LP3873: LP3873-1.8

Hi Bob, 

The recommended lowest Vin for this device is 2.5V. Were you trying to use the device with Vin=2.35V for a 1.8V output with 9mA of load? 

Regards, 
Jason Song

Our configuration in with a 5.0V Supply Input with local 330uF & 33 uF Bulk decoupling.

The concern is the datasheet turn-on OUTPUT characteristic.

The datasheet stating it "typically" will overshoot to +2.2V.  If it exceeds +2.35V, we may be exceeding the ABSOLUTE MAXIMUM of a very expensive device we are interfacing to.  We are hoping to complete PCB Layout this week.  The Very light load (Output Current Draw, 9 mA.) may lend to larger than Typical overshoot?  I checked for simulation models on the website and didn't find anything.

Looking for things I can do to reduce risk of larger overshoot.  We have 10.6 uF output capacitance.  Would increasing that help or would added capacitance increase inrush creating a larger overshoot?  Depends on the device response time during the turn-on.  

Normally this would be out +5V rise time, but we also have a power sequencing requirement so there is a second scenario where we would be Disabling Shutdown after the 5V has settled.

Bob

Hi Bob, 

The datasheet is trying to say for output option less than 2.0, during the startup, the part could overshoot and the voltage could exceed the target output voltage. For example, for 1.8V, during startup, the output could exceed 1.8V. 

In general, the big the output capacitor, the smaller the overshoots become. It also depends on the Vin rising time. More discussion can be found in this app note: 

Due to each board condition is different, if possible, I would suggest evaluating the LDO with the Vin supply you plan to give to the LDO and capture some scope-shoots to share here. Do you think that's something you could do? 

Regards, 
Jason Song

It appears my initial response was lost in the E2E system...  I am supplying my design implementation with 333uF total Input (+5V) and total load capacitance of 20.6uF (Now Doubled) @ 1.8V / 9mA.  Do You have a way to simulate your devices response to both the Input (+5V) being turned on?  What about With everything stable, and then de-asserting SD?  Is there an overshoot under that condition?  I need a worst case overshoot.  I cannot afford to have the expensive part the supplied devices interface too damaged.  Should I switch to an alternate LDO with Soft Start?  Do You have a recommendation?

Hi Bob, 

If your next stage circuit is very sensitive to the overshoot, you should probably pick up one device that has the soft-start function. For most of the LDO, having the Vin rail fully on before tunning on the EN/SD pin will help, but since this device specifically mentions the overshoot for the voltage option you pick, that means, the overshoot is due to the design. You may use a higher output capacitor to reduce the spike, but it would be very difficult to quantify with the maximum overshoot number. I would suggest using an LDO with soft-start built-in.

From our previous discussion, the Vin is 2.85V, and the output is 1.8V and the load current is around 9mA of current. Correct? 

Regards, 
Jason Song

Vin =5.0V, 

and yes, Vout = 1.8V with 9 mA load .

Do You have a recommended 5V->1.8V LDO with soft start?

Bob