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# oscillator frequency range Tl5001

Other Parts Discussed in Thread: TL5001, TPS40200, TL494, UCC35701, SG2524

We are designing a PWM voltage mode control circuit for a buck converter using TL5001. The design requires a frequency of 10 KHz(KW power IGBT application).

There is a discrepancy in the data sheet of TL5001 (SLVS084F). The highlights on the first page say that the oscillator frequency is 20 KHz to 500 KHz. On page 3 in the "oscillator/PWM" section it says that for the range of 20KHz to 500KHz, Rt ranges from 15K to 250K. But in the graph in Figure 5 it suggests that for the frequency of 20 KHz we require Rt of about 500K.

Please clarify whether we can go to 20 KHz (Rt = 500K) and also 10 KHz (Rt = 800K)

Similarly, in the case of tps40200 (SLUS659G), can we go down to 10 KHz. It will be really helpful if you can shed light on this.

• You have picked a really old device.  I doubt that you will be able to get a definitive answer.  Also, why would you want to operate at such a low frequency?  I would suggest that you set Fsw higher in the range of RT less than 250 kohm.  In the mean time, I'll see if anyone is still around who has worked with TL5001, but I bet they are long since left TI.

• Hello Anish,

According the different available reference designs, the graph shows which resistor value you need. John is right, when he mentions that this part is somewhat old and that your switching frequency is very low.

If you are using resistors in the range of or above 500 kOhm, you will get some influence on the switching frequency due to noise in the resistor and noise on your board. Especially when you think about a switch node not far away from the resistor, I think noise coupling will become the biggest issue with these high resistors.

Why do you want to switch such low?

Best regards,
Brigitte

• We have an existing design with IGBTs and so we cannot go with higher frequency.

The new ICs have very high frequency and so I went with this old IC.

If you know about any new ICs that work with frequency range near 10KHz, can you please tell me?
• For new ICs, I think you will find only higher frequencies.  For lower frequencies, TL494 (an even older device) can switch from 1 kHz to 300 kHz.  For TL5001, low frequencies require high values of RT which may result in noise issue as stated above.

• In reference to  the noise problem you mentioned, there will be two types of noise :-

1. Switching noise
2. White noise

With reference to switching noise, our driver card is a separate card with ground plane underneath. The IGBT gate pulses are given through isolated drivers. So, this noise should not be a problem.

Regarding the white noise, using metal film resistor for RT should minimize this noise also.

Do you think the noise problem will be overcome due to above facts.

Also, please let me know your thoughts about using UCC35701 (Introduced in January 2000) PWM IC as an alternate choice for frequencies below 20 KHz and up to 10 KHz.

Please refer SLUS293C, page 8, figure 3, we can change RT or CT. Increasing CT should avoid noise problems.

Thank you,

Anish

• UCC35701 should work well down to 20 kHz. It may work lower, but the graph of figure 3 clearly shows operation down to 20 kHz.
• In reference to  the noise problem (for TL5001) you mentioned, there will be two types of noise :-

1. Switching noise
2. White noise

With reference to switching noise, our driver card is a separate card with ground plane underneath. The IGBT gate pulses are given through isolated drivers. So, this noise should not be a problem.

Regarding the white noise, using metal film resistor for RT should minimize this noise also.

Do you think the noise problem will be overcome due to above facts?

Thanks you,

Anish

• With very careful layout it should be ok. You will probably want a separate analog ground and connect it to your bottom side ground plane at a single point. But there is no test data to support it. I think you will be the first to try it at that low frequency.
• TL494 - The error amplifier in the IC has severe capacitive loading restrictions on the amplifier output. ref: SLVA001E, page 11. We need capacitive load (type III compensation) at the amplifier output.

UCC35701 is not recommended below 20KHz. Also, there is no error amplifier inside the IC.

SG2524 (SLVS077E) seems to solve these problems. So I think this IC will do.

I would like to get your thoughts on this.