UCC35702

Raffaele Raimo

Other Parts Discussed in Thread: UCC35702

Hi,

I'm woking on a DC Buck converter with UCC35702 pwm controlled by a DC level.

I made a simply test circuit to check the IC but actually i'm stuck about the pwm signal generation: I can't get the IC working propely.

The voltage settings seems to be under the behavior ratings (0,6V<VFF<3,2V; Vdd>UVLO start threshold), Vsclamp is set at 50% of Ct voltage ramp.

It starts the voltage ramp on the capacitor Ct, but nothing happens on the Output pin.

Cs of the SoftStart pin is always at 0.002V and it never charges on startup time.

For the test purpose Ilim and Count pin have been bypassed to gnd.

I can't figure out my fault.

Could you help me to get this circuit working?

Thanks a lot,

Raffaele

over 12 years ago

0 John Bottrill over 12 years ago

TI__Mastermind 25130 points

Raffaele,

First reduce Vdd to no more than 14.3 volts and change R6 to 510 ohms. Put 3 1N4148 in series across R6 (anode to source cathode to Pin 3). This is necessary as you donot have an alternate energy source once the converter has started

Calculate the values of R1 and R2 to get a voltage of 3.5 volts on the Vff pin (Pin 6).

With the Vff voltage set to 3.5 volts using R1 and R2.. Ct should be between 0.2 volts and 3.5 volts.

Try this and see if the IC starts.

Regards,

John

0 Raffaele Raimo over 12 years ago in reply to John Bottrill

Prodigy 90 points

Hi John,

thanks for your suggestion. I did what you said and the IC starts wih the Ct ramp but still have no output.

All voltages are in each own range: Vdd=13.16V; Vctmax=3.5V;

Now Cs Vcs=5V if Vfb=0 and it falls to 0.6V with a Vfb=3V.

From Vfb=0 to Vfb=3V the IC outputs 0V

Here is an oscilloscope screenshot:

0 John Bottrill over 12 years ago in reply to Raffaele Raimo

TI__Mastermind 25130 points

Raffaele,

Put a 10 k resistor between Vref and vfb and another from Vfb to ground and try again.

John

0 Raffaele Raimo over 12 years ago in reply to John Bottrill

Prodigy 90 points

John,

I will get hands on circuit on next Monday.

I have just a question still running in my minds:

Does UCC35702 need the optocoupler on the FB pin or can I drive it as well with a simpler voltage level?

0 John Bottrill over 12 years ago in reply to Raffaele Raimo

TI__Mastermind 25130 points

Raffaelo,

As long as you keep the error amp output below Vref and above ground.

You should start to get pulses once the FB voltage is above about 2 volts (not exact).

Regards,

John

0 Raffaele Raimo over 12 years ago in reply to John Bottrill

Prodigy 90 points

Thank you John. It Worked!

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UCC35702