LP3944: 25mA each LED rule

Hi Pragash,

25mA is the peak current limitation. If you want larger current ability, you could parallel multiple output channels or refer to other devices like LP5861 or TLC59116.

Best Regards,

Aaron

Hi Aaron,

I think it's impossible to select a LED that I want with a peak current of less than 25mA. However, I think TI has a solution in their datasheet. They have a current limiting resistor to protect LP3944.  Please let me know your thoughts.

Hi Pragash,

Would you provide the LED part number on your selection list, elaborate on what's the goal of large current (brightness req), and what end equipment is being built with LP3944? The serial resistors with LEDs are usually for power dissipation purposes and cannot increase the 25mA current output from the chip side anyway. I will investigate the LED spec sheet and suggest some other suitable devices.

Best Regards,

Aaron

Hi Aaron,

The series resistor can be used to current-limit an LED. I will copy Wikipedia text here and provide the source also.

Wikipedia Text:

Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor.

Miniature indicator LEDs are normally driven from low voltage DC via a current-limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents,^[4] but indicators running on 100 μA are still practical.

source: https://en.wikipedia.org/wiki/LED_circuit

I have attached the datasheet of the LEDs I'm using here. 

IN-S124AR_Series_V1.0-1488257.pdf

SJW1151CKS-100-TR.pdf

I have two questions:

1) based on Wikipedia, if I calculate and set the resistor value correctly, it will limit the current and protect the LED dividers. Is this correct?

2) when I calculate based on Wikipedia, I'm getting 85 ohms. Isn't it very low? 

Hi Pragash,

pragash sangaran78 said:

Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor.

Thats exactly what I've mentioned "serial LEDs are for power dissipation, but not for increasing the output current." I didn't say that serial resistors can't be used for current-limit. The LED forward current is capable of 20mA, so I can understand that you tend to use the serial resistors, but it doesn't echo with your question that "most LEDs have peak currents much higher than this." 

1. Yes, it is correct. But I didn't see any connection of this with your original question. It seemed to me that you were doubting the current capability of LP3944 and were looking for devices with larger current output. 

2. That's reasonable values; did you use the 5V VLED for calculation? Plz show that how you get this result.

Hi Aaron,

I think you miss understood my question from day 1. My concern from day 1 is whether the LED driver will blow up if I connect the LEDs mentioned above because the LED driver's specs say the peak forward current must be 25mA or less. But if you look at the LED spec, the peak forward current is always more than 25mA even though the average forward current is 20mA.

I know you didn't say the series resistor cannot be used to limit the current, but you didn't say that the series resistor can be used to limit the current. You told me the peak current needs to be 25mA or less or change the LED or the driver IC. That's why I did my study and found out that series resistors can be a help. I want to confirm that and confirm if the resistor value is correct.