(Maybe off topic here: But that´s exactely why I don´t like to use VCC as VRef of an ADC. VCC is not that precise ... and the ADC readings are not precise, too.)This is called load regulation,
It´s not that worse:desired drop and estimate n for -nT exponential .e.g. 1T=37%, 5T=10%, 50T?=1%.
This is hard to explain with finger typing and no graphs, but any RC series filter on the output of an LDO is also part of the feedback path and forms a lag/lead filter. This is good for stability to prevent ringing on a step load response, but is poor for peak transient error due to the step. Therefore it is a performance tradeoff. In other words, Too little ESR where the RC break point is higher f than the unity gain bandwidth of the LDO reduces loop phase margin by integrating the current pulses. Too much ESR causes a linear voltage transient dip due to load R/(R+ESR) *100%. Thus assuming a 6Ω load and 1R added ESR you can expect a 14% poor transient drop.The hardware communicates to the mobile via bluetooth. I noticed that this variation is only for a particular mobile (Intex Make) and not for some others. (I need to check it throughly though to reach this conclusion)
Why would there be this variation only for a perticular mobile. I am checking it as said by sunnyskyguy and will update here.
Meanwhile I am designing a new board (this time its a 4 layer rather than 2 as earlier)
The LDO design is as follows
ΩΩΩ
Part Numbers used:
22uF/10V : CC1206MKX5R6BB226
10uF/10V " CC1206MKX5R6BB106
There are input caps for LDO too. I have a DC to DC that gives 5V, the output of that is connected to LDO
Part Number
47uF/16V : GRM32ER61C476KE15K
1R/1% (0805 package) resistor to maintain ESR. If I remember it right, datasheet says ESR to be between 0.3 to 22R
Am I doing it right??
The load I guess will draw about 500-600mA @ 3.3V (Approximated. I am unable to check on this and hence this approx value)
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I tried having saparate supplies for Bluetooth module and main board. It even works with Intex. This isnt going rt at all...
2) if above response is ringing more than the ESR / LOAD ratio then separate input and output grounds using an additional series ESR R between R27 and 1R on the 100uF cap, see spec figure 12 for details.
Let me check that and do the changes. I will update the sch
Meanwhile in old schematic
VDM is battery voltage (7.4V, Li Ion)
I am not using 5V anywhere. Used it just to drop the voltage so that 3V3 LDO works fine.
issues:
3V3 is used for bluetooth MCU etc.
Load is not a constant current consumption device. Its dynamic, variable.
Some boards work and some dont.
I did a few changes
I removed 5V LDO and instead place a 3V3 LDO in place of a 5V and left 3V3 LDO open and so that I could shory PIN 2 and 3 and take 3V3 from there.
LDO's are aub-optimal solution here for high pulse loads. -Buck would be much more efficient
Before shorting 2 and 3 of U2, I noticed this
Output of U1 (now a 3V3 LDO), gave 3.3V output
and since PIN 2,4 of U2 are not connected to 3V3, it should show 0V but was showing 0.9V
What might be causing this??
Ground Noise???
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What are you referring to when you say R27 and 1R. Figure 12???
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Update: In the old circuit that I am explaining. (1117 LDO)
I also was checking the battery voltage (Voltage divider on Battery connected to MCU pins). Removed these resistors and I saw the voltage drop to 0 instead of 0.9 (that I was getting earlier)
I was under the impression that I can connect a voltage diver directly to the MCU analog pin but it seems like it is adding some variations or error if I can call it so
I am using 7.4V Li Ion Battery, followed by a 5V (1117) regulator, the output of which is given to 1117 (3.3V)
The input (Li Ion) voltage depending on load is flatuating (Dropping to say 6.8V).
I am connecting a bluetooth module also at 3.3V.
Observations
I closely looked and I saw that 3.3V drops 200-300mV (Approx) for about 7ms and comes back to 3.3V and some spikes (lots of up down lines on CRO) of few mV variations.
This I think is efefcting bluetooth throughput
How can i make sure that the voltage is stable
If I use a saparate supply only for bluetooth with common ground ofcourse, it works fine
Don't care. This comes from the battery voltage divider --> ADC input pin --> internal protection diode -->ADC VCC pin --> (unconnected) VCC supply. The voltage (0.9V in your case) is not predictable. It depends an many items.and since PIN 2,4 of U2 are not connected to 3V3, it should show 0V but was showing 0.9V
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