Reverse Output Current through a LT3045

Question

Hello, 
 I would like to use the output of an LT3045 LDO (set at +5V in this case) as the upper voltage rail for a protection diode as pictured below:

At an input voltage around 5.7V (on the TRIG-SYNC In line) the diode begins to turn on (1 mA current), and at an input voltage of 6.0V the diode is at it's maximum current (200 mA). 
 
 My question is, in the broadest terms, will this work? 
 
 Particularly, how much reverse current will the LT3045 output tolerate and where does that current go? 
 
 Thank you very much for your help, 
 Tony

Achim · Accepted Answer

Hello Tony, 
 tonyr said: Particularly, how much reverse current will the LT3045 output tolerate and where does that current go? 
 my answer for a standard LDO would be: "it will not work, the LDO output can source current, but it cannot sink any current." But the LT3045 is not standard, because it has an "output overshoot recovery" included. This will indeed sink some current when the output voltage rises too high. If you look at the block diagram on page 14 of the datasheet you can find an npn-transistor close to the OUT-pin which conducts the sinked current to GND. 
 Regarding the size of the sink current the datasheet says: "As illustrated in the Block Diagram, the LT3045 incorporates an overshoot recovery circuitry that turns on a current sink to discharge the output capacitor in the event OUTS is higher than SET. This current is typically about 4mA." On page 9 of the datasheet you can find a diagramm that shows, how this current depends on the magnitude of the overvoltage and on temperature. 
 So for currents in the single-digit mA range your idea might work. But I don't see any specification for the "switching speed" of this recovery circuit, and for currents >10mA it surely doesn't work. 
 best regards 
 Achim)

Heke · Answer

Hi, 
 To supplement Achim 's spot-on advice: 
 If the TRIG-SYNC comes from an external (unknown) source, it may be preferable to use a comparator instead of a logic gate as the front-end. If the input signal hovers halfway the supply voltage, the input stage of the logic gate will draw considerable current from the supply. A comparator won't have that problem. That is, have a comparator together with a voltage divider at the input, with some positive feedback (for hysteresis). Place the diode protection circuit to that comparator input (You want to put the protection circuit behind some series resistance in order to limit the clamp current). 
 Cheers, 
 heke, AsamaLab

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Reverse Output Current through a LT3045