LM2758 EN low period after Flash Time-Out

Thank you for the question Sasaki-san,

100us is too fast to re-start the device after a timeout.  The output must fall below Vin before restarting, and that may take 10-20ms if I remember correctly.

I have a few questions for me to better understand the customer's application and possible solutions:

1. Do you know what current the customer wants to use for the Flash Pulse?

2. Is the customer using the flash timeout function, or are they only worried about flash timeout turning on and shutting off the output?

3. What is the input voltage and LED Vf (at what current) they are using in the application?

The reason for question 1 is that if the current is too high, they could cause the device to thermal shutdown if they try to continually keep the high flash current active.  For currents above 400mA typ continuous, it is not suggested to keep the flash current on longer than the flash timeout.  Normally, flash pulses are suggested to be 10%-30% of the pulse period to allow for proper thermal dissipation.

The reason for question 2 is that if the application is for the LED to remain on for a long time, and not time out, they could use "Torch" mode which does not use the timeout timer.  However, this is only suggested if the LED current is 400mA or below.  Otherwise, if they want to use the flash timeout to create a "pulse", and then create another pulse again right away (like a "pre-flash" red eye reduction), then we will have to try a different solution.

The reason for question 3 is to better understand if there is a way to optimize the LM2758 application circuit to work for the customer.

Regards,

Karl

Karl-san,

The answer for your questions are the below.

1, Their Flash Pulse current is 211mA. So I think that the possibility of thermal problem is low. I suggested them to keep using Torch Mode, however the hardware change is difficult for them because their hardware design has been already finished. Firmware change is possible.

2, The issue is that Flash turns-off by timeout before taking a picture, thus they would like to turn on the Flash again as quick as a possible.

3, Vf: min 2.8V  typ 3.3V  max3.8V(at ILED=500mA)

Why is 10ms-20ms needed for Flash timeout reset? Can't LM2758 response to short low signal of EN? Or is it only concern for thermal dissipation?

If it is only thermal dissipation, I believe that 211mA Flash current is no problem.

Could you let me know the min value of device response time for EN signal if my thinking is right?

Sasaki-san,

The 10-20ms I was talking about was not for the timeout reset, but for the Vout to fall down low enough for the device to restart the proper turn on sequence.  The only discharge path for Vout when the part is off is the internal shutdown pulldown (high resistance). One thing I forgot to consider that when the part is re-enabled, the LED load will quickly (~100-200us) pull Vout down low enough for the part to restart.  However, the startup process for Flash still takes up to 2ms (typ). 

There might be a possible solution because the timeout reset is done every time the signal on any EN pin changes.  Therefore, the best solution would be for them to apply the 100us low pulse to the EN1 pin, while holding EN2 logic HIGH.  This will cause Torch mode to turn on for a very short time, and then go back to Flash mode, with the flash timeout timer reset.  Because the duration in Torch mode is so small, the output current should be very close to the desired Flash current value.  This is the preferred method.

If the customer can not control EN1 separate from EN2, then the part will have to go through its startup process every time the part turns back on.  A low signal on both EN1&2 will quickly turn off the LED, but Vout will still be high (slow discharge through internal pulldown).  When a new HIGH pulse is applied to EN1&2, Vout is discharged down below Vin by the LED load, and then restarts the start-up routine.  This startup process can take up to 2ms (typ) before ILED reaches the target value as stated above.  Therefore, if they plan to pulse both EN1 and EN2, the frequency that they choose to create the 100us low pulse will directly affect the current through the LED. [1 - (2ms/Tperiod)] x ISET = ILED.

Regards,
Karl

Karl-san,

Thank you for your solution proposal.

They can control EN1 separate from EN2, thus I will suggest the EN1 pin toggle method. My understanding about this method is the following. Is this right?

1, EN1=0, EN2=1, device operates in Torch mode.

2, EN1=1, EN2=1, device turns on the Flash.

3, Make EN1 to low from high before Flash timeout occurs, and device returns to Torch mode.

4, Repeat the above procedure from 1 to 3.

About the period that keep EN1 to Low, is 100us enough? Can the device response?

Karl-san,

Thank you for your support.

I suggested EN1 toggle method to my customer, and they said that they can work around the issue by that method.

If I get additional question, please let me ask.

Best Regards,

Kohei Sasaki