17mm / 20mm MTN-MAXlp
22mm MTN-MAX
The first "torture" subject! (earlier PCB revision).
22mm MTN-MAX
THIS POST IS A WORK IN PROGRESS!
I will continue filling it out as time permits. Please feel free to ask questions.
Special Thanks!
First off, I need to give special thanks to all of those who have helped along the way with this project and many others. I have learned how to do most of this stuff through the examples, successes, failures, and other contributions of many others on this forum and elsewhere.
What Is It?
A low profile 17mm/20mm 22mm (20mm temporarily pulled) fully programmable buck driver, with a 4A maximum reliable output (with proper heatsinking and care) and low voltage protection. This driver is meant to fit in shallow 17mm pills, such as a P60. There is another version of this driver [upcoming] that is possibly good for 5A+, but requires more pill depth. The higher profile driver will also be higher efficiency at lower currents, due to the possibility of a bigger inductor.
What LEDs does it work with?
It is generally designed for a 3V LED (please see the note about maximum amperage, below), but it can also be used with 6V LEDs with some limitations. The driver is thermally limited with 6V LEDs since the wattage passing through the driver is higher than with a 3V LED.
I have not tested the upper limit yet with 6V LEDs, but with the 17mm driver I am guessing that ~3A is the upper limit with 3S input. I have only thoroughly tested 2A with 2S and 3S input. The inductor chosen will ideally be different for a 6V LED, especially with 2S cells, since the input and output voltages are closer together.
Does It Work?
Yes. While there's no guarantee that it will work in your particular setup, or with your particular components, this driver has been tested thoroughly at currents from 2A up to 3.8A to the LED. Anything above 3A requires particularly good heatsinking to the pill.
Specifications:
- Low profile (only [mm] total height height, without a spring)
- Configurable for 1A to 4A peak output, depending on the inductor and sense resistor combination
- Can be configured for 2S-6S cells (5V-28V), depending on the inductor and capacitors used
- Efficiency decreases and as the difference between the input and output voltage increases
- Can be used with pretty much any emitter configuration, provided that the input voltage is greater than the output voltage
- Not recommended for momentary use (high quiescent current drain)
- RSENSE: Sense Resistor
- 17mm: 2010; 1W or Higher
- 20mm: 2512; 1W or Higher
- Formula: 0.2/R=I
- An Additional Word of Caution Regarding Maximum Amperage. The MAX16820 is a hysteretic converter with a 10% current ripple, meaning that from the center, or average, current set point, the LED will momentarily experience an additional 5% current over the average set current. This means that if you have an LED that can only handle 6A, you need to set the sense resistor at least 5% below that number.
- Example: 6.1A maximum LED, 0.033 Ohm resistor. If all of the components were ideal and perfect, the 0.033 Ohm resistor should give us 6.06A average current. This sounds safe, but it's not. The LED will see peak currents of up to 6.36A, which will blow our theoretical 6.1A maximum LED.
- You also need to take into account the precision of your sense resistor, temperature variances, and several other factors.
- Summary: don't specify a sense resistor that is on the ragged edge of what the LED string can handle.
- ~2A = 0.1 Ohm
- ~3A = 0.068 Ohm
- ~3.4A = 0.058 Ohm
- ~4A = 0.05 Ohm
- CIN: Input Capacitor
- 2S-3S cells: 1206, 10uF, 16V (or higher), X7R
- 4S-5S cells: 1206, 10uF, 25V (or higher), X5R or better
- CMAX: Buck IC Bypass Capacitor
- 2S-3S cells: 0805, 10uF, 16V (or higher), X6S or better
- 4S-5S cells: 0805, 10uF, 25V (or higher), X6S or better
- C_OUT: Output Capacitor
- Optional, can slightly lower ripple.
- 0805, 1uF, 10V (or higher if needed for higher output voltage), X7R
- CVCC; CMCU: LDO Output/MCU Bypass
- 0603, 10uF, 10V (or higher), X5R or higher
- OTC: Off-time Capacitor
- 0805, 1uF, 10V (or higher), X7R
- MCU: MCU attiny13a-MMU
- MAX: Buck IC
- Maxim Integrated MAX16820
- FET: Main Switch N-Channel MOSFET
- NXP PSMN2R4-YLDX LFPAK33
- D1: Flyback/Freewheel Diode
- 17mm: Diodes Inc. SBR3U40P1-7
- 20mm: Diodes Inc. PowerDI-5
- L1: Inductor.
- INDUCTOR CHOICE DEPENDS ON SEVERAL FACTORS
- Difference between input and output voltage
- Target switching speed (must stay within ~0.3MHz and ~1.5MHz, but as switching speed increases, switching losses also increase, decreasing efficiency)
- Resistance vs. inductance vs. size (you can usually only have two of these, so you have to prioritize based on the application and size restraints)
- Inductor must be able to handle the peak inductor current without saturating
- MAXIMUM SIZE= ~6mm x 6mm. Pads are designed to allow many different inductors to fit and work. I have tested this driver with over 10 different inductors with different pad layouts.
- RECOMMENDED INDUCTORS:
- Coilcraft XAL series inductors are the best performance for their size, but they are also the most expensive, at around $2.50 each, plus shipping. The XAL6060 series work great!
