LDO as a voltage limitation.

[doraemon]

Hello!

I am designing a board that can drive DC motors and servos. The servos are plain RC model servos,
and they can be powered from "3.5 to 8 V under load" (that's what is written on the box).
My board can drive basically any DC motor up to 500 mA and from 3V to 15V (There is an input voltage
measurement so that the PWM adapts automatically for low voltage motors. Of course, you have to set
up the motor voltage).

So, assuming that the board will be powered between, say, 3.6 and 15V (3.6 would be 3 NimH batteries),
I need a system to limit the servo voltage. So I was thinking about a 7.5V LDO. This would work fine
if the input voltage is higher than 7.5 (+ dropout), but what about lower voltages?
Is there any problem in feeding a 7.5V LDO with, say, 4.5V? My feeling is that the LDO would pass
everything because the input is lower that its nominal value, but is there a hidden problem in doing that?

Thanks,

Dora.

 

[smijesh]

LDO cant do this, Try buck converter

https://www.ti.com/lit/ds/symlink/lm2576.pdf

 

[doraemon]

Hello!

Thank you for your reply.
Could you elaborate a little bit? Why can't a LDO do this?

The problem is that I _CAN_ do that with an LDO, it works perfectly well on my breadboard,
but I want to know if there are "hidden" problems that will happen in some unusual condition.

What I am using right now:

1. 4.5 V battery
2. 1.5 V vbattery
3. MCP 1700 (3.3V).

A.If I use 4.5 V battery and a MCP1700-33, I get 3.3 V at the output, and that's fine, that's what it's made for.
B. If I use 1.5V battery and MCP 1700-33, then I get something close to 1.5V and that's also fine.

The (A) is fine because MCP1700 is supposed to drop the voltage.

My question is about (B). If I put 1.5V at the input of a 3.3V regulator, my experiment seems to indicate that
it's fine: I get 1.5V, and the MCP1700 is useless, that's what I want. But will there be a hidden problem
at some point?

Basically what I want is:

if(voltage < reg_voltage) {
output = copy input;
}
else {
output = reg_voltage;
}

reg_voltage is the voltage I don't want to cross. Lower is fine.

Thanks,

Dora.

 

[schmitt trigger]

The problem with classic LDOs (using PNPs as the series pass element), is that their ground current becomes significant when the input voltage is near the dropout voltage.

Now, more modern LDOs like the MCP1700, employ Mosfets and this issue is lessened. However, they have traditionally been available only in low current devices. But I've not done an exhaustive datasheet search across all manufacturers lately. This could have changed.

You are correct that when operating below its input dropout voltage, the output will essentially mimic the input. However at that level, the protection features of the regulator (over temperature, short circuit protection, etc), it is unlikely that they would operate correctly. Perhaps the maximum current capabilities will be also reduced.

 

[doraemon]

Hello!

Thanks for your reply. Anyway I don't have much choice. If I want to do a discrete limitation
with a FET transistor and some comparison, then I would reinvent the LDO, so I think I will
use a LDO.

Dora