UCC3808-1: Soft start for UCC3808

Enzo Illiano

Part Number: UCC3808-1
Other Parts Discussed in Thread: UCC38C44

Hallo,

I need to implement a soft start on your UCC3808. I would like to have the possibility to set the soft start time (3.5 ms of the internal soft start are not enough)

As usual I added a capacitor to slow the voltage increase on “COMP”. This is allowed on the UCC38C44.

I imagine that it is also allowed for the UCC3808 and it seems to work properly (I tested it, the soft start time can be easily varied changing the value of C64)

Can you confirm us that this is not an improper use of the IC? In other words the IC should no be damaged if the COMP pin is connected to a large capacitor (about 10 uF)?

Thank you and best regards!

over 6 years ago

0 Colin Gillmor over 6 years ago

TI__Mastermind 25570 points

Hello Enzo

Using COMP to control the SS time is OK and adding a large capacitor at this pin won't cause any difficulty. Please look at section 7.3.1 of the datasheet (below) where it says the the COMP pin is internally current limited. This current is 250uA when COMP is at 3.5V (COMP source current in the Electrical Characteristics table on P5)

Regards

Colin

0 Enzo Illiano over 6 years ago in reply to Colin Gillmor

Prodigy 110 points

Thank you very much!

0 Enzo Illiano over 6 years ago in reply to Enzo Illiano

Prodigy 110 points

Dear Colin,

Just a last question. In the circuitry I proposed the capacitor voltage is "kept high" during nomal operation by 1 M ohm resistor. When the circuit is turned off it is discharged by the diode.

Can we avoid the 1 mOhm resistor? Do you suggest to connect the discharge diode to VCC or to the RC PIN?

Thank you!

Enzo

0 Enzo Illiano over 6 years ago in reply to Enzo Illiano

Prodigy 110 points

Dear Colin,

It would be nice if you could give me an answer asap since I need to freeze the circuit schematics.

Don't hesitate to contact me if the question was not clear.

Thank you! Happy new year!

0 Peter Meaney over 6 years ago in reply to Enzo Illiano

TI__Mastermind 31580 points

Hi Enzo,

Colin is on holiday this week also I do not have a copy of your schematic to refer to. If you just need a DC rail then Vcc should be fine, but please ensure the voltage rating of the pins are not exceeded.

Regards

Peter

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UCC3808-1: Soft start for UCC3808