Shutdown negative power regulator LDO

[karbiuch]

Hi


I dont understand application note of lt1964. In my board I need to using one atmega pin to turn off -5V stabilizer. The documentation says that:

Note 8: Positive SHDN pin current flows into the SHDN pin. SHDN pin current is included in the GND pin current specifi cation.

"SHDN: The SHDN Pin is used to put the LT1964 into a Low
Power Shutdown State. The SHDN pin is referenced to
the GND pin for regulator control, allowing the LT1964 to
be driven by either positive or negative logic. The output
of the LT1964 will be off when the SHDN pin is pulled
within ±0.8V of GND. Pulling the SHDN pin more than
–1.9V or +1.6V will turn the LT1964 on. The SHDN pin
can be driven by 5V logic or open collector logic with a
pull-up resistor. The pull-up resistor is required to supply
the pull-up current of the open collector gate, normally
several microamperes, and the SHDN pin current, typically
3μA out of the pin (for negative logic) or 6μA into
the pin (for positive logic). If unused, the SHDN pin must
be connected to VIN"

http://cds.linear.com/docs/en/datasheet/1964fb.pdf

This means that when you give to the shutdown pin 5V stabilizer is turned on? Do I have to connect +5V to shutdown pin via a resistor or without? I understand that if I gave -5V to shutdown pin IC will turn on. But as I said, atmega is able to generate in pin output 0 or 5V. How to shutdown/not shutdown LT1964 with positive logicl?

 

[d123]

Hi,

I think it means that shutdown >+1.6V = output on OR shutdown >-1.9V = output on, so at 25ºC between -1.9V and +1.6V the LT1964 is off. It looks like you can use positive or negative logic, it's all the same, to turn it on.

It's an active low pin.

"This means that when you give to the shutdown pin 5V stabilizer is turned on?
Do I have to connect +5V to shutdown pin via a resistor or without?
I understand that if I gave -5V to shutdown pin IC will turn on. But as I said, atmega is able to generate in pin output 0 or 5V. How to shutdown/not shutdown LT1964 with positive logicl?"

Yes, +5 or -5. Look at the datasheet, shows SHDN connected to Vin " If unused, the SHDN pin must be connected to VIN."
You only need a pull-up resistor if the ATMEGA output is an OPEN COLLECTOR (BJT collector is the output, that's why you need the pull-up, and it would function as an inverter - open collectors are high with low input and low with high input).
You can turn it ON with a +5V signal, OFF with a 0V signal, so the ATMEGA 0/5 should work fine, I think.

Page 6 of datasheet shows graph: "SHDN Pin Thresholds" for visual interpretation, and how temperature affects thresholds, and page 4 specifications section: "Shutdown Threshold" explains min, typ, max thresholds in + and - Volts.