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# LM2941: Will it work for non-typical dropouts of ~2 to 2.2V ??

Part Number: LM2941

Hi,

I am proposing to use the LM2941S/NOPB in a design where the input voltage is 10V nominal and the output is required to be 7.8V nominal so I'm looking to have a dropout voltage of about 2.2 V between input and output.

I was initially attracted to use this device because it mentioned on page 1 section 3 of the datasheet (SNVS770I –JUNE 1999–REVISED JANUARY 2015) that dropout voltages of ≤ 3V could be achieved but that higher quiescent currents than 30mA would be present, which was tolerable. However, at the start of section 3 it says ".....ability to source 1 A of output current with a typical dropout voltage of 0.5 V and a maximum of 1 V over the entire temperature range." I take this to mean, only for 1 amp output as the example here. For my design the load will draw 0.4A maximum.

In section 11.1.1 it defines the dropout voltage as "The input-voltage differential at which the circuit ceases to regulate against further reduction in input voltage. Measured when the output voltage has dropped 100 mV from the nominal value obtained at (VOUT + 5 V) input, dropout voltage is dependent upon load current and junction temperature." Should this not be "The Input-to-output voltage differential...."?

So for my design, VOUT + 5 V - 100mV = 12.9 V   so in my design no voltages will be near this value so my understanding from this statement is that voltage regulation will occur.

Looking at Figures 1 and 2 Typical dropouts illustrated are much lower (< 1 V). Why is this when the datasheet section 9 says "..........the input supply should be at least VOUT + 2 V"

This seems anomalous and am having a small doubt that my design might not work.

• Hi Philip,

The maximum dropout for LM2941 is 1 V over load and temperature. As you noted in Typical Characteristics curves, dropout is proportional to output current. The lower your output current, the lower the dropout requirement will be; therefore, you can have less headroom (Vin - Vout).

The statement of Vout + 2 V is meant to refer to the test condition for the line regulation specification in the Electrical Characteristics table.

In your application, you have 2.2 V of headroom so you are satisfying the dropout requirement with ample margin.

Very Respectfully,
Ryan