TPS40057: About the relationship between pwm modulator DC gain and VIN

Victoria Qin

Part Number: TPS40057
Other Parts Discussed in Thread: LM25145

Hi Team,

I',m studying about VFF control mode recently. There is a document shows the pwm modulator gain which is the second picture. We can see from it the VIN is the den of Fm so that it can be canceled by VIN in Gd transfer function part. It makes sense and easy to understand.

But in TPS40057 datasheet , I'm kinda confused about this, as seen in picture below. Why here says the VIN is the num of the modulator DC gain which conflicts to the upper document. And meanwhile I can't understand this sentence well 'with VIN(min) being the minimum input voltage required to cause the ramp excursion to reach the maximum ramp amplitude of VRAMP'. Could u please kindly help to clarify it?

Thanks a lot! Have a nice holiday!

Victoria

over 2 years ago

+1 Timothy Hegarty over 2 years ago

TI__Guru 72782 points

Hi Victoria,

The effective DC gain is Vin/Vramp, which should be constant over the Vin range if Vramp = 1/Vin.

Regards,

Tim

+1 Timothy Hegarty over 2 years ago in reply to Timothy Hegarty

TI__Guru 72782 points

I recommend using the LM25145 42V voltage-mode controller. It has better performance and more cost effective vs. the TPS40057. The LM25145 als has more detail on its line feedforward feature.

Regards,

Tim

0 Victoria Qin over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2610 points

Hi Tim,

Appreciate a lot for your reply. I would like to make it more clear that the effective DC gain is constant and equals to Vin(min)/Vramp over whole Vin range. Is it right?

Best regards,

Victoria

0 Timothy Hegarty over 2 years ago in reply to Victoria Qin

TI__Guru 72782 points

Hi Victoria,

I doubt Vin(min) should be in that expression. The gain of Vin/Vramp is a constant. This is because Vramp changes inversely with Vin (Vramp = 1/Vin).

Regards,

Tim

0 Victoria Qin over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2610 points

Hi Tim,

In TPS40057 datasheet, here says the Vramp amplitude is constant and independent from Vin. It's a little different from what u said. Could u please help to make it more clear?

Best regards,

Victoria

0 Victoria Qin over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2610 points

Hi Tim,

I read about LM25145 datasheet, I found the Vramp indeed varies with Vin... So does VFF control mode have two ways to ensure a constant Vin/Vramp? One is to vary Vramp amplitude with Vin but keep Vramp slope constant, another is to vary Vramp slope with Vin meanwhile keep constant Vramp amplitude. Is it right?

0 Timothy Hegarty over 2 years ago in reply to Victoria Qin

TI__Guru 72782 points

Hi Victoria,

If the slope increases, then the peak amplitude of the ramp must also increase (for a given switching period). I think the mention of constant amplitude in the TPS40057 datasheet is a bit confusing. The only way the slope increases and amplitude stays constant is the pulse width decreases.

In any event, the LM(2)5145 is the preferred voltage-mode controller here.

Regards,

Tim

0 Victoria Qin over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2610 points

Hi Tim,

Here is the VFF effect diagram in TPS40057 datasheet. ton1>ton2 which aligns with decreasing pulse width. In LM25145 datasheet, I 'm not able to find a diagram clarifying the principle about how VFF effects PWM duty cycle. Could u please offer any diagram to clarify it?

BR,

Victoria

+1 Timothy Hegarty over 2 years ago in reply to Victoria Qin

TI__Guru 72782 points

Hi Victoria,

It's effectively the same diagram for the LM25145. The ramp can be pulled low after the on-time (once the PWM comparator trips) or it can extend for the full switching period such that the full ramp amplitude is observed.

My understanding is the two ICs have effectively the same functionality from a feedforward standpoint. Both compensate for the change in gain with input voltage.

Tim

0 Victoria Qin over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2610 points

Hi Tim,

About what u said ' if Vramp = 1/Vin', Vramp changes inversely with Vin. But in LM25145 datasheet it says as below, Vramp increases as the Vin increases.

It makes me confused. Could u please help to explain that?

Best regards,

Victoria

+1 Timothy Hegarty over 2 years ago in reply to Victoria Qin

TI__Guru 72782 points

I should have said Vramp is proportional to Vin.

See the LM25145 EC table where it gives the feed forward gain as Vin/Vramp = kFF = 15

Hope this clarifies it. Thanks for the question, Victoria.

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TPS40057: About the relationship between pwm modulator DC gain and VIN