SparkFun Forums

[Hami]

Hello,

i want a circuit without a on switch. The schematik is attached below. A battery may be connected through K8 (Power In) and be charged trough an external charger circuit connected at k9 (Bat_load). There are 2 LDOs. U2 powers an MCU. U1 powers some sensors. To switch it on, an external signal on K3 (On) may power the MCU. Afterwards, the MCU keeps hisself enebled through the POWER_ON signal. The external signal at K3 may be diconnected. The MCU enebles U1 through "Sens ON".

Power.gif

Will the circuit work as i want it to do?

Thank you in advance
Hami

[Mee_n_Mac]

It appears to be OK. W/o knowing what the LDOs are I can't say whether the Power_On signal (presumably a 3.3V level) will work after a diode drop.

[skimask]

The LT1129 and MCP1825 types that I generally use are a ...'pull low to turn off'... rather than a ...'pull high to turn on'..., so should work even after a few diode drops @ 3.3v

[Hami]

Mee_n_Mac and skimask, thank you very much for your response. I did not see the problem with the forward voltage drop. Thus, i did not report the parts, i want to use, sorry.

I want to use as diode the CDSQR4448:
http://www.mouser.com/ds/2/80/CDSQR4448 ... -27238.pdf

and as LDO the MIC 5205:
http://www.micrel.com/_PDF/mic5205.pdf

It appears to be OK. W/o knowing what the LDOs are I can't say whether the Power_On signal (presumably a 3.3V level) will work after a diode drop.

Yes, signals are at a 3.3V basis. The diode has a forward voltage drop of max. 0.72V (at If=5mA) and the LDO switches on at a enable input logic-high voltage of 2.0V. Therefore, the circuit should work? Is my choice for the LDO and diode ok?

Thanks in advance
Hami

[MichaelN]

Those regulators are not designed to be used with ceramic output capacitors, which have very low ESR and can lead to instability. You can either use tantalum capacitors, or add small resistors in series with ceramic capacitors. Personally, I only use regulators designed for ceramic caps.

The datasheet states: "For low-dropout regulators that are stable with ceramic output capacitors, see the μCap MIC5245/6/7 family"

[Hami]

MichaelN, thank you, very good point (I should do better reserach next time). I will consider it. Unfortunately Mouser does not sell (or at least they are not in stock) this family. Which LDOs are u using? May you recommend a LDO?

Best
Hami

[dlotton]

Something to consider...

I'm not sure what how much current your sensors require. You could use a single regulator and a P-Channel MOSFET (or a load switch) to switch power to the sensor(s). This solution could reduce part count and/or cost... not that the part count on a LDO is particularly onerous.

Just another approach to consider.

DL

[MichaelN]

Which LDOs are u using? May you recommend a LDO?

I've used LP2985 for quite a few years, but there are lots of others (eg MIC5203).

[Hami]

MichaelN,

thank you very much!

Best
Hami