TPS92691-Q1 design query

Hello Prahlad,

1. The TPS92691 is a very good fit. Another option is the TPS92690 depending on the topology you select. You obviously need a buck-boost of some sort.

2. From and EMI/EMC perspective the Cuk topology is the best as it has continuous current at both the input and output. But it requires a coupled inductor. SEPIC and buck-boost/boost-to-battery have discontinuous output current so EMI/EMC can be a little harder. SEPIC also requires a coupled inductor. Buck-boost/boost-to-battery only requires one. The 690 is capable of Cuk and SEPIC. The 691 is capable of both plus buck-boost/boost-to-battery due to its high or low side current sense.

3. For buck-boost/boost-to-battery the LED- connection is common to Vin. In SEPIC LED- is common to ground with the 691 if you use high side sense. In Cuk LED+ is common to ground.

4. Either device can have the switching frequency synced. So either can be synced in phase or out of phase. All of these topologies have continuous input current however so neither may be necessary. But either are possible if desired.

5. There is a boost calculator in mathcad format for the 690. There will be an excel calculator for the boost 691 available within the next day or two. They are very similar devices. If you want to do buck-boost/boost-to-battery with either they will work fine. Just set the input voltage as Vin and the output voltage as Vin+Vout and the calculators work fine. There is a buck-boost/boost-to-battery EVM available for the 691 and Cuk reference design for the 690 available. For 3A some of the components for either will need to be beefed up in the end.

Hi Jensen,
Thank you for your reply.

Hi Clinton Jensen,

Thanks for the information.

As we are using 3 LEDs in series its coming out 12.3 is max forward voltage. w.r.t Battery specifications we have to go with Buck-boost or Sepic Topology. Can you please provide more information coupled inductor calculation  and AC capacitor value calculation.

In datasheet i did not see the formula for calculating Coupled inductor and CAC between two inductors.

as per our calculation for to support VBAT_MIN, VOUT_MIN inductor value is 100uH is getting for Minimum LED current 200mA-250mA. so we have to use 100uH (1:1) coupled inductor or 50uH coupled inductor? Please confirm.

What are the advantages over coupled inductor(Sepic) and two uncoupled inductors(buck-boost)? please provide if you have any comparison data.

Regards,

Narasimha LV

That is a video you might find useful. I'm not a master of sepic converters, I just choose the inductor value like I would in a boost but use total Vout as Vin+Vled. Then I basically just use a 4.7uF capacitor and it is generally enough but I'll bump it up if I need to or reduce it if I can.

In any case if you calculate 100uH you would use a 100uH coupled inductor. A coupled inductor makes operation much better (a lot less ringing during transients in particular) than two separate inductors. That is true for both Cuk and sepic. A standard buck-boost (or boost-to-battery, they are the same other than where you place the output capacitors) only requires one inductor. I don't really have any useful comparison data other than what you could find in various reference designs or datasheets, but the performance is similar. The buck-boost will have a slight advantage in efficiency due to less components and it is a more simple design that is also easier to stabilize.

1. As mentioned, please share the "boost calculator in Mathcad format for the 690" and excel calculator for the boost 691" if its done.

2. On PSIPE SIMULATION we noticed the LED driver configured for the SEPIC MODEL with 100uH coupled inductor, resets itself always, due to Vis at the IS pin, exceeding 550mV, because of which we went ahead and increased both the current limit series resistor of the coupled inductor from0.069 ohms to  3.2 Ohms, which was the default value of the PSPICE MODEL, that was downloaded from TI Website, to limit the Vis threshold to 550mV so that the LED Driver does not reset itself often. Is this the right practice or is the Led Driver designed to reset itself always, so that the average Current value is used to drive the LED LOAD.

3. When we set the Viadj voltage to 0.14V which is supposed to give an Iout of 200mA, but on simulation it was noticed it generated 1.2A of current.

4. The Css as per the datasheet is calculated for 8ms start up time, is this mandatory, or can we alter the Css to support the soft start time to 1ms ?

1. They are both available on the web.

2. Which model is this? Could you provide a picture of your schematic and point out which resistor you changed to 3.2 ohms?

3. Again I would need to see a schematic.

4. Sure you can change it. You can omit it altogether if you would rather. At some point the COMP capacitor will be the dominant factor. It will act as its own softstart of sorts since that capacitor has to be charged up from the COMP pin.

Hi Clinton,

Could you please provide design calculation file Sepic topology, in web its available only for Boost topology.

We are facing few issue in simulation can you help on this.

Could you please review or take up with design team for Sepic design Calculations and provide your comments on this.

Kindly confirm to go ahead with design

SEPIC.ZIPSEPIC_TPS92691_DRIVER.pdfLED DRIVER CIRCUIT WITH PWM 50.docx

Regards,

Narasimha LV

Hi Clinton,

PFA ppt for LED driver TPS92691 simulations  and Queries.

 

Please clarify the below for to complete the design:

 

1. No Sepic calculation formulas in datasheet, Please specify which document to consider for Design calculations.

2. We have used “AN-1484 Designing A SEPIC Converter”  for design calculations, as per calculation coupled inductor value is arrived 10uH (min 6uH and Maximum 13uH), please confirm whether is this correct document?

3. Gate is switching ON as soon as PWM is ON, due to this 11A ripple is coming in 20us. Please let us know how to avoid this.

4. As per calculation the Primary and Secondary current is 6.2A+3.6A=9.8A but we have observed in 11A in primary side itself, Please advise how to reduce this.

5. From Output high Current 3A to Low Current 200mA, Output LED current ramp up is getting delay.  3A – 600us in 1^st PWM, 1.5A – 1ms in 1^st WPM and 200mA -2 PWM and 200us in 3^rd PWM.   Please advise how to reduce this delay?

 

Regards,

Narasimha LV

LED DRIVER SIMULATION.ppt

Sorry for the slow reply. I have been told to close this out since this is being supported offline directly by our team. Please work with Prahlad to coordinate. Thank you.