MaxFlex Driver Technical Information

MaxFlex5 and MaxFlex5A Jan 2010 Latest MaxFlex5A (shipped 2010) increases output voltage to 29.5V and introduces 1300mA current table.

Note: MaxFlex5A uses the same PCB as maxFlex5 so the silkscreen will still say V5.0. The maxflex5A version increases the voltage to 29V by 3 component changes on the PCB rather than a change to the actual PCB design.

25 Jan 2010: New UI-UNI2 firmware released. Instruction manual is here maxflex5 V2 manual.

The new Thermal design White Paper (reading is highly recommended) can be found here: max4/5_thermal_guide.pdf

The picture below shows the top view of MaxFlex5. The exposed rectangular gold area just above the (C) TaskLED is a direct thermal interface to the bottom of the switcher IC (on the other side of the PCB). If the power dissipation of the MaxFlex5 board exceeds about 1W it is highly recommended to affix a heatsink or copper tab to a heatsink to the exposed area. The exposed area is at Ground potential - i.e. electrically connected to the battery GND and LED- pads. This means it is safe to mount the tab to a heatsink that connects to the body of a flashlight, IF the body of the flashlight is the same as the Battery negative or ground.

The two holes below VIN+ (anticlockwise) are both ground pads (input ground and LED- output). MaxFlex5 has LED- common to battery ground input.

The SWA hole below LED+ (anticlockwise) is one side of an external momentary action switch. The other side of the switch must be wired to SWG. It is CRITICAL where the switch Ground is wired on high current/high power LED configurations. MaxFlex5 has introduced the new SWG solder pad/hole that is specifically provided as the ground connection for the momentary action switch. The switch only switches a control signal and carries at most 300 microamps.

Note: it is recommended to keep the maximum wire length between the switch and maxFlex5 less than around 4" (10cm). Having wire leads that are too long can cause the wires to act as an antenna and cause maxFlex5 to respond irregularly or turn on/off by itself. Longer wires may work, it is up to the user to determine whether they work in their specific application.

Note: There are 2 GND connections provided on the PCB, but most installations will require 3 connections to ground (Battery negative/ground, LED- and ground for the STAT LED- if used). It is recommended to wire LED- and Battery negative to the same PCB ground hole and wire STAT LED- to the other PCB ground point. Both PCB ground points are equivalent.

Note the pin labeled STAT in the bottom/center of the board. Refer to the operating manual above for how STAT can be used to drive a Red or Amber LED to warn of low voltage.

Note, MaxFlex5 is a Boost regulator (step up), so input voltage must be less than the output voltage to ensure MaxFlex5 remains in regulation. If the input voltage exceeds the output voltage (at the dialed in drive current), MaxFlex5 will no longer regulate and the input voltage will go through the series inductor and schottky diode directly to the load. This will cause the output current to a LED to rise rapidly since LEDs have a very steep Current vs Voltage curve (Vf).

Note, Input_current should be around 3A or less for optimal performance of MaxFlex5. When running at high output power it is recommended to solder or use thermal epoxy to mount a tab to the pad on the back of the MaxFlex5 board (the pad is at ground potential) and to thermal epoxy/solder/bolt it to a heatsink or the body of the flashlight.

Below are some example measurements with 4 and 6 LEDs (older less efficient inductor of maxflex2).

Input Volts	Input Amps	# of LEDs	Output Volts	Output Amps	Efficiency
5.5	2.25	4	14.4	720mA	84%
6.58	1.82	4	14.4	720mA	87%
7.65	1.54	4	14.4	720mA	88%
9.78	1.18	4	14.4	720mA	90%
11.74	1.66	4	15.3	1190mA	93%
5.89	2.86	4	14.4	975mA	83%
7.07	2.29	4	14.5	982mA	88%
9.52	1.65	4	14.5	990mA	91%
9.41	2.91	6	23.3	1024mA	87%
12.0	2.21	6	23.3	1030mA	90%
14.3	1.84	6	23.2	1038mA	91%

Note: it is not recommended to run maxFlex5 at >3A input current, the above tests were to demonstrate the reduced efficiency at the higher input currents.

To determine the heat being dissipated on the maxFlex5 driver, calculate the output power as in the above equation (Step 1). Use the chart above to approximate the efficiency that the driver will be running at.

