Frequently Asked Questions

TASKLED - Smart LED drivers

In this web page we try to provide answers to some of the most common questions about LED drivers in general and some specific TaskLED drivers. Last updated 10-Feb-2014

Common Questions
Choosing a Driver
Common Assembly Issues

First we'll start with some common questions:

Q: What is a Boost driver and when would you use one?

A: A Boost driver wil step the input/battery voltage up to drive a series string of LEDs whose total Vf is higher than the supplied input voltage. An example would be using 2 li-ion batteries in series (8.4V freshly off the charger) to drive say 4 series connect white LEDs with a total Vf of around 13.2V (4 x 3.3V). A boost driver will draw higher current from the input than what it will provide at the output. As the battery voltage drops, the boost driver will draw even more current and this tends to put more strain on a battery pack that is already fairly discharged. Having a protected battery pack (internal) or a way for the driver to stop running before it draws the battery pack down is essential to protect rechargeable cells (especially li-ion where overdischarge can lead to a dangerous situation when trying to recharge the cells). The user must be aware that the battery voltage (when fully charged) must be lower than the LED Vf at the target current (especially important if the output current can be lowered for dimming to say 50mA). In this case the user must determine the total LED Vf at say 50mA and ensure that is greater than the fully charged battery voltage. If this requirement isn't met, the driver will go direct drive and the battery voltage will flow through the DC path of the boost driver to the LED load.

Q: What is a Buck driver and when would you use one?

A: A Buck driver will step the input/battery voltage down to drive a series string of LEDs whose total Vf is lower than the supplied input voltage. An example would be using 4 li-ion batteries in series (16.8V freshly off the charger) to drive say 3 series connect white LEDs with a total Vf of around 13.2V (4 x 3.3V). A buck driver will draw lower current from the input than what it will provide at the output. As the battery voltage drops, the buck driver will draw more current, but never more than the output current. Having a protected battery pack (internal) or a way for the driver to stop running before it draws the battery pack down is essential to protect rechargeable cells (especially li-ion where overdischarge can lead to a dangerous situation when trying to recharge the cells). If the battery voltage drops until it is the same as the LED Vf (or just above it), the driver will stop bucking and will transition to direct drive (or close to it, just like a boost driver), but in this case as the battery voltage drops lower the LEDs will dim since the driver can only step down and the battery voltage is already lower than the LED Vf for the target current. So, a buck driver will deal gracefully with the battery voltage dropping.

Q: Why doesn't TaskLED make a buck-boost driver?

A: Combined buck-boost drivers tend to be less efficient that a straight buck or boost driver. Where a typical buck or boost driver can run at 90% efficiency (or higher) a buck-boost would likely be around 80%. There are some efficient buck/boost designs, but they cost more to manufacturer.

Q: What are the various intermediate current levels of the various Flex UI's, i.e. what is L2,L3,L4 for UIB multimode of the lFlex driver?

A: TaskLED does not document these values since a) there are so many combinations and modes (e.g. UIB multimode, UIF 8 level, lFlex, maxFlex6, h6Flex etc etc) and b) the intermediate levels are also subject to change with different firmware and/or hardware revisions. What can be stated is that the levels are chosen to provide reasonably uniform changes in intensity for each level/step change. The human eye responds in a somewhat logarithmic fashion to light intensity (similar to our ears and sound), so those curves are plotted and then intermediate levels are chosen with equal spacing in intensity. Those chosen levels are then mapped to the appropriate drive current for the particular level and current table. Testing is then performed and the values are again adjusted as necessary to "look good". If a customer wants to know the current of a particular level, then connecting a meter on the amp range and inserting it in series with the LED(s) and taking measurements is the best approach.

Q: What is PWM and how is it used in various drivers?

A: PWM means Pulse Width Modulation and is term that describes varying the on/off time (duty cycle) of a waveform. Many buck and boost drivers will use a fixed frequency to run the power conversion core of the driver and use PWM to vary the time the inductor is being pumped with energy. This PWM runs at very high frequency (typically > 100kHz) and has nothing to do with Dimming PWM. Dimming PWM is where the output current to the LED is turned on and off at a lower frequency (say >100Hz). Varying the on time versus the off time of the Dimming PWM means that the LED gets more or less average current and as such can be brightened or dimmed. The dimming PWM frequency is chosen to provide flicker free dimming and preferably runs >200Hz.

Q: Is PWM dimming the best way to dim an LED?

A: No, for many applications, like video and photographic use, PWM can cause problems due to the relatively low frequency that is used. This low frequency PWM can 'beat' with the camera frame time to cause banding as the LED turns on and off within a single frame. For these applications it is best to use current regulation to dim, i.e. the current to the LED is adjusted to the appropriate brightness but remains constant once adjusted. W ith phosphor based white LEDs there can be tint shifts as the constant current is varied and this is one of the main drawbacks if the user requires consistent tint over the entire dimming range.

