PWM Dimming an LED with the TPS63020

Terry,  

What are your specifications? e.g. what is your input voltage range, how much current do you need for the LED, what is the forward voltage drop of the LED, what is your primary design goal (efficiency, cost, simplicity).

One note is that the TPS63020, TPS63010, and TPS63030 have built in OVP whereas the TPS63000 does not.  

Depending on your specs, the TPS61200 may be a possible solution since it has down mode and OVP. 

Method 1, as you described above, is preferred over method 2; as we have had customers reporting mixed success using method 2.  

Here are links to a number of existing app notes on the topic of LED drivers, some related to the TPS630xx: 

High Efficiency Battery Powered High Brightness LED Driver Using the TPS63000: http://www.ti.com/lit/ug/slva268/slva268.pdf

Different Methods to Drive LEDs Using TPS630xx Buck-Boost Converters: http://www.ti.com/lit/an/slva419a/slva419a.pdf

LED-driver consideration: http://www.ti.com/lit/an/slyt084/slyt084.pdf

Daniel,

Thanks for your response! I had previously read through all of the app notes and did not see anything that specifically described how to integrate dimming.

The specs that I am trying to achieve is an LED @ 1.8A with a forward voltage of 3.1V driven by a Li-Ion battery. The battery we have chosen has usable power from about 3.2V to 4.21V. I believe that the only buck-boost driver that will work is the TPS63020 however I must be able to integrate PWM dimming from 1-100% for this particular application.

I am wondering if putting a low RDSon mosfet in series with the LED and following scheme 2 of "SLVA419A: High Efficiency Battery Powered High Brightness LED Driver Using the TPS63000" would be all I need to accomplish this?

We are planing an initial production run of around 5000 units and we are on an extremely tight time scheduale. I messed around with the SwitcherPro software a bit and was disapointed that it only works for fixed voltage designs not fixed current.

Anyway, you help and feedback is greatly appriciated

Terry Cooke

Robohobbies.

Any feedback??

Most simple solution is taking the circuit of Figure 2 in the app note SLVA419A and apply the PWM signal to the enable pin (EN) of the device. This solution usually works fine at PWM frequencies of 100Hz to 200Hz. At higher frequencies the circuit is too slow causing nonlinear brightness control at high and low duty cycles. A PWM frequency of 100Hz is not noticeable to the human eye. If for some reasons higher PWM frequencies are required then I suggest circuit 23 of the TPS61040 datasheet. Either apply a PWM signal with defined amplitude directly or use a PWM signal and RC filter to apply a analog signal to the feedback network. Both solutions work.

Putting a MOSFET in series to R3 in Figure 2 of SLVA419A has two problems: Output voltage is not regulated during the off phase and secondly high inrush current from the output cap will occure during the on phase. Those issues can be solved by adding more complexity but does not lead to a simple solution with good performance. Same applies for a high side MOSFET switch where you also have to deal with the output capacitor. Because of this we do not recommended these type of solutions.

Another option is to use two different current sense resistor values.  Use a high value to set a very low LED current.  Then switch in a lower value through an N-ch MOSFET to set the higher current.  With this configuration, the power supply is always regulating the LED current to one of the two discrete values.  The LED current accuracy will not be affected by the PWM signal amplitude.  EVMs are available for these ICs, so you will be able to order one and evaluate the different options to find out which works best for you in your application.

with the dual current set point method, would pwm'ing the n-channel vary the current between the two set points?

Terry

yes