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/etc... Amplifiers & Other Linear Forum
TLV225 Output Current
For the TLV2252 series amps I see an IO (output current) max at 50mA. When I look at Figure 18 it shows a IOH at ~220uA @ 4.5V @ ~50 C. I need to drive the base of a bjt using 0.52mA. It looks like the min value I could see out of the amp is 4.65V. So taking into account the VBE I would use a 2k resistor. Figure 18 seems to limit it below what I need. Am I mis-interpreting the figure?
I follow your reasoning when the TLV2252 VOH output voltage is ~4.5V at 220uA IOH (TA 50C). However, I don't understand the 4.65V VOH when IOH is increased to 520uA. It appears the most the TLV2252 can deliver is about ~500uA IOH and that is when the TA is -40C. Therefore, it is unlikely the IOH drive would be enough at warmer temperatures.
Take a look at the TLV2452. It has similar specifications to the TLV2252, but also much higher output drive capability. Also, take a look at the OPA330. It is a high performance device with similar operating current requirements. If these devices do not meet all your requirements, let me know and we can identify another one that will.
PA - Linear Applications Engineering
Thomas KuehlHowever, I don't understand the 4.65V VOH when IOH is increased to 520uA.
What I was referring to was at the transistor base I believe I need 520uA to fully saturate the BJT under worst case conditions of 5.25mA @ Ic.
Currently the design I inherited (which is working at ambient) has a 20k resistor limiting base current to ~230uA. The Amps aren't warm to the touch, but I'm trying to define that the 20k is sufficient and that the current 225uA is sufficient to fully sink the current. It is currently working properly, but I just wanted to make sure.
I'm starting to think I may be wrong on my Ic. Basically I have U1 (amp) powering the base of Q1 (bjt) through 20k resistor. Q1 is in parallel to a 10k pull down resistor. Up stream Q1C & Rpulldown are tied to a 1k Rup. My understanding was that the current through Rup would go through Q1 when Q1 had power applied at the base.
So at this point I believe the design is insufficient if what I'm stating above makes sense.
It appears from your explanation that the NPN transistor is receiving enough base drive for it to saturate and pull the collector low through the 10k pull-down. The result would indicate that the base current demanded is likely much less than 520uA - also meaning the current gain (beta) of the NPN is much higher than 10x. The base current required is probably closer to 200uA in the circuit. TLV2252, figure 18, indicates that the amplifier should be able to deliver that level of current at all but the highest temperatures.
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