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Looking for a Voltage Supervisory IC that disconnects Vcc instead of asserting reset

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Noggin

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I have a project on which we made the unfortunate decision to slap a super cap on the Vcc rail to ensure there is enough power available to allow the system to shut down properly without corrupting an SD card. Unfortunately, this severely dampens the Vcc rise when power is applied, which can cause the MCU to not start up properly. Asserting reset does not let the MCU recover. I've read that the sharp rise of the Vcc rail is used to start the crystal oscillating properly, if the rise is too slow then then crystal isn't guaranteed to start.

I'm also seeing other issues with the Real Time Clock (off-chip) not responding to I2C commands until a full power cycle. I suspect that this may also be due to a slowly rising Vcc.

I've been searching for a voltage supervisory (V-Sup) IC that passes through Vcc instead of just asserting a reset pin, but haven't had any luck. I imagine it should be relatively simple to put a couple of FETs in circuit to disconnect Vcc using the V-Sup's reset output, but it would be great to have a 1 component solution. This is a very tight board design already using two double sided, stacked boards. Something like an SOT-23-6 with an adjustable threshold would be the best fit.

Is there such a device?
 

ericgibbs

Well-Known Member
Most Helpful Member
hi N,
Would a device like this PDF type be suitable.? I have used them in MCU power supply control.?

E
 

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Noggin

Member
Yeah, that looks pretty much like what I'm looking for, but much lower power capabilities and a 2.5v or so threshold. I've drawn up a circuit using an LTC2915, 2 resistors, and two FETs that'll do what I want. While its more packages than I desire, I should be able to dead bug this into the circuit for testing and we should be able to squeeze it onto the board somewhere.
 

dougy83

Well-Known Member
Can you just put a parallel resistor and diode between the super capacitor and VCC (the diode anode to the cap, cathode to VCC)? The resistor slows down the charge rate of the cap when power is applied, and the diode allows enough current to supply the device when power is removed.
 

Noggin

Member
That was a consideration early on, but in practice it won't work. The diode drop can't be tolerated unfortunately. The cap was sized such that with the lowest Vin, worst case SD card voltage requirements, worst case SD card write time, worst case temperature, as well as long term aging that we will always have enough time to complete the write, close the file, and unmount the SD card. By putting a diode in place, we'd either need a larger super cap (already at 47mF) or we'd need to use a FET's to short out the diode....

Hmmm... using a FET to short the diode might actually work. We know pretty doggone quick when power is lost. By the time we figure it out, the supercap voltage shouldn't have dropped very much. We could drive the FET in an interrupt. I'm off the project for the time being, but will be back on it in a few weeks. I may look into that solutions. Thanks!
 

crutschow

Well-Known Member
Most Helpful Member
I think you don't need the diode, just a couple resistors and a P-MOSFET (with possibly an added BJT driver for the MOSFET gate).
Below is an LTspice simulation of that approach.
For simulation purposes the control voltage Cnt is used to both cut off the supply voltage and turn on the MOSFET at same time. In reality there will be a small time delay between the two for the time it takes to detect the power loss. During that time the MOSFET substrate diode will carry the current.
The charging resistor value is arbitrary, depending upon the peak C2 charging current you can tolerate.
M1 should be a logic-level type MOSFET if the supply voltage is less than 10V.

upload_2016-6-9_19-35-38.png
 

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KeepItSimpleStupid

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A voltage detector acts a bit differently. Maybe it would work?

I had a similar issue with an SBC (single board computer) Switching supply was OK. Linear was not. Changing an IC driving the reset pin to a schmdt trigger version worked. This was the 80's.
 

atferrari

Well-Known Member
to complete the write, close the file, and unmount the SD card.
I am not questioning your design at all. Just out of curiosity, is the unmount needed? Aren't you shutting down the sytem?
 

ChrisP58

Well-Known Member
Most Helpful Member
I would go with crutschow's circuit concept, with a couple of changes.
1) Move the charge resistor and mosfet to the negative side of the supercap. That way you can use a direct driven n-channel mosfet.
2) Don't wait to switch the mosfet on until the input voltage is lost. Turn it on as soon as the supercap is fully charged. You could measure it with an A2D channel, but since you the values of R and C are known, you can just use a delay timed from turn-on.
 

spec

Well-Known Member
Most Helpful Member
Another approach.

spec

2016_06_15_Iss1_ETO_SUPPLY_ON-OFF_ver1.png
NOTES
(1) The supply turn on voltage is set by RV1
(2) Recommended turn on voltage is 4.5V; this will give a turn off voltage of 4.4V due to the 100mV hysteresis provided by R12 and R13. (can be altered to change the hysteresis)
(3) N1 reference voltage is 1.182V
(4) Omitting R10, the circuit consumes 9uA maximum
(5) R9 is a gate stopper and should be mounted directly to the PMOSFET gate terminal


DATA SHEETS

(1) http://cds.linear.com/docs/en/datasheet/1540fas.pdf
(2) https://datasheets.maximintegrated.com/en/ds/MAX921-MAX924.pdf
(3) https://datasheets.maximintegrated.com/en/ds/MAX931-MAX934.pdf
(4) http://www.vishay.com/docs/62864/sia453edj.pdf
 
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