Lets assume 4 LEDs running at a nominal 1A from a 9.6V NiMH battery pack. From above we have the following:

As power being dissipated increases beyond 1.5W the thermal path to the heatsink becomes critical to reliable operation. Most of the heat being dissipated in the driver is from the switcher IC, U1 and the inductor L1. The thermal interface (the gold area) is tied directly to the bottom of U1 using thermal vias. Mounting an adequate heatsink with a non-conductive thermal epoxy is recommended. Use a "thin" smear of non-conductive thermal epoxy (like Arctic Alumina) rather than a thick layer.

Additional heatsinking to the top surface of U1 (the plastic package) can help a little, but not nearly as much as providing a good heatsink to the gold area.

Beyond 2W of dissipation thermal management becomes more challenging and it is highly recommend that the thermal protection be enabled in the menu system. Start with 50°C in the menu and refine as needed. A thermal path from the top of the inductor will enable reliable operation at up to 3.5W dissipation.

NOTE 1: The thermal protection will not be able to deal with a fast rise in temperature of the switcher IC since the thermal sensing is performed inside the microcontroller and it will take time for its temperature to rise is response to the switcher IC getting hot.

NOTE 2: For high power applications that have multiple LEDs running at high current (e.g. 700mA or more), it will help to have a battery input voltage that is as high as possible (less than the total Vf of the LEDs when dimmed) to improve the efficiency of the switcher IC. By improving the efficiency, the heat dissipation of the switcher IC will be lower.

The following section will give an overview of the thermal issues that can present themselves in a high powered LED application.

The switcher IC that is mounted to the gold thermal pad has a maximum junction temperature specification of 125C. The thermal coefficient is 37C/W (junction to ambient).

So, if we have 2W of dissipation in the switcher IC, that means the junction to the thermal pad is 37C/W x 2 = 74C. Put simply the junction would be 74C hotter than the thermal pad area. There is also thermal resistance from the through the PCB to the gold thermal pad area of the PCB. Even if we assume that thermal resistance is a low value like 14C/W we have a futher 14C/W x 2W = 28C temperature differential through the PCB to user attached heatsink. We are now looking at 74C + 28C = 102C hotter junction than heatsink. So, it can be seen that keeping the heatsink as cool as possible is essential to running high power through the maxFlex5 driver.

New information is available in the maxFlex thermal guide White Paper - see top of this webpage for downloadable pdf.

Due to some folk having thermal problems with maxFlex5, several tests we run to monitor the temperature of the top plastic package of the switcher IC (U1) versus the thermal pad temperature. The test configuration utilized a heatsink attached only to the gold thermal area on the bottom side of the maxFlex5 driver.

The test fixture was a LED load of approximately 21V of total Vf run at 750mA and 1000mA nominal. Measurements are below of power in, power out and efficiency:

The 2nd test (11.6V input at 2.2A) caused the switcher IC to overheat (due to the 2.4W heat loss) after several minutes. Given the 37C/W thermal resistance of the IC package that would have the junction temperature 37 x 2.4 = 89C hotter than ambient. Given the plastic package temperature was measured at 50C+ this means the junction was at least 89+50 = 140C when the IC overheated and cut off (failure mode). Of course this is a worst case calculation since it assumes all the losses are in the switcher IC, when in fact some will be in the inductor and schottky diode. But, it illustrates what can cause the driver to fail even though the package temperature may only be 50C.

The other two tests had the driver working with stable case temperatures around 40C. The conclusion from the above tests is that the key parameter defining whether the driver will operate reliably or fail is the Power Loss in the switcher IC. All tests had the same heatsink attachment (thermal pad material and a large heatsink).

From temperature measurements and calculations, results show that the PCB has <14C/W thermal resistance, e.g. with 1.4W being dissipated by the switcher IC, inductor and schottky, the switcher IC 'ambient' will be around 20C hotter than the heatsink thermal interface. The ~14W/C thermal resistance is the resistance from the bottom of the switcher IC package through the PCB to the heatsink attachment 'gold' surface.

Areas of the circuitry on MaxFlex5 utilize high impedance paths and if potting (not required) is to be utilized, the user must ensure than the compound is non-conductive and non-capacitive otherwise correct operation may be compromised.

