# CS ( current sense) INPUT LED DRIVER JOB

#### 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

#### ericgibbs

Joined Jan 29, 2010
18,072
h
This clip from the datasheet explains.
E

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#### DAISY1988

Joined May 2, 2021
3
h
This clip from the datasheet explains.
E

Thank you Mr Eric

Actually what i dont understnd is why the LED current is sensed throught an external resistor ? what is the infrmation that CS provides ?? I really ccan't figure this out

#### Ya’akov

Joined Jan 27, 2019
8,186
To measure current you measure the voltage drop across a resistor.

#### DickCappels

Joined Aug 21, 2008
10,068
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
2,757
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.

#### 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 :
and the CS Resistor is only mentioned in the slope compensation of the sub-harmonic oscillation

( we have a duty-cycle that is > 50%)