# What are SD/SS, delay, and flag pins on LM2596?

I'm studying the TI datasheet for an LM2596.

On page 4, at the top left in section 7.1 Absolute Maximum Ratings, it mentions:

• $\overline{\mathrm{SD}}/\mathrm{SS}$ pin input voltage (6V)
• Delay pin voltage (1.5V)
• Flag pin voltage (-0.3 - 45V)

..without definition.

Googling on 'sd/ss pin' and 'flag pin' produce unusual results that don't help in this case :)

The chip has an enable pin, labeled $\overline{\mathrm{ON}}/\mathrm{OFF}$ in the schematic and elsewhere. The others are $V_{IN}$, ${Output}$, ${Ground}$, and ${Feedback}$ (it is a 5-pin device).

Can anyone explain what these three pins are and why they're named that way? I have to assume they are each either an alternate name or a different use for one of the pins because there just aren't enough pins to go around otherwise :)

• That datasheet is messed up in several ways. – Ignacio Vazquez-Abrams Jul 2 '16 at 1:20
• Shutdown/soft-start probably. This relationship is made on page 6. It seems that it is probably a relic from when the datasheet was rewritten - it was originally a Nat-Semi part. – Tom Carpenter Jul 2 '16 at 1:22

It seems that TI updated a number of its LM25XX datasheets in May, and they made several goofups in and between them. The pins you ask about are found in the LM2593HV and LM2599 devices, but not in the LM2596. The previous revision of the datasheet (revision C, document SNVS124C) does not contain the unusual additions.

• What do those pins do? So I know what I'm missing :) – scanny Jul 2 '16 at 1:39
• "This dual function pin provides the following features: (a) Allows the switching regulator circuit to be shut down using logic level signals thus dropping the total input supply current to approximately 80 μA. (b) Adding a capacitor to this pin provides a soft-start feature which minimizes start-up current and provides a controlled ramp up of the output voltage." "At power-up, this pin can be used to provide a time delay between the time the regulated output voltage reaches 95% of the nominal output voltage, and the time the error flag output goes high." – Ignacio Vazquez-Abrams Jul 2 '16 at 1:40
• "Open-collector output that provides a low signal (flag transistor ON) when the regulated output voltage drops more than 5% from the nominal output voltage. On start up, Error Flag is low until V OUT reaches 95% of the nominal output voltage and a delay time determined by the Delay pin capacitor. This signal can be used as a reset to a microprocessor on power- up." – Ignacio Vazquez-Abrams Jul 2 '16 at 1:40

I've accepted Ignacio's answer, this is just to organize the bits in the comments and form them into a straightforward reply to the question. They do appear on the datasheet for the LM2593V which has more details on them.

1. These three pins are real and distinct, but they do not appear on the LM2596. Their appearance on the datasheet is mistaken, perhaps a copy-paste error.

2. $\overline{SD}/SS$ stands for shutdown/soft-start. This is similar to the $\overline{ON}/OFF$ pin on the LM2596, but allows delayed start-up (allowing input capacitor to charge before starting switch, reducing start-up surge current) to be implemented with a single capacitor rather than two resistors and a capacitor.

3. The error flag pin indicates when the output voltage is out of regulation (<95% of nominal). High (>1V) indicates power good.

4. The delay pin allows setting a delay between the moment the output voltage reaches regulation and when the error flag pin goes high (power good).