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Linear Regulator Clarifications

electronicsLearner77

Jul 2, 2015
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I want to convert from 5V to 3.3V using LT1117-3.3 IC, the data sheet is Linear Regulator. I have attached the LT spice model, few questions are, do i need to use resistor or not at the output side, i get 3.3V output in all cases. In the data sheet resistors are used. Please advise.
1677691831220.png
 

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kellys_eye

Jun 25, 2010
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The schematic you show is the one for the 'adjustable' output, not the fixed voltage versions which don't require the external resistors.
 

crutschow

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The schematic you show is the one for the 'adjustable' output, not the fixed voltage versions which don't require the external resistors.
The fixed voltage versions have the resistors internally contained in the chip.
 

Harald Kapp

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The specific hint is in the type numer: LT1117-3.3 - teh bold suffix indicates a 3.3 V regulator. The datasheet shows this clearly:
1677741431044.png
 

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electronicsLearner77

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I am using the below circuit the IC is AMS1117-3.3V
1681226544382.png
Along with the above circuit i have other micro related section populated on the board. But as soon as i give power of 5V the voltage drops and it does not go beyond 1.2V, the power supply goes to CC mode. How do i find the problem on the board. Please advise.
 

kellys_eye

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What is the load on the output? Are you using a suitable low-ESR capacitor on the output? Any stray capacitance? Is your PSU outputting enough current?
 

electronicsLearner77

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What is the load on the output? Are you using a suitable low-ESR capacitor on the output? Any stray capacitance? Is your PSU outputting enough current?
As per the theoretical calculation the load is less than 700mA, but i don't know how to measure the actual value of load current. I see now most of the micro pins are floating, does it consume current?
Are you using a suitable low-ESR capacitor on the output?
Are you referring to 10uF as shown or it is something else?
Any stray capacitance?
I need to know how to check this, I have to google it.
Is your PSU outputting enough current?
The PSU is 5V/1A, but if goes to CC mode the voltage is less but the current goes to little high value like 1.2A. Is it the correct PSU?
Please advise.
 

Harald Kapp

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I see now most of the micro pins are floating, does it consume current?
Possibly. Floating pins should be avoided. To be specific: floating input pins. Since a µC's unused GPIO pins typically are inputs this rule applies. However, many µCs have default so called weak pull-ups (or weak pull-downs) on their pins to avoid specifically this issue. You'll have to check the datasheet of your µC to make sure this is the case here. In that case, the current caused by the floating inputs is usually negligible.
Are you referring to 10uF
Yes, notably the one at the output. Although the datasheet of the AMS1117 doesn't explicitly state the requirement for a low ESR capacitor. A tantalum capacitor is used in the datasheet.
I need to know how to check this, I have to google it.
Stray capacitance is the capacitance resultimng from the construction. Any wires or PCB traces or planes the are parallel for a significant length create a capacitance between the two of them. Tis is typically in the pF range and hard to measure without sensitive equipment. If yu avoid long paralle traces, wires or planes, this should not be an issue here. But if you have long wires, the resulting capacitances and inductances can cause all kinds of undesired effects like e.g. oscillations.
The PSU is 5V/1A, but if goes to CC mode the voltage is less but the current goes to little high value like 1.2A. Is it the correct PSU?
Please advise.
Next to impossible to give detailed advice without knowing the load's current requirements. If your load current is less than 1 A the PSU you have should be sufficient. If your load current is higher than 1 A the PSU is insufficient. But without knowing your load we cannot judge whether 1 A is a meaningful limit or not.
 

kellys_eye

Jun 25, 2010
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Although the datasheet of the AMS1117 doesn't explicitly state the requirement for a low ESR capacitor.

Quick and simple test would be to replace the PSU with one of higher output capability (say 2A or 3A). If it still doesn't work there are other problems than what you 'think' has happened.....
 

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bertus

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Hello,

Are you sure that you have the AMS1117-3.3 and not the AMS1117?
AMS1117_output voltage.png
The AMS1117 will produce 1.25 Volts on its output, when connected as shown.

Bertus
 

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electronicsLearner77

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Are you sure that you have the AMS1117-3.3 and not the AMS1117?
Yes it is AMS1117-3.3. I connected the circuit independently only with this IC still the same behavior. What are the possibilities the ICs are bad around 10 numbers . I am planning to replace with LM series and check the output.
 

Harald Kapp

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Check the regulator without load to ensure the circuit on your breadboard is correct.
 

electronicsLearner77

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I wanted to understand is there a mechanism to know if an IC is damaged by checking the resistances between the terminals.
 

Harald Kapp

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is there a mechanism to know if an IC is damaged by checking the resistances between the terminals.
Generally: no.
There may be a few special cases where this may be possible, but usually not.
To check correct operation of an IC you need a test circuit that supplies power, some external components, often one or more signal sources and measuring instruments. A typical test setup would be shown in the datasheet of the IC. If not, a basic application circuit can be used.
 
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