 # Feedback on BJT Saturation Calculation

I'm trying to use the PIMN31 as a switch and buffer for my outputs.

This is a resistor equipped transistor with the base resistor being 1K and the base to ground/emitter resistor being 10K. The issue is that I can only buy these in bulk but I want to be sure they work OK before I buy them.

Now, I understand that the first thing to do is find out the load current i.e. the collector current. Here's a schematic: My worst-case load is really about a 100 2K pull up resistors in parallel. So the effective resistance becomes just 20 Ohms. I note from the datasheet that the max. collector-emitter saturation voltage is 0.3V. Hence, Ic becomes:

Ic = (3.3 - 0.3) / 20 = 150mA - well below the limiting rating of 500mA.

My next step was to find the required base current:

This is where I'm slightly confused... the min Hfe is given as 70. But from the graph in page 5 of the datasheet we find this graph:

I note that the Hfe near at 100mA is about 300 (I"m looking at curve 2). So what Hfe value should I use?

If I use 70, I get the required base current to be:

Ib = Ic/B = 150 / 70 = 2.1mA

If I use 300, I get:

Ib = 150/300 = 0.5mA.

However, my actual base current is going to be 2.64mA. So while I'm well saturated if I use a hfe of 300, but near the limit when I use a value of 70.

I'd appreciate any feedback regarding this calculation.

Nevermind - figured it out. 300 Hfe was typical, not min. If I use min, and do another worst-case analysis, this transistor won't saturate! I think I'll go with FDG6317NZ - it's smaller in anycase, so thats a plus.