EngineersForge
Newbie level 1
Hi all:
I am hopping to solve a pesky configuration of a switching power supply, part number LT1374.
In our design, we need this switcher and ignition circuit to do the following:
1) Default OFF, ignition signal is not present.
2) Ignition signal is sensed, optocupler turns ON switcher
3) Switcher provides 3.45Vdc to a LDO at 2.8Vdc that keeps the switcher ON indefinitely
4) Ignition signal is removed optocupler turns OFF; LDO keeps switcher ON until power is turn OFF by microcontroller after it performs controlled power-down.
The problem is that the switcher’s shutdown pin is self-biasing if left open. The pin has two thresholds. At 2.38Vdc the switcher goes into lockout, and at 400mVdc it turns off completely. It needs to be turn off completely to achieve minimum power draw.
The sample (sorry it is very basic) does not work properly although it should. The problem seems that the switcher’s shutdown pin sources WAY too much current (about 300uA and is specified only for 3.5uA). For whatever reason, any high-side switch configuration becomes partially turn on?
The question:
Is there a better or simpler way to control the shutdown pin of the LT1374, without using a germanium type low-side switch? Any suggestions would be greatly appreciated!
Many thanks,
MK
I am hopping to solve a pesky configuration of a switching power supply, part number LT1374.
In our design, we need this switcher and ignition circuit to do the following:
1) Default OFF, ignition signal is not present.
2) Ignition signal is sensed, optocupler turns ON switcher
3) Switcher provides 3.45Vdc to a LDO at 2.8Vdc that keeps the switcher ON indefinitely
4) Ignition signal is removed optocupler turns OFF; LDO keeps switcher ON until power is turn OFF by microcontroller after it performs controlled power-down.
The problem is that the switcher’s shutdown pin is self-biasing if left open. The pin has two thresholds. At 2.38Vdc the switcher goes into lockout, and at 400mVdc it turns off completely. It needs to be turn off completely to achieve minimum power draw.
The sample (sorry it is very basic) does not work properly although it should. The problem seems that the switcher’s shutdown pin sources WAY too much current (about 300uA and is specified only for 3.5uA). For whatever reason, any high-side switch configuration becomes partially turn on?
The question:
Is there a better or simpler way to control the shutdown pin of the LT1374, without using a germanium type low-side switch? Any suggestions would be greatly appreciated!
Many thanks,
MK
