CS ( current sense) INPUT LED DRIVER JOB

Thread Starter

DAISY1988

Joined May 2, 2021
3
Hello circuits experts

I have to work with the OB3353 LED DRIVER and i have some difficulties on understanding what is the job of the CS pin input,5th,( Current sens)!!!!, what is the relation between The feedback and the CS input and how can we choose the resistor's values !!!??????? Any help please
ob3353.png
 

DickCappels

Joined Aug 21, 2008
7,743
The reason it is sensed through an external resistor is that the resistor will dissipate power, also, resistors are "expensive" inside integrated circuits because they use a large area on the chip.
 

ronsimpson

Joined Oct 7, 2019
1,475
There are two currents that are being measured. There is a slow and a fast loop that measures current.
The LED current is measured by Rfb and is related to the brightness you see. The IC responds to the current slowly. (mS)
The inductor current, shown in blue, is also related to the LED current but some what indirectly. The IC responds to the current very fast. (35nS)
A cycle starts out with no current in the inductor or MOSFET. The MOSFET turns on and inductor current builds up to some point. (say maybe 100mA) Exactly when the current reaches the trip point the MOSFET turns off and the current continues to flow from Vin, fuse, inductor, diode and into the storage capacitor.
1619974675294.png
 

Thread Starter

DAISY1988

Joined May 2, 2021
3
There are two currents that are being measured. There is a slow and a fast loop that measures current.
The LED current is measured by Rfb and is related to the brightness you see. The IC responds to the current slowly. (mS)
The inductor current, shown in blue, is also related to the LED current but some what indirectly. The IC responds to the current very fast. (35nS)
A cycle starts out with no current in the inductor or MOSFET. The MOSFET turns on and inductor current builds up to some point. (say maybe 100mA) Exactly when the current reaches the trip point the MOSFET turns off and the current continues to flow from Vin, fuse, inductor, diode and into the storage capacitor.
View attachment 237306


Thank you very much for your explanation it's more clear for me now.you encourage me to ask more questions.
i can't see the relation between the first loop FB and the CS, here is the LED current :9.PNG
and the CS Resistor is only mentioned in the slope compensation of the sub-harmonic oscillation

( we have a duty-cycle that is > 50%)8.PNG7.PNG
 
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