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Need some clarification on the working of LDO

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Nov 20, 2005
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about ldo?????????????

Please someone helps me with that point
the micro controller works with an avg of 40 mA to 100 mA (max) does the LDO produces diff current
for example when the when the microcontroller needs just 40 it will give it 40 or it produces a constant current (acts a current source) if not is it's max is 100 mA

please give me (example of this LDO)

Re: about ldo?????????????

The LDO is a Low Dropout Regulator, the current is parameter, but this current can be 10 mA or 10 A. This is not a current source, the producer give the maximum current, some device can have a limit for the minimum current, but this give not a constant current, the current is depending only from the load.

about ldo?????????????

LDO give same quantity as request , until maximum allowed for him. It's work as constant voltage source, not constant current. You must care about only three things :
LDO output voltage equal VCC of controller
LDO maximum current greater then maximum
required for controller
LDO dissipated power :
when you are using LDO be sure, that
thermal conditions is safety, because dissipated power on LDO is : (Vi-Vout)*Imax .
Usually LDO mounting on PCB square pad with
width/length 1cmx1cm for 1Wt of dissipated power.

Re: about ldo?????????????

for mr vvvvvvvvvvvvv

LDO maximum current greater then maximum of microcontroler

i think it isn't true cause if i enter current > device(microcontroller , WIFI......) input current device will be burnt

about ldo?????????????

LDO give same quantity as controller want, but if
controller has short current loop, then current, requested from LDO, will be large . In this case
there is LDO with short current protect built in
LDO. LDO never , never give current more then
you request from it.
As a primer imagine a bottle with water standing on the table. Water don't leakege itself from bottle until you begin pump it from bottle by tubule.

In the LDO similar, current is a most safety aspect.
If your circuits has not short, nothing worry.


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Re: about ldo?????????????

what the meaning of short current loop?
thanks for all help

Re: about ldo?????????????

Short current loop is the case when you connect VCC wire to GND and
resistance between VCC and GND = 0.
( it's happen on PCB when you accidentally short trace by probe or screwdriver, or when there is solder bridge between traces , or when your schematic is wrong and there is two bjt, one connected to VCC, other to GND and both connected with each other and open in the same time )

In this case LDO connected to VCC see then (Ohm law) request for current is :

I = VCC/R = VCC/0 = infinite

And try to provide infinite current. Next, maximum LDO current, flowing through
our short wire, warming that wire and eventually burn LDO or wire ( win strongest).

Actually take place next processes :

LDO works as constant voltage source (not current), when you connect to LDO
wire with very small resistance , say 0.1ohm , LDO try to send very small current , say 0.1 ma, and LDO see on his output voltage :

V = I*R = 0.1ma*0.1Ohm = 0.0001V
But LDO know, then he must provide 3.3V and try to perform this task and
adds some current to output , say 10 ma :
V = I*R = 10ma*0.1Ohm = 0.001V
This is too small , LDO adds more , 100ma :
V = I*R = 100ma*0.1Ohm = 0.01V
Let LDO maximum current 1.25A , he give all him current :
V = I*R = 1.250ma*0.1Ohm = 1.25*0.1 = 0.125V

And LDO remain in this state, attempting to provide 3.3V to him output.

Well now calculate Power dissipation :

Power on short resistor (0.1ohm) = 0.1*1.25^2 = 0.15w this is not much
Power on LDO (input 5V) = (5V-0.125V)*1.25 = 6w this is toooo much

30 seconds and your LDO is dead, if he has not current short protection.
There is LDO with current short protection .
For your schematics , i think, very suitable LDO from Linear Technology LT1763 :
Output Current: 500mA
Wide Input Voltage Range: 1.8V to 20V
No Protection Diodes Needed (if you change input polarity)
Fixed Output Voltages: 1.5V, 1.8V, 2.5V, 3V, 3.3V, 5V
Adjustable Output from 1.22V to 20V
Reverse Battery Protection (if you change output polarity)
Overcurrent and Overtemperature Protected ( this is case current short loop)

you can request free sample from the site, it's work.


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about ldo?????????????

thanks for detailed description

Re: about ldo?????????????

thanks for your help but i just want sure from (if my microcontroller haven't short current loop )i don't care from input current becouse my controller suck which need current from ldo even ldo current large than wanted for controller.


thanks vvvvv

about ldo?????????????

PCB is plate where you place details , connected
together with trace and solder.


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