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Hello
i would like to use the LMR16030SDDAR and i have 2 questions about this part:
attached are my schematic and the datasheet...
my parameters are : input = 18-60V , output = 15V (3A)
1) for the EN/UVLO enable threshold is 1.2 V as stated in the datasheet (page 13), so i have used 2 resistor divider (R22=RENT, R=23RENB) to make this pin 1.2V when VIN is 18V,
BUT in the datasheet (page 14) there is a different formula for the 2 resistor divider (R22=RENT, R=23RENB) which is not understood.
what is the correct way to set UVLO for 18V ?
2) can the Soft Start be more than 10mS (WEBENCH® POWER DESIGNER limit to 10mS only) ?
what is the Css or the Soft Start time limit ?
thanks in advance
Hello
The equations in the data sheet are correct.
If you wish a Von=18V and a Voff=17V (example)
then you would use RT=274k and RB=19.1k (standard values)
What soft-start time are you looking for ?
Thanks
Hi Frank
thank you for your fast reply, can you tell me please how and what values did you use in the equations to get these resistors ?
about the soft start i would like to know what is the IC limit, there is no reference about it in the datasheet also Vref is also not known ?
i would like it to be as long as possible but below the limit for proper operation...
Hello
See attached calculation.
I don't know why Webench limits the soft-start to 10ms.
One thing to be careful about is the size of the soft-start
capacitor. The discharge current may become large with
larger values of capacitor.
I think I would limit the time to 50ms or 100ms.
Thanks
Hi Frank
thank you very much, about the soft start WEBENCH is limiting to 10mS also other DC/DC converters IC this is very strange (example tps563210a) !
anyway if i want to increase the soft start what would be the value because in the datasheet Vref is not mentioned ?
220nF ? and if yes isn't it a high value ?
Hello
The reference voltage is 0.75V
I think that 220nF is OK; that would give about 55ms.
Thanks