TPS61500: TPS61500 Min. PWM freq.

Yaacov Rothschild

Part Number: TPS61500

We are using the TPS61500 as a LED driver for IR illumination.
The circuit drives 4 LEDs at a 2.2A synchronized with the camera shutter.
It's straight out of the datasheet. Input voltage is 12v, pulse time is 2ms.
LED combined forward voltage is ~13.5v

When the frame rate is > 50 fps the circuit works fine. At somewhere
around 47 fps the current suddenly falls to around 1A.

I have tried different values for the soft start cap, the input caps, the output caps
and the freq resistor. I have also tried removing the soft start cap altogether, all of
this to no avail. Always at around the same 47Hz frequency, the output current suddenly
drops.

What causes this sudden drop ? Is there any way around it ?

Our new cameras run at 20 fps so I need to find a solution.

Thanks,
Yaacov

over 8 years ago

0 Rodney Jones over 8 years ago

TI__Prodigy 600 points

This thread had been assigned, it will be replied to in an short period of time.

0 Caryss Li over 8 years ago

TI__Expert 4550 points

Hi Yaacov,
I'm afraid the dimming frequency you use is much lower than our recommedation, the min PWM frequency is 200Hz for both PWM dimming and Analog dimming, 50Hz is already too low, and 47Hz or 20Hz is much lower. We don't recommend work in that frequency. If you want, you may check the output voltage, after such long off time, the voltage may not be regulated.

0 Yaacov Rothschild over 8 years ago in reply to Caryss Li

Prodigy 40 points

Thanks for the info. Is there another product you could suggest that would meet our requirements ?

0 Caryss Li over 8 years ago in reply to Yaacov Rothschild

TI__Expert 4550 points

Hi Yaacov,
The LED config is 4 leds in series, and led current is 2.2A? When led is on, the inductor current will over 2.7A, that's quite large, I'm afraid we don't have other device support such high current.

0 Yaacov Rothschild over 8 years ago in reply to Caryss Li

Prodigy 40 points

Hi Caryss,

So here is a possible solution. We can just add keep-alive pulses during the off time.
The chip would then receive one long pulse followed by a bunch of shorter ones for each cycle.

Do you foresee any problems doing this ?
Obviously our major concern is heat, what is the minimum recommended duty cycle/pulse length for a 200 Hz signal ?
Would a higher frequency allow a lower duty cycle ?

Thanks,
Yaacov

0 Caryss Li over 8 years ago in reply to Yaacov Rothschild

TI__Expert 4550 points

Hi Yaacov,
Are you using the device working in pure PWM dimming mode? Then the min duty for 200hz could be 1%.

0 Yaacov Rothschild over 8 years ago in reply to Caryss Li

Prodigy 40 points

It's working !

I tried adding a 200Hz 2% duty signal as a keep alive during the off period of my main trigger signal..
Besides for a slight 200Hz buzz from the inductor, it's working fine. Now I just need to re-design the
trigger circuit.

Thanks for your help.

Power management

Power management forum

TPS61500: TPS61500 Min. PWM freq.