# Understanding the difference between Linear, Buck, Boost and Direct Drive drivers

A long time ago I promised I would do a topic to explain to the interested layman the difference between Linear, Buck, Boost and Direct Drive drivers. Well, here it is.

Direct Drive

This is the simplest and cheapest design. As its name implies, it is a direct path to the LED, just like old incandescent flashlights. Sometimes there is a control chip and transistor to create modes (High, Low, Strobe, etc.) by rapidly switching ON and OFF the LED. That’s called PWM.
This is used by people who want to make extremely powerful flashlights that pull 6A or even more.
It is also used in cheap flashlights, but they add a resistor that will limit the current to a lower value. This is not good as the current will vary with the battery voltage. (Same as incandescent, but worst because of the LED)

In a direct drive flashlight, the battery voltage must be equal or higher than the LED voltage (An alkaline or NiMH battery can’t power a 3V LED using a direct drive driver).

Cheap

Current is varying greatly with battery voltage!
If a resistor is added to limit the current, the efficiency may be lower than a linear driver.
Voltage of the battery must be higher than the LED voltage (~3V) but not too much above. You can’t use this if you have multiple batteries in series.

Linear

This is the equivalent of a direct drive flashlight with a resistor to limit current… But smarter.
The only difference is that the resistor value is constantly adjusted to make a constant current. How is that possible? Well, that’s the job of the linear regulator. Most drivers use 7135 chips as linear regulators. Each chip will supply a constant 0.35A to the LED. A driver with four 7135 chips will supply a constant 1.4A to the LED.
This is also a very simple design. You can see the controller (that produces modes) surrounded by 7135 chips.

Don’t forget! Even though this is slightly smarter, it’s basically still a resistor to limit the current! This can never boost a lower voltage to give the LED the voltage it needs! That’s usually 3 - 3.3V.
This is the perfect driver for flashlights using single LED and single lithium battery. The battery has a voltage of 3 to 4.2V and the LED about 3 to 3.3V. So for most of the discharge the current will be constant.

Quick maths: What is the efficiency of a Linear driver?
Well, the linear driver has a variable resistance that burns off any excess power to reduce the voltage to 3V for the LED. That means that with a fully charged battery the efficiency will be lower than when the battery is discharged.
Efficiency=VLED/VBattery
Fully charged: Efficiency=3.3V/4.2V=78%
Half discharged: Efficiency=3.3V/3.7V=89%
Almost discharged: Efficiency=3.3V/3.3V =100% (Approximation, not taking into account all the parasitic resistances.)

When the battery voltage becomes too low, the linear driver will reduce the resistance to its minimum, to power the LED until the end (but dimmer).

“Why can’t I use a linear driver with two lithium batteries in series??”
Let’s do the maths:
Efficiency=3.3V/7.4V=45%!!! More power is wasted in the driver than fed to the LED! That will reduce battery life and the driver will overheat. On top of that the 7135 chip will fail above 6V…

Simple
Robust
Efficient if used in a single cell single LED configuration
Constant current for most of the discharge of the battery

Voltage of the battery must be higher than the LED voltage (~3V) but not too much above. You can’t use this if you have multiple Li-ion cells in series.

Buck

This is also called a step down driver. and is part of the SMPS (Switched Mode Power Supply) family.
It is easily recognizable thanks to the big inductor. Sometimes the inductor is black.

This is a more complicated design. I won’t go into details as it is well explained on Wikipedia .
In a nutshell, it uses an inductor and capacitor to step down a voltage. Compared to a linear (7135) driver, the Buck driver will have a fairly constant efficiency, even with a battery voltage much higher the LED voltage. It can be used to power a single LED with 2 or more batteries in series.

What about the efficiency? Well, it is usually between 75% and 90%. That depends on the quality of the design.

For info, this is wildly used in many devices (PC, TV, Smartphone, Tablet, etc.) because it is efficient.

Can be used with batteries that have a voltage much higher than the LED voltage. For example three lithium batteries in series will produce about 11V. In that case you need a Buck driver to drive an LED that needs ~3V.
Good efficiency
If well designed it can produce a true PWM less low mode. That’s good for sea sickness and for the LED efficiency. (More on that below)

Slightly more expensive
Voltage of the battery must be higher than the LED voltage (~3V).
Bulky

Boost

This is also called a step up driver. and is part of the SMPS (Switched Mode Power Supply) family.
It is easily recognizable thanks to the big inductor. Sometimes the inductor is black.