Rather than potting, it is recommended to use the thermal interface pad on maxFlex5 to connect to a heatsink. A thermal epoxy product like Arctic Alumina is recommended since it is non-conductive and non-capacitive.

Soldering is also another option if copper or tinplated material is to be used as the tab. Please ensure that the board is not overheated during the soldering process - if in doubt use thermal epoxy instead.

MaxFlex4 top and bottom view pictures provided for reference. Maxflex4 is very similar to the maxFlex5 driver, the key difference is the addition of the SWG connection point on MaxFlex5.

The picture below shows the top view of MaxFlex2. The exposed rectangular gold area just above the (C) TaskLED is a direct thermal interface to the bottom of the switcher IC (on the other side of the PCB). If the power dissipation of the MaxFlex2 board exceeds about 1W it is recommended to affix a heatsink or copper tab to a heatsink to the exposed area. The exposed area is at Ground potential - i.e. electrically connected to the battery GND and LED- pads. This means it is safe to mount the tab to a heatsink that connects to the body of a flashlight, IF the body of the flashlight is the same as the Battery negative or ground.

The two holes below VIN+ (anticlockwise) are both ground pads (input ground and LED- output). MaxFlex2 has LED- common to battery ground input.

The SW hole below LED+ (anticlockwise) is one side of an external momentary action switch. The other side of the switch must be wired to Ground.

Note: it is recommended to keep the maximum wire length between the switch and maxFlex2 less than around 4" (10cm). Having wire leads that are too long can cause the wires to act as an antenna and cause maxFlex2 to respond irregularly or turn on/off by itself.

Note: There are 2 GND connections provided on the PCB, but most installations will require 4 connections to ground (Battery negative/ground, LED-, SW ground and ground for the STAT LED- if used). It is recommended to wire LED- and Battery negative to the same PCB ground hole and wire STAT LED- and SW ground to the other PCB ground point. Both PCB ground points are equivalent.

Note the pin labeled STAT in the bottom/center of the board. Refer to the operating manual above for how STAT can be used to drive a Red or Amber LED to warn of low voltage.

Note, MaxFlex2 is a Boost regulator (step up), so input voltage must be less than the output voltage to ensure MaxFlex2 remains in regulation. If the input voltage exceeds the output voltage (at the dialed in drive current), MaxFlex2 will no longer regulate and the input voltage will go through the series inductor and schottky diode directly to the load. This will cause the output current to a LED to rise rapidly since LEDs have a very steep Current vs Voltage curve (Vf).

Note, Input_current should be around 3A or less for optimal performance of MaxFlex2. When running at high output power it is recommended to solder or use thermal epoxy to mount a tab to the pad on the back of the MaxFlex2 board (the pad is at ground potential) and to thermal epoxy/solder/bolt it to a heatsink or the body of the flashlight.

Input Volts	Input Amps	# of LEDs	Output Volts	Output Amps	Efficiency
5.5	2.25	4	14.4	720mA	84%
6.58	1.82	4	14.4	720mA	87%
7.65	1.54	4	14.4	720mA	88%
9.78	1.18	4	14.4	720mA	90%
11.74	1.66	4	15.3	1190mA	93%
5.89	2.86	4	14.4	975mA	83%
7.07	2.29	4	14.5	982mA	88%
9.52	1.65	4	14.5	990mA	91%
9.41	2.91	6	23.3	1024mA	87%
12.0	2.21	6	23.3	1030mA	90%
14.3	1.84	6	23.2	1038mA	91%

Note: it is not recommended to run maxFlex2 at >3A input current, the above tests were to demonstrate the reduced efficiency at the higher input currents.

To determine the heat being dissipated on the maxFlex2 driver, calculate the output power as in the above equation (Step 1). Use the chart above to approximate the efficiency that the driver will be running at.

Lets assume 4 LEDs running at a nominal 1A from a 9.6V NiMH battery pack. From above we have the following:

As power being dissipated increases beyond 1.5W the thermal path to the heatsink becomes critical to reliable operation. Most of the heat being dissipated in the driver is from the switcher IC, U1. The thermal interface (the gold area) is tied directly to the bottom of U1 using thermal vias. Mounting an adequate heatsink with a non-conductive thermal epoxy is recommended. Use a "thin" smear of non-conductive thermal epoxy (like Arctic Alumina) rather than a thick layer.

