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Interesting feature of LDOs?

Maarten Verhage46
Prodigy 100 points
Other Parts Discussed in Thread: TPS3422

Dear TI people,

My application calls for a special feature of an LDO which I am unsure whether it exists. Please read with me, so I can clearly explain what I’m looking for.

I’m working on a 1-10V dimmer for fluorescent lamps; Designed to be fitted into a junction box behind two pushbuttons.

The system basically consists of a microcontroller that responds to the pushbuttons and adjusts the dimming level by transferring a 8-bits value to a DAC. After the 5V DAC there is an opamp circuit that doubles the voltage to have full 1 to 10V control range.

I’m using a very small flyback design to generate 5.5V and 10.5V DC. The intention is to generate 5V and 10V with LDOs.

But most of the time the lamps are off and I want the dimmer to consume as little power as possible. As I don’t need the analog DC output at that moment I can disable the 10V LDO.

However I still need to provide power to the microcontroller because anytime the pushbuttons could be pressed. The microcontroller is set to sleep when the lamp is off. I realized the 5.5V with higher ripple voltage is adequate for the microcontroller VDD pin when the lamp is off. The 5V DAC can also tolerate 5.5V but again it doesn’t need to do anything then.

So, what I’m looking for is an LDO which can be set transparent (this is my term for the feature). With this I mean the pass element doesn’t need to operate in its linear region. It just needs to be completely on and stays there. The LDO should have a special input pin to enable this transparency mode.

Ok, my questions:

1. Does this exist?
2. If yes, what is the correct term for this feature?
3. If yes, which LDO devices could you recommend for this?
4. If not possible, would it be doable to make an LDO with discrete devices? Any info/docs would be highly appreciated then.

Best regards,
Maarten Verhage

  • 0
    TI__Expert 8905 points

    Hi Maarten,

    An LDO that is fully on is in drop-out, which has the drawback of significantly increasing the supply current required by the LDO.  A few other options for you to consider:

    (1)Use really low Iq LDOs for the 5.5V to 5V conversion - TPS70950 would be a great choice for this. This will reduce the current consumed by the LDO to a nearly very low level (3.2uA).

    (2) Based on your needs you may want to consider using 3 LDOs and generate a lower voltage rail to power the micro-controller, so you can disable the 5V and 10V LDO. The TPS78233 is a great LDO for really low Iq MCU operation.

    (3) For really low standby consumption, you could use a push-button controller (TPS3422) for your standby mode - when a button is pressed, it enables the 5V and 10V LDO's. Then use the TPS70950 to generate the 5V rail and the TPS7A4701, or if you need less current, the TPS71501, to provide your 10V rail.

    Regards,

    David