I'd like to place a 5v and a 4.096v LT1461 in series driven from the same 12v rail to get a 9.096v output at 50mA. Is this possible or will I have issues floating one above ground in this fashion?
In order to stack voltage references the part must be capable of sinking current since the supply current of the upper voltage reference will flow into the output of the bottom voltage reference. The LT1461 cannot sink current. This fact can be seen from the load regulation specifications where there is only a specification for sourcing current. It can also be seen from the simplified schematic that the output stage only has a PNP transistor and so can only source current.
It is possible to work around this by placing a resistor for the supply current to flow through - that is achieved by placing a resistor to ground on the output of the bottom reference. The supply current for the top reference will then flow through this load resistor.
Pick a resistor value which will give you at least the maximum supply current of the LT1461 (70uA) plus some headroom for resistor tolerance. I would recommend to put the 4.096V reference on the bottom since this will allow you to choose a smaller resistor and lower the overall power consumption of the circuit.
The current sourcing capability of the top voltage reference output stage will not be affected.
Thanks Rob! I had experimented with a similar topology using the resistor as you say with a different reference so I am familiar with the idea. That particular topology had problems with my output current draw and capacitance loading needs that I couldn't solve, hence my interest in the 1461 since it has a very high 50mA output drive.
As a follow on question, is it possible to use a series reference that can sink current, such as the ADR43x series, in a zener diode shunt like fashion? The application I'm trying to adapt this stacked voltage reference topology to is currently using a zener shunt limited to ~17mA total current (including load current) at Vz. I had the thought that a reference that could sink current could be used like a zener but with a third connection for power. Doing this could reduce the strain placed on the reference itself, as it'd only need to sink enough current to regulate the shunt voltage as opposed to supplying all of the current for the load as in the full series implementation. The load in this case is quite constant (once up to temperature the zener shunt is very stable, it is the temperature related voltage deviations I am trying to eliminate) so I believe this approach may be a reasonable solution if it is feasible. I am a novice at best with circuit designs so I am learning as I go here.
Normally you would connect the voltage reference using the three pins VIN, VOUT and GND. Since the IC is placed between VIN and VOUT this is commonly referred to as series mode connection. However, some series mode references can also be connected in a shunt mode where VIN and VOUT are tied together. This is only possible when you are only sinking current. For example, the LT6657 can be used in shunt mode and it is described in the data sheet how to do this. The LT1461 cannot be used in shunt mode.
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