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# UC2845A: start-up time

Part Number: UC2845A

Hi, Team.

i've been asked by Customer as below.

1. is it realistically happened that It takes 20sec for start up time as below condition.

DC180V

start up R...188kohm(there are 4 resister(47k) in series.

Cap for Vcc..470uF

basically it takes 4sec for start up time but customer has seen that it took 20sec just one time.

i know it is difficult to assume though, what root cause can we expect ?

when customer replaced cap for Vcc, customer had seen it one time never happened ever.

• Hello Masaharu,
The startup time relates directly to the R and C values on the Vcc pin.
You can expect a time constant of 88 seconds with the values you used.
This is the root cause you are looking for.

Regards
John
• Hi, Thanks.

as my calc, the 180Vdc reaches 9V (UVLO turn on) by 4.5ms.
how did it take 20ms ..?
• Hi,

In addition, May I ask you to know how we measure the time from power off to turn on for IC ?
it takes a time for making the voltage from IC though,
Customer was telling about the time that IC wakes up(it would be 4ms if the conditions are 180Vdc ,188kohm and 470uF)

then, what might it be root cause for the situation customer has seen "20ms" was taken for IC wake up
• Hello Masaharu Takahashi,

With a 180V input the capacitor will take about 4 seconds to charge up to Vcc(on) = 8.4V.

To verify this you will need to use a scope and obtain a plot of Vin and Vcc.

If you are seeing a longer time you need to verify the R and C values as well as the input voltage.

Is Vin = 180V as soon as the power is applied ?

You need to verify the test conditions with some actual measurements.

Regards

John

• Hi,

Since this was just one time and customer doesnt have it such as waveforme.
customer had checked Vcc and Vin to measure the time.

what factor may occur for increasing "wake up time" from 4s to 20s?

Is start up current related with this phenomenon ?
• Hello Masaharu Takahashi,
If the test is not repeatable then it may be a mechanical issue.
Perhaps an intermittent solder connection
Regards
John
• Hi, John-san