Q: Is PWM dimming used in any of the Taskled drivers?

A. Yes, it is used on the lower dimming settings of the h6Flex, b3Flex and HBFlex. The drivers utilise constant current regulation and most of the dimming range is performed by varying the current. At lower current levels, PWM it used in conjunction with constant current to provide consistent tint at low levels and also more accurate current regulation.

In the case of the h6Flex (which can be used with the SST50/90 from Luminus) the dimming will go as low at 1000mA, and then transition to PWM of 1000mA current. This is required due to the minimum current spec of the Luminus devices, they are not guaranteed to light below 1000mA. Luminus recommends using PWM at 1000mA to dim below the 1000mA level. The h6flex PWM frequency is 400Hz and the duty cycle can vary from 5% to 100% (lowest/highest).

The b3Flex uses current regulation down to 500mA and then transitions to PWM of 400Hz with duty cycles from 5% to 100%. b3Flex also supports optional ultralow output for L1 and it transistions to 200mA constant current and PWM of 400Hz with duty cycles of 5% to 20%.

The HBFlex uses current regulation down to 500mA and then transistors to PWM of 400Hz with duty cycles from 10% to 100%. For the HBFlex another advantage of using PWM at 500mA versus lower constant current is that at the dimmest level the LEDs will still be receiving 500mA (at varying duty cycle), so the minimum LED Vf would be at 500mA. This allows for a higher battery voltage while still providing full range dimming to 50mA without risk of the HBFlex going direct drive. See description of what a Boost Driver is at the top of this page.

Q: What is the difference between a H6cc and a H6Flex driver?

A: The H6cc is a basic buck driver. The output current can be set via an onboard trimpot (variable resistor) and can also have an external Potentiometer connected to provide continuous variable output current. It also has a PWM input that can be connected to a user provided PWM signal. The H6Flex driver uses the H6cc LED driver circuitry and also adds a microcontroller circuit that provides the user with digital dimming control, voltage monitoring/cutoff, temperature monitoring and various user interfaces.

Q: Do the Flex drivers all operate the same?

A: Yes, all Flex drivers utilise the same base firmware features and other than specific menu option differences (such as the number of current tables supported) operate with the same user interface. This means you could have a light running with a maxFlex6 driver and another with a h6Flex driver and if configure to use the same UI, would function identically to each other. This is one key advantage of the Flex architecture - a consistent user interface and operating experience across the whole Flex family line of drivers.

Q: Can I use one switch to control 2 or more of the same Flex drivers?

A: No, is the short answer. The main issue is that each driver operates internally at a slightly different frequency and that difference means one driver might see a press as being 1.0 seconds and another as 1.1 seconds. Then when you press to change a level (say your are in UIF) and release the button at 1.05 second, the first driver has changed levels while the second assumes is was a click and turns off. Of course what exactly happens depends on what UI you are in and what is mis-interpreted, but the end result is that the drivers will eventually get out of sync with each other.

Q: Can I use a Flex driver with a flashlight/torch that has a tail switch?

A: A tail switch interrupts power to the driver by interrupting the negative end of the battery from the body of the light. Since the driver only sees power being applied (tail switch on) or being interrupted (tail switch off), all the Flex driver can do is turn on or off. No level changes (dimming) will occur and no access to the menu system is possible. The Flex driver can be pre-configured with a momentary action switch to select an appropriate current table, set thermal protection, set voltage warning levels etc etc., but when used with the tail switch it can only be turned on or off in the same was as if the battery was connected/disconnected. To have access to all the UI and dimming features a dedicated momentary action switch is required that ONLY connects to the SWA/SWB connection points of the Flex driver.

Q: Can I use TaskLED drivers in an automotive environment (car/truck/motorcycle/boat etc)

A: Unless the driver specifically states that it is rated for automotive use (e.g. CC1A), then TaskLED does NOT recommend its use in such an environment. A driver designed for automotive use must have reverse polarity protection AND spike/surge protection to at least 40V minimum.

Next we'll discuss how to choose a driver and provide some examples.

Choosing a TaskLED driver requires the user to decide on battery voltage and chemistry (and capacity) and the number of LEDs and what the maximum drive current will be. Also, deciding on how the LEDs are wired, for example, all in series, all in parallel or a combination of series/parallel strings.

The main issue with wiring LEDs in parallel is current matching. Similar LEDs will have similar Vf versus I curves (forward voltage versus drive current), but there can still be sufficient variation that for the case of say 2 parallel LEDs, one LED will draw more current than the other. Example, 3A provided to 2 parallel wired XPG does NOT guarantee that each LED will receive exactly 1.5A each.

