Soft start latching switch for DC-DC converter

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I'm looking at the following DC-DC converter device: XT1861

 

 

The datasheet is very basic and I could not find the enable pin threshold voltage, however the application circuit shows the enable pin shorted to the output so I assume the voltage needs to be around output level. I'm interested to power-on the device using a momentary switch and an AVR will turn it off. I've found similar topics online but none were exactly like this.

 

Suggestions are appreciated!

 

 

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Look at the last graph in the datasheet. Around 0.6V.

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Kartman wrote:

Look at the last graph in the datasheet. Around 0.6V.

 

Nice catch - so this means that the enable pin can be hooked up to pretty much anything. I've seen this circuit:

A series resistor should be added to the switch so it won't short as the converter powers up. Anything else?

 

 

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You are gonna short the output to ground through that mosfet? Put a resistor in series with the diode maybe at least.

Jim

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jgmdesign wrote:

You are gonna short the output to ground through that mosfet? Put a resistor in series with the diode maybe at least.

Jim

 

Or just replace it with a resistor?

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NO that is a mess!

 

Hook Vbatt to switch to schottky diode 1 to EN  ...pressing  switch brings up EN & the power to the AVR.  An AVR pin will then be quickly set high by the program (once it decides power should be maintained).

Hook AVR pin to schottky diode 2 to EN   ....when the AVR pin goes high, it will keep EN high & power on.

 

this part will contain both diodes, the common goes to EN

 

When the AVR pin goes low, nothing will keep EN high &  it will shut off.  Add a 10K-50K pulldown from EN to gnd.....they don't spec an EN pin idle current, so you maybe can't go too high ohms.

You prob don't want the pin to float, unless the datasheet shows something otherwise.    Lower ohms, is extra load on the battery (when AVR pin is high).    

 

Note that many boost chips (especially cheap ones) only stop boosting....they can't shut off the output from the input (so VOUTminimum =Vin) ...this chip looks like it has a switch, but they give very little verbiage about it.  

 

You are planning on including the voltage-boosting inductor, aren't you?  (since  the purpose of the chip is to control the inductor to form a booster)

.

==========

If the AVR is powered by something else & you only want it to control the booster:

1) Your circuit is a disaster, since Vout will be applied to the battery when the switch is pressed--the battery will try to charge itself !!! (Patent this energy generator!)

 

2)  Tie your switch to the AVR & let it decide what to do:

         Tie Avr pin to EN via a 1K resistor

         When the switch interacts with the AVR program , it can decide to take action.  If the pin is set low, it will pull EN low & shut off the power, if the pin is set high, the power will be on.

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. Nov 30, 2019 - 10:53 PM
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With a little more thought, you can use two resistors and no mosfet.
You might want to look at some Torex parts rather than what could be chinese copies.