Looking at the simplified schematic (as depicted inside the IC symbol), one should be able to see that *the LT308x device maintains the voltage at the OUT pin to be equal to the voltage at the SET pin*. In the simplest form of normal operation, a resistor from SET to ground is all that is needed to provide this reference voltage, due to the current source which sources 50 micro Amp of current out of the SET pin. Another way to program an output voltage is to drive the SET pin to a specific desired voltage (ensuring that the circuit driving it can sink that 50 uA).

In the booster designs, the voltage at the set pin is *not *determined by that simple V=IR resistor arrangement. Instead, it is explicitly set to track the output voltage of the other regulator (LT1963-3.3). The output of that regulator has a voltage divider (8.2 & 6200 ohms or 42 & 33000 ohms) which provides the reference voltage to the SET pin of the LT308x. The actual value of the lower leg resistor does not directly determine output voltage, since the equivalent impedance at this node is dominated by the smaller value (upper leg) resistor going to the regulator output, rather than the lower leg resistor to ground. The 50 microAmp current flows through this resistor into the load. The LT308x maintains its output to be at that voltage which is slightly lower (due to the voltage divider) than the output of the LT1963. It will only source current when the voltage drop across the 20 mOhm resistor in series with the the LT1963 output exceeds 4 mV, and at that point, the two regulators will share current based on the ratio of their output resistors as well as their respective output voltages.

Analyzing the resistor network (in post #1) deeper, assume that the output of the LT1963 is fixed at exactly 3.3 V. The current through the 33 k resistor is the sum of the current through the 42 Ohm resistor plus 50 uA (from the LT3081 booster SET pin). Assume that the voltage at this node is approximately 3.3 V (due to the relatively low impedance to the 3.3 V node, the output voltage). Then the current flowing through the 33000 ohm resistor is ~ 100 uA (remember, 50 uA of that comes from the SET pin). Then, about 50 uA needs to flow through the 42 Ohm resistor. This makes the voltage at that node approximately (42 * 50e-6) 2.1 mV lower than the output node. You can see that altering the value of the 33 k resistor will change the (mV level) offset of the booster compared to the LT1963, but will not directly set its output voltage in the "normal" way with V=(50uA*R).

Many thanks for this reply, macboy

I have delved into it some more & there are two factors that I believe are in operation:

- the parts must have a minimum load to stay in regulation - 1mA load for LT3083 & 5mA for LT3081 which begins to explain the differences seen on the schematic between the voltage dividers used on these parts in their role as current booster to the same 3.3V LT1963

- the other factor is that it tries to keep the voltage on the SET pin lower by 4mV than the OUT pin - this seems to be to turn off the current output from LT308X when this difference falls below 4mV differential? Is this correct? I'm not sure how this works, though i.e the how is there a different voltage differential between SET & OUT when load or no-load?

So for LT3083 acting as current booster to 3.3V would require R to gnd of 3.3K (for 1mA load) & hence 4.2ohm as the second R in the voltage divider (to give this 4mV drop) - the datasheet shows X 10 of these values which reduces this load by 10 to 0.1mA so what gives?

There's also a differential between the R connecting the SET pin to load (20mohm) & OUT pin to load R (10mohm) but I doubt this comes into play?

Same applies to LT3081 - as current booster for 3.3V, R to gnd of 660ohms (for 5mA load) & hence 800ohm for other resistor in voltage divider - using X10 would give 6.6K (6.2K closest shown in the datasheet which only gives a load of 0.5mA - so again what gives?). here, there's NO differential between the R connecting the SET pin to load (20mohm) & OUT pin to load R (20mohm)

Is my analysis correct? If so what's up with the datasheet values??

I guess whether the SET pin is 2.1mV or 4mV below OUT pin is not of much consequence but the min load requirement of 1mA seems to be important & the datasheet R value of 33K only gives 0.1mA load - hence no regulation would result?