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TPS61194: Switching frequency is not as expected

Dominik Schegg

Prodigy 50 points

Part Number: TPS61194
Other Parts Discussed in Thread: , TPS92200

Dear all

I am currently developing a circuit in which an LCD with 4 LED strings is used as backlight.

Uin = 16 - 36V
string voltage = 10V
string current = 55mA

Based on this information I choosed the component TPS61194 in SEPIC mode.
With the help of the Eval Board TPS61194EVM I have realized the corresponding setup.

Sting current is at 55mA, as expected, also the string voltage is at 10V. Both output values seems to be ok (blue an green curves).

According to equation 2 from the datasheet, the switching frequency is calculated by:
f_sw = 67600 / (R_fset + 6.4)
f_sw = 2MHz for R_fset = 27kOhm

Why is the measured frequency at signal SW (yellow curve) not at fsw as expected at 2MHz?

over 4 years ago

0 Yuting Shao over 4 years ago

TI__Genius 13001 points

Hi,

Can you please upload waveforms again since they are broken.

BR,

Robin

0 Dominik Schegg over 4 years ago in reply to Yuting Shao

Prodigy 50 points

Hi Robin

Sure, here are the waveforms:

The pic above shows the waveform with frequency setting Rfset = 27kOhm

Curve 1: SW Signal (IC11, Pin 18)
Curve 2: Boost Voltage
Curve 4: String current at OUT4 channel

As you can see on the yellow curve, fsw is at around 1/7us = 142kHz.

The waveform below is with Rfset = 160kOhm:

There is no stable swtiching freqency.

Many Thanks!

0 Yuting Shao over 4 years ago in reply to Dominik Schegg

TI__Genius 13001 points

Hi,

This device is a booster, but for your application, Vout>Vin, so device operates at pulse skip mode, that means the switching frequency is lower.

BR,

Robin

0 Dominik Schegg over 4 years ago in reply to Yuting Shao

Prodigy 50 points

Hi Robin

Just to be sure: in my application Vin is at 24V (battery) and led string voltage is at 10V, so Vout < Vin

Can you confirm this waveform is plausible?

Would it be necessary to make improvements?

kind regards

0 Yuting Shao over 4 years ago in reply to Dominik Schegg

TI__Genius 13001 points

Hi,

It seems OK. BTW, could you please share your circuit schematic? And why not considering of buck LED driver?

BR,

Robin

0 Dominik Schegg over 4 years ago in reply to Yuting Shao

Prodigy 50 points

This will be the final application:

Because I couldn't find a suitable buck converter.

If you can recommend a component, I am open to it!

Thanks Robin

0 Dominik Schegg over 4 years ago in reply to Dominik Schegg

Prodigy 50 points

I try it again:

0 Yuting Shao over 4 years ago in reply to Dominik Schegg

TI__Genius 13001 points

Hi,

Please take a look at TPS92200 for high efficiency and fewer external components, and you can set the current sense resistor 100mV/220mA.

BR,

Robin