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Part Number: TPS92515

Hi Team,

I have a question about TPS92515 analog dimming.

My understanding is TPS92515 VIADJ has some offset and supporting turn-on-calibration so even if we supply 0V to VIADJ, there is some LED current thr LED based on below link.
e2e.ti.com/.../3673435

in order to reduce that LED current with 0VIADJ, I think there are three ways but do you know any other way to reduce LED current?
I listed each pros/cons but if there are any other pros/cons, could you let me know?
1: use smaller Inductor
> Pros: higher tr/tf so RMS current becomes lower.
> Cons: easy to hit minimum on time
2: longer toff
> Pros: in DCM, longer toff means longer off time so RMS current becomes lower.
> Cons: longer switching frequency.
3: PWM pin turns Low during 0VADJ
> Pros: no switching so true 0A
> Cons: it takes transient time from no switching mode to switching mode.

Regards,
Kai

• Hello Kai,

1)  This is true under certain circumstances.  If you hit minimum on-time the energy in the inductor is 1/2*L*i^2.  If 'L' goes down 'i' goes up since minimum on-time is a fixed value. Since the 'i' term is squared and the 'L' term is not the energy goes up proportionally with 'i'.  Other than your statement is correct since you are regulating peak current.  The 'i^2' stays constant and 'L' drops so the energy will be lower.

2)  This is also true however if switching at lower frequency and trying to keep the ripple constant 'L' has to go up defeating the decrease in average current.  This can be offset by using more output capacitance.

3)  This is partially true.  There really isn't transient time from no switching to switching, if PWM goes high the TPS92515 will turn on the MOSFET until it hits peak current and be in regulation after the first on-time.  If there is output capacitance there can be delay however if you apply a duty cycle of PWM at a reasonable PWM frequency you will get reduced current.

Note that I stated average current, not RMS current.  Calculating current when driving LEDs uses average current not RMS current.  This is because the load is a fixed voltage not a resistor load.

You may want to consider using the TPS92515A instead of the TPS92515 due to the power on calibration circuit.  The benefit of the TPS92515 is a more accurate set-point for regulation.  The Con is that some parts can power up in a different calibration state if close to a bit transition.  This can cause a 5 mV change in offset voltage.  At full brightness it has a more accurate set-point, at low dim levels it can be a noticeable light change during power cycling even though it is a more accurate set-point.  The TPS92515A does not have the calibration circuit which makes the offset error higher but you will not see some TPS92515A change regulation set-point during power cycling.

Best Regards,

• Thanks Irwin,

Regarding A version, is it TPS92515AHV-Q1?

Also is only the difference between non A version and A version calibration function?

Regards,
Kai

• Hello Kai,

Yes, that is the only difference.  If you compare the datasheets you will see the error amplifier offset is what is different, Vcst in the electrical characteristics table.

Best Regards,

• Hi Irwin,

Thanks. I will discuss what is the best for customer system.

Regards,

Kai