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LM2591, LM2596, LM2941 - Add Start up Delay

Other Parts Discussed in Thread: LM2596, LM2941

Hello TI,

I have a customer trying to do a design change to add start up delay to the on board power supplies to reduce inrush current at start up. The input voltage to the board is 15V.

LM2596 was used as the switcher with 5V output.

LM2591 (switcher) was used as the negative regulator for -13V. So the effective input voltage to this switcher is 15+13=28V.

LM2941 was used as the LDO for +13V.

 

Also see data sheets below

http://www.mouser.com/ds/2/405/lm2596-405261.pdf

http://www.mouser.com/ds/2/405/lm2591hv-405258.pdf

http://www.ti.com/lit/ds/symlink/lm2941.pdf

 

The typical start up delay circuit and the threshold parameters for the ON/OFF pin are pretty much the same for LM2596 and LM2591. For example, it showed that 0.1uF, 47K, 47K can be used to form the discharge circuit at ON/OFF pin. I clearly understand the threshold voltage (typical 1.3V). But I'm not sure about the threshold current. Does it mean that the resistor values have to be able to provide this much current to make sure the ON/OFF pin is properly biased? I guess it will need more current for the OFF state and less current for the ON state?

 

The LM2941 data sheet only gave the ON/OFF pin parameters. The threshold current of 50uA is a lot higher than the typical 5uA of the switchers. Also there is no start up delay circuit example given in the LM2941 data sheet. So can we arrange similar start up delay circuit for LM2941? Can TI provide some typical values to use for LM2941? Should we use smaller value resistors than 47K since the threshold current is now 50uA?

 

It's really not clear to me what the threshold current means. Could you please explain it?

Thanks,

Dan