Not getting Desired Output current from LDO

Hi Jehan,

To assist you I will need more information about your setup.

I see in your table you have listed the min and max allowed input voltages for each device, but do not mention the input voltage that was used for the testing. 

Can you explain what the "Working Temperature" is referring to? Is that the temperature measured at the top of the case during normal operation? Also, is the shutdown temperature the temperature at which the device begins cycling in and out of thermal shutdown when the load is increased above the "working" current? 

From the information you have provided it seems that there is an issue with power dissipation. Board layout is very important for thermal performance, and without knowing what the layout looks like that you did these tests with, it is difficult to diagnose the problem. Can you share a screenshot of the board layout in the immediate area surrounding the LDO?

Regards,

Nick

Hi Nick,

Thanks for your response.

We have tested TPS7B8733QKVURQ1, TPS7B8833QKVURQ1, TPS7B8633QKVURQ1 & TL760M33QKTTRQ1 LDO part at 24VDC.

TLV2217-33KVURG3 we tested at 12 VDC input voltage.

Working temperature is temperature measured at top of the epoxy case during normal operation.

Shutdown temperature is temperature measured at top of the epoxy case when voltage and current getting on-off.

Right now, we are testing this LDO on General Purpose Board.

We are already aware about P(d) Power dissipation of each packages TO-252/ TO-263/ TO-220/ TO-3P/ TO-247

We are manufacturers of Solid State Relay where we simulate and test such packages.

We have specifically chosen lower drop out voltage for lesser power dissipation.

Point is choosing surface mount package or we should proceed with TO-220 with smaller heatsink.

Our constant current requirement is 1A for supply at 3.3 VDC.

We are yet to design PCB because of initial testing.

If you recommend to make thermal pad on PCB than we will proceed with other package selection

Please recommend heat dissipation area requirement for constant current of 1A.

Let me know if you require any more details.

Thanks and Best Regards,

Jehan Patel

Hi Jehan,

With 12V or 24V input and 3.3V output, the dropout voltage is not relevant because there is plenty of headroom to be above the dropout condition. To get the advantage of the low dropout voltage in the context of thermal performance, the input should not be significantly larger than the set output, i.e. VIN should be larger than VOUT + VDO with some margin, and not much more to avoid excessive power dissipation. 

If your current requirement is 1A DC, do you intend on using 2 (or more) LDOs in parallel to boost the current drive capability? I ask because all of these LDOs are 500mA devices. 

Assuming the above is true, i.e. that each LDO will drive 500mA, the power dissipated in each for the 24V input case is (24V-3.3V) ⋅ 500mA = 10.35W. This will likely generate too much heat to try and dissipate with a surface mount part, even with good layout optimized for thermal performance. If you can get the 24V down to something closer to VOUT + VDO with something like a buck DC/DC, the power dissipation will be significantly less and this could become a viable option. 

Regards,

Nick