Another choice the user needs to make is whether a 'dumb' or 'smart' driver is needed. A 'dumb' driver is one that is just a constant current driver that may have the option for an external Potentiometer and/or PWM to be provided by the user such as a H6cc or Hyperboost driver. A 'smart' driver is one of the Flex based drivers that contain a microcontroller that provides digital dimming control, voltage sensing, temperature sensing and various user operating modes.

Example 1.

4 series li-ion cells to drive 3 series XPG LEDs at up to 1.5A

4 series li-ion cells provide a voltage range from 4.2V to 3.2V per cell (charged to discharged). 3.2V is a safe voltage to choose as discharged and will provide many recharge cycles, versus discharging a cell down to 2.7V.

The 4 li-ion cells provide a total input voltage from 16.8V down to 12.8V.

3 series XPG LEDs will have a Vf around 10V maximum (typical 3.3V per LED at 1.5A drive current).

Since input voltage is higher than output voltage a buck driver is required.

This provide and input to output headroom of (12.8V - 10V) = 2.8V worst case and that is sufficient for either the h6flex or b3flex to remain in full current regulation for the entire battery runtime. A H6cc could also be used if a 'dumb' driver is all that the user requires for a light system.

Example 2.

4 series li-ion cells to drive 7 series XPG LEDs at up to 1A.

As before, 4 series li-ion cells provide a total input volate range from 16.8V down to 12.8V.

The 7 series XPG LEDs will have a total Vf around (23.1V).

Since input voltage is lower than output voltage a boost driver is required. Either a maxflex6 or hyperboost driver can provide a solution for this battery/LED combination.

For a boost driver we also need to calculate the maximum input current to ensure we are not exceeding the boost driver's maximum input current specification. This calculation is not require in Example 1, because with a buck driver the maximum input current will never exceed the output current.

So, let us proceed with the input current calculation. The worst case will occur when the input voltage is at its minimum, and for this example that would be 12.8V.

The worst case also occurs when the output current is set to the maximum, which is 1A for this example.

So, our output power is:

Output_power = Output_voltage x Output_current

Output_power = 23.1V x 1A = 23.1Watts.

We now need to calculate input power and we can assume 90% efficiency as a conservative number for the applied input voltage versus output voltage (see the maxflex6 technical section for more details).

Input_power = Output_power / efficiency

Input_power = 23.1W / 0.90 = 25.7W input

Power dissipation (heat loss) in the maxflex6 driver will be (25.7W - 23.1W) = 2.6W. This is within the capabilities of the driver but will require the user to provide a GOOD thermal path from the driver to a heatsink.

Next we calculate the input current. For a worst case calculation we use the lowest battery voltage, 12.8V in this example.

Input_current = Input_power / Input_voltage

Input_current = 25.7W / 12.8V = 2A.

An input current of 2A is well within the capabilities of the maxflex6 driver (and easily handled by the hyperboost driver).

So, in summary, either driver can be used to provide a solution depending on the functional needs of the user.

Common Assembly Issues.

All Flex drivers (except the lFlex):

The Flex drivers utilise a current regulated DC:DC switching regulator core. They all contain a shielded inductor that is the main power conversion device.

Even though they utilise a shielded inductor, it is possible for some of the magnetic field to 'escape' if a wire is run in close proximity to the inductor and this should be avoided.
The inductor should be kept away from nearby strong magnetic fields such as a nearby magnetically operated hall switch or reed switch. Maintain a 1" distance from the inductor to strong magnets.

The Flex drivers require a momentary action normally open switch for control purposes. The wiring to the switch should be routed away from high current wiring such as the wiring to the LEDs and the wiring to the batteries/power source. High current pulses on those wires could couple on to the switch wiring (SWA/SWB) and cause the driver to incorrectly detect a switch click. A 0.1uF capacitor wired across the SWA/SWB pads on the Flex driver can help to 'filter' these erroneous events.

All Drivers:

At the higher power levels that LED drivers operate at, a good thermal path to a heatsink is necessary to prevent risk of damage to the driver electronics. All TaskLED drivers have a thermal path and recommendations on how to mount the driver to a heatsink (please refer to the technical section of the driver you are using). Many TaskLED drivers ship with a piece of double sided adhesive lined thermal tape that has proven to be more than adequate for use with that driver.

Some drivers offer additional thermal path challenges and these are the b3flex and maxflex6. Both these drivers can be ordered with an option ($1) aluminium shim that can be used to provide a 'flat' mounting surface to a heatsink. In the case of these two drivers, it is highly recommended to use a product such as Arctic Alumina 2 part epoxy (applied as a thin uniform film) to provide the thermal interface between the driver and the shim and then between the shim and the heatsink. If a pedestal is designed/machined into the housing to allow direct attachment to the b3flex/maxflex6 thermal point, then the shim isn't required, but Arctic Alumina 2 part epoxy is still recommended as the thermal 'glue'.