This is very similar the the Buck driver, but as its name implies, it will increase the voltage.
This is typically used in flashlights using one or two AA/AAA batteries. Two AA batteries in series will have a voltage varying between 2 and 3V. The LED needs ~3.3V so the voltage needs to be stepped up. There is no other solution!
I measured the efficiency of a single AA flashlight and typically found:
At the battery: Vin=1.2V ; Iin=2.2A
At the LED: Vled=3.2V ; Iled=0.35A

Let’s do some maths:
Efficiency=(Vled*Iled)/(Vin*Iin)=(3.2*0.35)/(1.2*2.2)=42%!!
That’s really bad! Well yes, but it’s hard to step up a voltage as low as 1.2V… The efficiency is better at 2.4V (2*AA). That means that if you choose a 2*AA flashlight you’ll get more than twice the runtime for the same brightness! That’s definitely something to consider.

It can also be used to power a string of LEDs (for example 3 LEDs in series will need about 10V) with a single lithium battery.

Can be used with batteries that have a voltage lower than the LED voltage.
If well designed it can produce a true PWM less low mode. That’s good for sea sickness and for the LED efficiency. (More on that below)

Slightly more expensive
Doesn’t work if the battery voltage is higher than the LED voltage.
Bulky

PWM means Pulse Width Modulation. It’s a way to control the brightness of a flashlight by rapidly switching it on and off. If it doesn’t switch rapidly enough (PWM frequency too low) it can be unpleasant to the eye. The picture above was taken while rapidly moving the flashlight to show the effect.
Something that is often overlooked is the fact that a light using PWM to make a low mode will be less efficient than one using a constant current. Why is that?

Well, as you can see above, the lumen output is not linearly dependant to the current. Let’s take an example:
Driver 1 is driving the LED at 2800mA at full mode. This driver also has a medium mode that is a duty cycle of 50%. That means that half of the time the LED is OFF and it’s half of time ON. We will get half the lumen of the full mode: about 500 lumen. On average it’ll consume 1400mA.
Driver 2 is also capable to drive at 2800mA. When in medium mode however, it uses a constant current of 1400mA. The average consumption is the same as Driver 1. The lumen output however will be about 600 lumen! That’s 20% more lumen.
All this to say that a flashlight that uses constant current on all modes will be more efficient!

Conclusion
If you have a flashlight with a single LED single lithium battery then get a Linear driver.
If you have a flashlight with one or two NiMH/Alkaline batteries, then you need a Boost driver.
If your battery voltage is much higher than the LED voltage then get a Buck driver

If you want to see the same thing explained by someone else, I invite you to read this.
Thank you for reading this. I hoped it was helpful to you and if it was please say thank you!

5 Thanks

This is a complicated subject. Even though I know how it works, I may have forgotten something or made mistakes. So please feel free to help me improve this topic!

Great effors, admittedly I only skimmed but I’ll read the entire thing start to finish later this evening.

One thing I see at first tho is your statement in the DD section

This is usually true however not a rule, if an LED with lower vF is used (and the driver components all accept ~1v input [most parts used have ~2.7v minimum]) then you can run that emitter from a lower power cell. For example I have an ATTiny 13V (low power version of 13A) MCU and use it to run a DD FET driver from a 1.8V lithium primary driving a red rebel LED. If you could find an led with a 1.2 or 1.5vf you could drive those with a DD setup.

True. I had in mind the Cree LEDs that need 3V…

Looks like there is an issue in the boost section under advantages.

Note I’m not trying to say you wrong about anything, thanks for your efforts here, just trying to help you improve this for others.

Thanks for doing this!

I’m not taking it the wrong way, you are helping me to produce something accurate and helpful to others! So thank you.
I corrected the mistakes you pointed out.

I feel like there is too much text. I just installed Matlab and KiCAD. I’ll try to make some nice graphs to explain the same things with less words… But not today.

Nice explanation, I think it is a good introduction for folks who are new to building flashlights, but I guess someone who is completely new to this should judge that :-)

Thanks for the write-up!

I don’t, it’s a in-depth subject.

If you want to redo it with less text that’d be cool but I urge you leave everything already up. Make a new post with the shortened version, you should never remove reference material.

I did also do an article about it a couple of years ago (Date says 2010): http://lygte-info.dk/info/DriverTypes%20UK.html

Thanks for the support guys! I see that somebody already linked to my topic so I’m happy.

I somehow never found this article on your website, even though I looked through it many times.

That article was only posted on CPF and I wanted to update it before posting it on my website, but has never gotten around to do that. Maybe you can use some ideas from it (I might still update it and post it sometime in the future).

Lagman, you may consider going a little more in-depth on the PWM section and go into duty-cycle some.

Thanks for taking the time to put all this great information together.

I might do a separate in depth topic on this. I don’t want this to be so long that it scares newbies.

On a side note, I have a doubt about my title. Would it be better to say:
“Understanding the difference between Linear, Buck, Boost and Direct Drive drivers”