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# LP5951: Input for the LP5951-3.3

Part Number: LP5951

We have just find an unwanted scenario that could affect one of our designs. More specificalli, we are using your LP5951-3.3 to regulate a 3.3V digital Power Supply signal. We have just discovered that the input for this component (under certain unexpected situation) could drop down to 3 V. Could this affect the functionality of the component? I guess that in terms of regulation performance gets worse, but how much? When can we expect a TOTAL failure in the delivery of the 3.3 V to the output? Thanks in advance for your close support.

• Hello Santiago,

Yes, by definition the low input voltage will force the output voltage out of regulation.

How far out of regulation will depend directly on how much current is being pulled through the LP5951 : Vout = (Vin - Vdo)

Vdo is determined by the Rds(on) of the internal pass PMOS transistor such that Rds(on) =  (250mV/150mA) = 1.667 Ohms

If the load current is near zero mA, then: Vout = Vin

If the load current is near the max of 150mA, then : Vout = (Vin - (1.667 Ohms x 150mA)) = (3.0V - 250mV) = 2.750V

If the load current is near mid-point at 75mA, then : Vout = (Vin - (1.667 Ohms x 75mA)) = (3.0V - 125mV) = 2.875V

Since the LP5951 minimum operating voltage is 1.8V, there will not be any total failure, or shutdown, of the device with Vin at 3.0V.

But ... Vout will track with the falling Vin whenever Vin < (Vout + Vdo)