Additional heatsinking to the top surface of U1 (the plastic package) can help a little, but not nearly as much as providing a good heatsink to the gold area.

Beyond 2W of dissipation thermal management becomes more challenging and it is highly recommend that the thermal protection be enabled in the menu system. Start with 70°C in the menu and refine as needed.

Areas of the circuitry on MaxFlex2 utilize high impedance paths and if potting (not required) is to be utilized, the user must ensure than the compound is non-conductive and non-capacitive otherwise correct operation may be compromised.

Rather than potting, it is recommended to use the thermal interface pad on maxFlex2 to connect to a heatsink. A thermal epoxy product like Arctic Alumina is recommended since it is non-conductive and non-capacitive.

The following section is historical reference data for the previous maxFlex PCB design.

The picture below shows the top view of MaxFlex. The exposed rectangular tinned area just above the (C) TaskLED is a direct thermal interface to the bottom of the switcher IC (on the other side of the PCB). If the power dissipation of the MaxFlex board exceeds about 1W it is recommended to affix a heatsink or copper tab to a heatsink to the exposed area. The exposed area is at Ground potential - i.e. electrically connected to the battery GND and LED- pads. This means it is safe to mount the tab to a heatsink that connects to the body of a flashlight, IF the body of the flashlight is the same as the Battery negative or ground.

The two holes below VIN+ (anticlockwise) are both ground pads (input ground and LED- output). MaxFlex has LED- common to battery ground input.

The SW hole below LED+ (anticlockwise) is one side of an external momentary action switch. The other side of the switch must be wired to Ground.

Note the pin labeled STAT in the bottom/center of the board. Refer to the operating manual above for how STAT can be used to drive a Red or Amber LED to warn of low voltage.

Note, MaxFlex is a Boost regulator (step up), so input voltage must be less than the output voltage to ensure MaxFlex remains in regulation. If the input voltage exceeds the output voltage (at the dialed in drive current), MaxFlex will no longer regulate and the input voltage will go through the series inductor and schottky diode directly to the load. This will cause the output current to a LED to rise rapidly since LEDs have a very steep Current vs Voltage curve (Vf).

Now, Input_current should be around 2.2A or less for optimal performance of MaxFlex. When running at high output power it is recommended to solder or use thermal epoxy to mount a tab to the pad on the back of the MaxFlex board (the pad is at ground potential) and to thermal epoxy/solder/bolt it to a heatsink or the body of the flashlight.

Input Volts	Input Amps	# of LEDs	Output Volts	Output Amps	Efficiency
5.5	2.25	4	14.4	720mA	84%
6.58	1.82	4	14.4	720mA	87%
7.65	1.54	4	14.4	720mA	88%
9.78	1.18	4	14.4	720mA	90%
11.74	1.66	4	15.3	1190mA	93%
5.89	2.86	4	14.4	975mA	83%
7.07	2.29	4	14.5	982mA	88%
9.52	1.65	4	14.5	990mA	91%
9.41	2.91	6	23.3	1024mA	87%
12.0	2.21	6	23.3	1030mA	90%
14.3	1.84	6	23.2	1038mA	91%

Note: it is not recommended to run maxFlex at >2.6A input current, the above tests were to demonstrate the reduced efficiency at the higher input currents.

To determine the heat being dissipated on the maxFlex driver, calculate the output power as in the above equation (Step 1). Use the chart above to approximate the efficiency that the driver will be running at.

Lets assume 4 LEDs running at a nominal 1A from a 9.6V NiMH battery pack. From above we have the following:

Areas of the circuitry on MaxFlex utilize high impedance paths and if potting (not required) is to be utilized, the user must ensure than the compound is non-conductive and non-capacitive otherwise correct operation may be compromised.

Rather than potting, it is recommended to use the thermal interface pad on maxFlex to connect to a heatsink. A thermal epoxy product like Arctic Alumina is recommended since it is non-conductive and non-capacitive.

Voltage In	Current In	Power In	Voltage Out	Current Out	Power Out	Power Loss	Efficiency
12V	1.38A	16.6W	20.5V	0.74A	15.2W	1.4W	>91%
11.6V	2.2A	25.5W	21.8V	1.06A	23.1W	2.4W	>90%
14.5V	1.67A	24.2W	21.6V	1.06A	22.9W	1.3W	>94%