TL2845: Frequency Setting

Santhosh Srinivasan

Part Number: TL2845
Other Parts Discussed in Thread: UCC28180

Hi,

I'd like to understand how the TL2845 controller sets the frequency with the combination of Rt and Ct. (I know there is a given formula that has it as 1.72/(Rt*Ct) but I'm wondering about its internal operation). So does it work similar to say, a controller like UCC28180, where the frequency is proportional to the current supplied by the pin (and as a result the resistance we connect to the pin)? If i were to supply a small current externally at the pin through the resistor Rt in TL2845, will the frequency change?

Thanks,

Santhosh

over 3 years ago

+1 Steven Mappus1 over 3 years ago

TI__Mastermind 33135 points

Hi Santhosh,

It's fairly simple as you've described. Frequency is proportional to dV/dt of the RAMP which is set by the amount of constant current available to charge CT. The RT sets the current into the CT for the linear charging portion of the RAMP. when the RAMP reaches the peak value, the CT is discharged internally. Yes, you should be able to apply an additional external current through RT and manipulate the frequency this way. Thanks for connecting through E2E.

Regards,

Steve

0 Santhosh Srinivasan over 3 years ago in reply to Steven Mappus1

Prodigy 30 points

Also just to confirm, the internal voltage at pin 8 (VREF) is just 5V, so for any resistance the current would be 5/Rt?

0 Steven Mappus1 over 3 years ago in reply to Santhosh Srinivasan

TI__Mastermind 33135 points

Not really 5V/Rt because you are charging a capacitor with a voltage source. The voltage at one end of Rt is fixed by VREF, the voltage at the other end of Rt is the RAMP. The charging current is approximated by I=Ct*dV/dt. Ct is what you choose for Ct, dV is ~1.7V which is from the data sheet oscillator RAMP peak-to-peak voltage and dt is almost 1/F if your discharge time is fast.

Regards,

Steve M

0 Santhosh Srinivasan over 3 years ago in reply to Steven Mappus1

Prodigy 30 points

What do you mean by "if your discharge time is fast enough"? Why could it change? Also, is the internal discharge automatic? So essentially the capacitor oscillates b/w some voltage V and V+1.7?

0 Steven Mappus1 over 3 years ago in reply to Santhosh Srinivasan

TI__Mastermind 33135 points

Hi Santhosh,

The discharge time is the downslope of the RAMP. The discharge time is proportional to Ct so for a large value of Ct, you have a longer discharge time, a small value of Ct gives a faster discharge time. After thinking about it some more, just use the estimate of Ichg=5V/Rt...once you have the circuit built work through the I=Ct*dV/dt to understand what's happening. I feel that I=Ct*dv/dt gives more insight into the RAMP generation. Thanks for connecting through E2E.

Regards,

Steve M

0 Santhosh Srinivasan over 3 years ago in reply to Steven Mappus1

Prodigy 30 points

Alright, thanks Steven!

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TL2845: Frequency Setting