TPS61165: TPS61165: Technical feasibility for LCD backlight driving 360mA , 18.2V

Hi Dave,

How about the input voltage?

BR,

Robin

Hi Robin ,

Thanks for your rely.

The input voltage is 5V as stated in (c) 

Appreciate your guidance !

Hi Dave,

For your application requirements, TPS61169 seems a good choice while TPS61500 current limit is a little high.

BR,

Robin

Hi Robin,

Currently we are have try TPS61169 , however the thermal dissipation for this chip does not seem to match the application requirement of single Chain 6s6p configuration.

With Voltage of 18.2V and Iled of 360mA , the chip is not able .

When i perform Pdmax calculation , it is way out. Hence we start considering TPS61165 as alternative.

But with operating range hit 55 degC , TPS61165 Pdmax again exceed.

Any good recommendation for our application ?

By the way with 6s6p backlight LED configuration , will there be Vled offset issue which resulted in brightness homogeneity issue ?

Thks & Rdgs

Dave 

Hi Dave,

Please have a check for layout to find if there is any possibility to improve thermal performance. For example, larger trace, lower Rdc of inductor, lower Vf of diode, larger GND area are all helpful.

For brightness issue, the answer is yes. Normally, adding series resister to each string is a simple solution. However, it will make the thermal performance worse. 

BR,

Robin

Hi Robin ,

Good day !

Just to confirm , am I right to say that both TPS-61169 and TPS-61165 can meet the requirement of Vout 18.2V and Iled 360mA with 6s6p for backlight driving application ? 

Is the Pdmax computation still a valid design check process ? because when i compute the Pdmax at Tambient of 50 degC , the junction to ambient thermal resistance of TPS-61169 is much worst than TPS-61165, and both is exceeding the Power consumption of TPS 61169 / 61165 assuming 80% efficiency.

Physical measurement on Tcase for TPS61169 show 78 degC at room temp driving at approximately 180mA.

Is there a need to change to another package to bridge the gap or solely by layout change can solve the problem ?

Appreciate your feedback soonest possible.

A million thanks in advanced. 

Thks & Rdgs

Dave Tan

Hi Dave,

For 5V output, TPS61165 current limit is a little low. And you can calculate the inductor peak current to make sure peak current won't above current limit. 

One thing that we need to make it clear is that the main power dissipation of device itself is from internal FET. So if we don't talk about the external inductor, diode and so on, the efficiency of device itself is high, not only 80%. 80%--90% is the efficiency for whole system including chip.

So I think the power for calculating device thermal performance is much lower and the rising temperature is not so high. For TPS61169, I haven't heard about the compliance about thermal issue.

BR,

Robin

Hi Robin,

Good day !

Thanks for your reply. Just a quick confirm , based on the specification current limit for TPS61165 and TPS61169 are both stated as 1.2A. Is that correct ?

Currently when we are driving TPS61169, the Vref (Vfb) measured 189mV with Rset tuned to 0.7 ohm, Iled approx 270mA . Tcase measured to be 78 degC at room temperature. When we further reduce Rset to 0.5 ohm to achieve high driving Iled , the circuit could not drive more than 270mA and Vfb seem to drop to a very low value ( below 10mV ) .

Can you see this attached image ?

A million thanks for your guidance !

Hi Robin , 

Just in case you could not see my attached image for the backlight circuitry . I have attached the pdf file for your perusal.

TPS61169 backlight driving circuit.pdf

Hi Dave,

You can download TPS6116p model for simulation, and the corresponding link is: 

And I have s simulation based on provided schematic. Device work normally and inductor peak current is about 700mA which is lower than current limit. For TPS61169, its current limit is 1.8A@typ while TPS61165 is 1.2A@typ.

How about the power supply for device? Do you set the current limit to it?

BR,

Robin

Hi Robin,

The Power supply input is 5V 2.5A , so it shouldn't be limiting the output current .

Based on your feedback , TPS61165 should be also ok right with a current limit of 1.2A ?

Thks & rdgs

Dave 

Hi Dave,

Yes, you're correct.

BR,

Robin

Hi Robin , 

Thanks for your reply.

Currently the Vref ( 0.2V )  drop below 100mV when i try to calibrate the Iled to 360mA by adjust the resistor value.

Any idea what could cause the Vref unable to hold to 200mV ? 

By the way is there anyway to link up with you via skype ? whatapp ? email ? to speed up the investigation ?

A million thanks in advanced.

Hi Dave,

If you want 18V/360mA output under 5V input, the average inductor current is above device current limit. So device is not able to hold the output voltage.

I will request friendship from you and then give you my email address.

BR,

Robin

Hi Robin,

Thanks for your reply.

Can you provide more details for the comment " average inductor current is above device current limit , so the device is not able to hold output voltage " ?

So the issue is with the inductor selection ?

My email is dave.tan@brtchip.com

Thks & Rdgs

Dave

Hi Dave,

Please check your email inbox.

BR,

Robin