UCC28070A: Questions about the UCC28070A

Ted Zhang

Intellectual 2280 points

Part Number: UCC28070A
Other Parts Discussed in Thread: UCC28180

Can you help with below several questions for UCC28070A from my customer?

Open‑Loop Detection
1. Datasheet mentioned open-loop detection, however there is no detailed description about this function in datasheet. Can you help to elaborate on this?
How should I connect CSA,CSB if I only need one-phase instead of two-phase interleaved?
See below line highlighted in red, does it mean if I connect RSYNTH to VREF, the CSA and CSB will automatically connect to respective current amplifiers?
What specifically does this falling percentage mean? Does it mean that output voltage should drop at around 93% to trigger slew-rate correction?
What does this parameter(phase mismatch) refer to? Can you help with more details?

Many thanks in advance!

over 7 years ago

0 Jankel over 7 years ago

Intellectual 731 points

Hi,

Another question is as follows:

You can see from 7.3.7 that the open loop protection is realized by the sink current of 250nA, but for instance, when VSENSE is open-loop, the 250nA will pull down the VSENSE to 0V? If so, Ton/T would be continuously increasing, I think this is not what we want. What we want should be totally opposite, that's to say, 0 Ton rather than a continuously increasing Ton.

0 John Griffin1 over 7 years ago

TI__Mastermind 34465 points

Hello Ted

(1) Open loop detection detection is a protection feature and it shuts down the PWM outputs when value of VSENSE drops below a nominal value of 0.75V.
This is important because there may be a situation where the upper resistor in the VSENSE divider chain is disconnected during a fault.
Without the open loop protection feature the UCC28070A will go to full duty cycle and cause dangerously high output voltages

(2) If you are only using one phase then you can just not connect up the other phase. For example connect GDA and CSA but leave GDB and CSB open.
If you only need one phase then you probably should not be using an interleaved controller.
It will be less expensive and simple to use an non-interleaved part such as the UCC28180

(3) What this sentence means that if there is an application where you do not require current synthesis then you should connect RSYNTH to VREF.
This is in an application where CSA,CSB are actually measuring the inductor current instead of the MOSFET drain current. In this situation you need to disable the synthesis function and connect CSA,CSB directly to the current amplifiers.

(4) There is a 100uA current source added to the output of the error amplifier (slew rate correction feature) This is added when the value of VSENSE is 93% of the regulated value of VSENSE (3V)

(5) There is a mismatch in the current amplifiers and the mismatch between phase A and phase B can be anywhere between -12mV to +12mV

For example it may require 12mV on CSA for CAOA to have a zero volt output and CAOB may have a zero volt output with zero volt input.
This is the "mismatch"

Hope this helps

Regards

John

0 Jankel over 7 years ago in reply to John Griffin1

Intellectual 731 points

Hi John,

Thanks for your reply.

There're still some questions as follows.

(2) I like UCC28070A more for these reasons compared to UCC28180(One Cycle Control Mode??????): a little higher freq( because I'm using GaN, so I want high freq controller(analog) ), dither function good for EMI, fast transient response, AC Input Voltage Feed-Forward, zero-power detection, start-up hold off, etc.

(3) What's the meaning of connecting CSA,CSB directly to the current amplifiers?? Shall we manually connect them(there's no pin outside the IC connected to the input of the amplifiers CA1 & CA2) or do you mean that when we disable the synthesis function, the CSA and CSB is automatically connected to the inverting terminals of respective current amp inside the IC?

(6) Pls help me with what I wrote in floor 2, Thanks.

0 John Griffin1 over 7 years ago in reply to Jankel

TI__Mastermind 34465 points

Hello Jankel,

I am glad to hear you are considering using the UCC28070A. It does have many useful features and is more flexible in applications than the UCC28180..

The dither function is useful in EMI and interleaving will help reduce the size of the X capacitors in the input EMI filter. However most pfc's use a switching frequency of less than 150kHz because the limit for conducted emissions starts at 150kHz. If you use a switching frequency of less than 150kHz then you only have to worry about the second harmonics and higher. Higher switching frequencies will seem to reduce the size of the magnetics but you may find that radiated emissions and the parasitics associated with pcb layout will cause common mode noise problems and the EMI filter will actually get bigger as you increase switching frequency.

Your next question is about CSA,CSB. The answer is no. You do not connect these signals.
Connect your current sense signals to CSA and CSB and disable RSYNTH as described. The connection is made internally by the UCC28070A

Regards

John

0 Jankel over 7 years ago in reply to John Griffin1

Intellectual 731 points

Hi John,

Could you help me with this question?

You can see from section 7.3.7 that the open loop protection is realized by the sink current of 250nA, but for instance, when VSENSE is open-loop, the 250nA will pull down the VSENSE to 0V? If so, duty cycle(Ton/T) would be continuously increasing until Max value, I think this is not what we want. What we want should be totally opposite, that's to say, 0 Ton rather than a continuously increasing or Max Ton.

So, how can this 250nA current source realize the VSENSE open-circuit protection???