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# LM2991: Clarification to its input and output voltage differential requirements.

Part Number: LM2991

Hi Experts,

We are seeking your conclusion on this query from customer about design application with LM2991, a negative LDO. They wanted it to convert -6.5Vin to -5.0Vout.

From Section 7.4.1 of the device datasheet:

Does it mean if designed with -5Vout, a minimum of -10Vin is required?

I have left my understanding below to the customer and this is my stand:

We have the same understanding with the Vin requirements of this device. When we treat this in positive convention (disregard the negative polarity), the input requirement should be 5V higher than the output up to 26V, to regulate the device up to 1A output.

We are seeing from device description that it has 600mV drop-out voltage (typical) when operating a 1A load current. This drop-out means that at 1A, Vout will be ~(-4.4V). I have this high regard that this device requires a minimum voltage differential of (5V) in order to operate this up to 1A at normal condition.

But, upon checking from Figure 12:

the device seems that it should work on their application ( Vin=-6, Vout=-5, @ 1A ). Which is contrary to the above assertions.

Can you help us verify this application notes? I am seeing from our E2E threads that they are designing the device lower than 5V differential voltage and some experts supported it.

Regards,
Archie A.

PS: I tried this design on Webench and their requirements shared seems functional.

• Hi Artzy,

I apologize for the delayed response.

The following does seem confusing:

After looking at this and the Data Sheet, I believe this section is just stating that when Vout is withing the range of -5V >=Vin>-26V, then having a Vin outside of these ranges, the device does not operate properly.

At first sight it does look like it is saying that for Vout=-5V Vin=-10, but this is not true.

The device should operate properly with Vin=-6.5V and Vout=-5V as a 1.5V of headroom is being provided.

Best,

Edgar Acosta

• Thanks for confirming, Edgar.

Hope our information sheet for LM2991 would be clearer on this on the next revision. Especially the test parameters set in the notes, only seeing a headroom greater than 5V differences.