducts the current through L1 while the SW-pin of MCP16301 is switched off. R3 sets the EN-pin to high to switch the switching converter on. The following list shows the total bill of materials for a 3,3V module:

IC1              MCP16301-I/CHY              1x          1,22€
R1               1MOhm - 0603                   1x          0,02€
R2               31,2kOhm - 0603               1x          0,02€
R3               10kOhm - 0603                  1x          0,02€
D1               1N4148                              1x          0,03€
D2               SS14L                                1x          0,07€
L1                15uH                                  1x          0,45€
X1                3-Pin Header                     1x          0,30€
C1                4,7uF - 1206                      1x          0,79€
C2                10uF - 1206                       1x          0,79€
C3                100nF - 5,08mm                1x          0,20€
                                               Total:  11 pieces,  3,91€
(all pieces from Farnell with german taxes, based on prices of 10 pieces or less)

Based on this design I developed a symmetrical power supply with one regulated and one unregulated voltage output. Just replace L1 with a subminiatur transformer like the LPD-series from Coilcraft and add a single bridge rectifier D3 with a buffer capacitor C4. Now you will have no more trouble with generating negative voltage for your Opamp designs.

These are the additional/replacement parts for this module: 

L1            LPD - Desired Val       1x   +1,10€
D3           SS14L                          1x     0,07€
C4           4,7uF - 1206                1x     0,79€
                               Total: +2 pieces,  +1,96€

The last module is a step up converter to power your device from a single NiMH/NiCd battery. This module has the same size and the same pinout like the previous step down converter! This module is based on the MCP1640 and might be very interesting for single solar-cell powered systems, because it can deliver your wished output voltage from a input voltage as low as 0.65V (once it powered up). So your device can even be powered during sunset with a pretty fair efficiency up to 96%.

Here is the part list for a 3,3V output step up module:

Here is the part list for a 3,3V output step up module:

IC1           MCP1640-I/CHY      1x   0,54€
L1             4,7uH                       1x   0,45€
R1            976kOhm - 0603      1x   0,02€
R2            562kOhm - 0603      1x   0,02€
X1            3-Pin Header            1x   0,30€
C1            4,7uF - 1206             1x   0,79€
C2            10uF - 1206              1x   0,79€
                               Total: 7 pieces, 2,91€

I hope you like my small and handy boards and I wish a lot of fun by reproducing them!

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