LP38692: Logic Control of the EN Input

Nicholas Ipri1

Part Number: LP38692
Other Parts Discussed in Thread: TPS709, LP38690

The LP38692 will be used to provide Vdd to my system including the microcontroller (MCU).

The MCU will have a GPIO, configured as output, connected to the EN pin of the 38692 to allow the MCU to power off the regulator / system

(There will be additional logic / circuitry OR'd with the GPIO to drive the EN pin from another 2nd supply input to allow the system to restart once powered off)

When that 2nd supply input isn't connected and the MCU is running, I want to implement a way to hold EN enabled while the MCU is rebooting. i.e. for the few seconds that the GPIO output is not driving the EN pin... during reboot, that GPIO may return to it's default as a hi Z input and would have to be reconfigured by firmware to drive EN again.

What would be the most reliable way to accomplish this? Perhaps an RC time constant or drive the EN pin using a MOSFET / pullup resistor to the input supply where the gate is controlled by the GPIO?

over 4 years ago

0 Jason Song over 4 years ago

TI__Mastermind 23361 points

Hi Nick,

One configuration you may try is to have something similar to an open-drain output, and the GPIO will be controlling a MOSET to pull down the EN voltage. Before your system reboot, you could first bring the GPIO low if it's not and then make it a hi Z during the reboot. This kind of open drain configuration will allow EN to stay high and only pulled low with a high GPIO signal.

The RC delay can technically work, but since your delay could be in seconds and this may not be that reliable.

Another option is to pick a LDO with a EN pin that has a pull-up circuit, which means when floating, the EN will be pulled high. If you could share your needs on the LDO, like Vin, Vout, Iload and other key parameters, we may be able to recommend you a LDO that have a pull-up EN pin.

Regards,

Jason Song

0 Nicholas Ipri1 over 4 years ago in reply to Jason Song

Prodigy 200 points

Thanks, an internally pulled up EN would be the best choice, by far.

The system is an asset tracking IoT application with GPS and LTE. I need a fixed 3.3v output with max Vin = 10v. Iload is typically < 150 uA (hence an LDO) but can peak at up to 650 mA for very brief periods when the LTE modem is transmitting. I am planning on having some additional capacitance on the supply output to help supply the sporadic load current demands.

0 Jason Song over 4 years ago in reply to Nicholas Ipri1

TI__Mastermind 23361 points

Hi Nick,

Thanks for providing the details. Please allow some time for us to track down if there is a LDO that has the pull-up EN and will meet your application need. We will try to get back to you by 3/17.

Regards,
Jason

0 Jason Song over 4 years ago in reply to Jason Song

TI__Mastermind 23361 points

Hi Nick,

After investigation, the closet we have is TPS709, but it cannot provide 650mA pulse current. You probably need to consider using the GPIO and a MOSET combination as we discussed.

Regards,
Jason Song

0 Nicholas Ipri1 over 4 years ago in reply to Jason Song

Prodigy 200 points

Thanks. In that case, I am thinking to implement the LP38692 as in the attached file. Can I get you to review this?

ti1.docx

0 Nicholas Ipri1 over 4 years ago in reply to Nicholas Ipri1

Prodigy 200 points

Vbat is sourced from a battery. cpu_vd_en and at_vdd_en are GPIOs from 2 different microcontrollers that I want to independently enable the power supply.

0 Nicholas Ipri1 over 4 years ago in reply to Nicholas Ipri1

Prodigy 200 points

Any feedback yet?

Also, I see that for the lp38690, there is a spec for minimum load current of 100 uA.

Our minimum load current will be well below 100uA. Will this par work or is there another that is more suited for this?

0 Jason Song over 4 years ago in reply to Nicholas Ipri1

TI__Mastermind 23361 points

Hi Nick,

You can have two GPIOs to control the EN, but I would suggest having a pull-down resistor to ground at EN pin to make sure EN pin is always in a known state. With the diodes, if you could have a floating EN when both GPIOs are low.

Everything else looks good.

Regards,
Jason

Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

Power management

Power management forum

LP38692: Logic Control of the EN Input