- Other inductors from Abracon, Bourns, and others will also work, but generally have more on resistance for a given inductance and size
- 2S Cells:
- 1A: 15uH-22uH
- 2A: 8.2uH-11uH
- 3A: 6.8uH-8.2uH
- 4A: 4.7uH-6.8uH
- 3S Cells:
- 1A: 18uH-22uH
- 2A: 8.2uH-15uH
- 3A: 6.8uH-10uH
- 4A: 4.7uH-8.2uH
- R1: 36K 0603 1% (or better)
- R2:
- 2S-3S: 4.7K 0603 1% (or better)
- 4S-5S: 2K 0603 1% (or better)
- RDIM: 100/200 ohm 0603
- PLDN: 19.1K 0603 (helps with mode switch timing; can also use 4.7K if faster switching is desired)
- RSENSE: Sense Resistor
- 22mm: 2512; 1W or Higher
- Formula: 0.2/R=I
- An Additional Word of Caution Regarding Maximum Amperage. The MAX16820 is a hysteretic converter with a 10% current ripple, meaning that from the center, or average, current set point, the LED will momentarily experience an additional 5% current over the average set current. This means that if you have an LED that can only handle 6A, you need to set the sense resistor at least 5% below that number.
- Example: 6.1A maximum LED, 0.033 Ohm resistor. If all of the components were ideal and perfect, the 0.033 Ohm resistor should give us 6.06A averagecurrent. This sounds safe, but it's not. The LED will see peak currents of up to 6.36A, which will blow our theoretical 6.1A maximum LED.
- You also need to take into account the precision of your sense resistor, temperature variances, and several other factors.
- Summary: don't specify a sense resistor that is on the ragged edge of what the LED string can handle.
- ~2A = 0.1 Ohm
- ~3A = 0.068 Ohm
- ~3.4A = 0.058 Ohm
- ~4A = 0.05 Ohm
- ~5A = 0.04 Ohm
- ~5.5A = 0.036 Ohm
- ~6A = 0.033 Ohm
- C1: Input Capacitor
- 2S-3S cells: 1206, 10uF, 16V (or higher), X7R
- 4S-5S cells: 1206, 10uF, 25V (or higher), X5R or better
- C2: Input Capacitor (optional)
- 2S-3S cells: 0805, 1uF, 16V (or higher), X7R
- 4S-5S cells: 0805, 1uF, 25V (or higher), X5R or better
- CMAX: Buck IC Bypass Capacitor
- 2S-3S cells: 0805, 10uF, 16V (or higher), X6S or better
- 4S-5S cells: 0805, 10uF, 25V (or higher), X6S or better
- C_OUT: Output Capacitor
- Optional, can slightly lower ripple.
- 1206, 1uF, 10V (or higher if needed for higher output voltage), X7R
- CVCC: LDO Output
- 0805, 10uF, 10V (or higher), X5R or higher
- CTINY: MCU Bypass
- 0805, 10uF 10V (or higher), X5R or higher
- OTC: Off-time Capacitor
- 0603, 1uF, 10V (or higher), X7R
- MCU: MCU attiny13a-MMU
- MAX: Buck IC
- Maxim Integrated MAX16820
- FET: Main Switch N-Channel MOSFET
- NXP PSMN2R4-YLDX LFPAK33
- D1: Flyback/Freewheel Diode
- 22mm: Diodes Inc. POWERDI-5
- L1: Inductor.
- INDUCTOR CHOICE DEPENDS ON SEVERAL FACTORS
- Difference between input and output voltage
- Target switching speed (must stay within ~0.3MHz and ~1.5MHz, but as switching speed increases, switching losses also increase, decreasing efficiency)
- Resistance vs. inductance vs. size (you can usually only have two of these, so you have to prioritize based on the application and size restraints)
- Inductor must be able to handle the peak inductor current without saturating
- MAXIMUM SIZE= ~6mm x 6mm.
- RECOMMENDED INDUCTORS:
- Coilcraft XAL series inductors are the best performance for their size, but they are also the most expensive, at around $2.50 each, plus shipping. The XAL6060 series work great!
- Other inductors from Abracon, Bourns, and others will also work, but generally have more on resistance for a given inductance and size
- External inductors also supported with through-hole vias
- 2S Cells:
- 1A: 15uH-22uH
- 2A: 8.2uH-11uH
- 3A: 6.8uH-8.2uH
- 4A: 4.7uH-6.8uH
- 5A: 4.7uH-6.8uH
- 6A: 4.7uH-6.8uH
- 3S Cells:
- 1A: 18uH-22uH
- 2A: 8.2uH-15uH
- 3A: 6.8uH-10uH
- 4A: 4.7uH-8.2uH
- R1: 36K 0603 1% (or better)
- R2:
- 2S-3S: 4.7K 0603 1% (or better)
- 4S-5S:2K 0603 1% (or better)
- RDIM: 200-4.7K ohm 0603
- PL: 19.1K 0603 (helps with mode switch timing; can also use 4.7K if faster switching is desired)
Parts Shopping Cart Links:
17mm v1.01 Mouser - 2S-3S Cells - NO INDUCTOR OR SENSE RESISTOR
17mm v1.01 Mouser - 2S-5S Cells - NO INDUCTOR OR SENSE RESISTOR
17mm v1.01 MTN Electronics - 2S-3S Cells
20mm v1.01 MTN Electronics - 2S-3S cells