TL2845: Does the TL2845 support frequency dithering?

Santhosh Srinivasan

Part Number: TL2845
Other Parts Discussed in Thread: UCC28180, TLV3201

Hi,

I'm using the TL2845 current mode PWM controller for a DC-DC converter operating at a certain constant frequency. I'm aiming to add a frequency dither so that I can reduce the EMI peaks. My requirement is very similar to how frequency is dithered on the UCC28180 on the SLUA704 document. However, the UCC28180 has a simple resistance where we supply an additional dither current to change the frequency. On the TL2845, it is not simple as there is an Rt-Ct combination that decides the switching frequency. Are there any design examples or procedures to follow to obtain dither on the TL2845? Is a frequency dither even possible on the TL2845?

Thanks

Santhosh

over 3 years ago

+1 Hong Huang over 3 years ago

TI__Guru 74115 points

Hi,

The same idea can be used for TL2845. You can use a circuit similar to TLV3201 to make dithering circuit based what you mentioned for UCC28180 (SLUA704) or this below (SLUA646). You can connect to RT/CT pin of TL2845. The capacitor on TL2845 RT/CT pin is connected to a capacitor to GND and a resistor to VREF. Now you can connect TLV3201 dithering circuit to RT/CT pin so the capacitor on RT/CT would have two charging currents, one from VREF and the other from TLV3201. TLV3201 circuit will provide a designed slower frequency charging current to superimpose to VREF charging current - the oscillator frequency of TL2845 then is dithered by TLV3201 circuit.

www.ti.com/.../slua646.pdf

0 Santhosh Srinivasan over 3 years ago in reply to Hong Huang

Prodigy 30 points

Okay, that sounds good. Also, what would the voltage internal to the Rt/Ct pin be? In the example document we have 2.5V internal to the Rt pin because of which we use a 5V Vref so that the dither is symmetric.

0 Hong Huang over 3 years ago in reply to Santhosh Srinivasan

TI__Guru 74115 points

Hi,

Yes, roughly RT/CT pin peak is about 2.5V at end of each clock cycle. The dither frequency is much lower than your switching frequency. So no need to make very accurate f_dither to fsw.

0 Santhosh Srinivasan over 3 years ago in reply to Hong Huang

Prodigy 30 points

Thank you!

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TL2845: Does the TL2845 support frequency dithering?