Hello,
We're working with the SN6505B and over-loading the output to see the behavior in the system.
Is this because of the internal thermal limit being reached?
Thank you, Keith
Hello,
Just a couple more questions:
Is the off time after a thermal shutdown a fixed time duration? During our tests we saw that it was approximately 13.404 ms across multiple cycles which prompted this question.
Is I_Lim as described in the datasheet the current where the chip will fault and stop switching? Or is I_Lim the peak current limit the chip enforces?
Following that question, what specific factors determine that current limit? The datasheets lists a wide range for the Current Clamp Limit, I_Lim of 1.42 – 2.15 A with a nominal value of 1.75 A for our operating Vcc as seen below.
Is there any additional documentation on the overcurrent and overtemperature protection available? We were unable to find any in the datasheet.
Thank you, Keith
Hi Keith,
Thank you for following up with additional questions. Please see my inputs below,
Is the off time after a thermal shutdown a fixed time duration? During our tests we saw that it was approximately 13.404 ms across multiple cycles which prompted this question.
The datasheet specs the thermal shutdown on and off thresholds, please see below screenshot. The device enters thermal shutdown when on-threshold is reached (typically 168C) and turns off. Once device is turned off, it starts cool and the temperature of device drops. Once the temperature drops below off-threshold (typically 150C), the device turns back on. Since the device only needs to cool down by about 18C, it turns back on quickly. Based on what you have shared, it looks like the time taken to cool down 18C from 168C to 150C is 13.4ms. Since the device is under same operating and ambient conditions, it is mostly taking the same amount of time to cool off.
If you try a new device or change ambient temperature, this time could be slightly more or less. But for a given device in given operating conditions, you will mostly see the same off time.
Is I_Lim as described in the datasheet the current where the chip will fault and stop switching? Or is I_Lim the peak current limit the chip enforces?
The current limit of SN6505 is a cycle-by-cycle current limit. When it detects that the drain current has reached ILIM, it turns off the switches for that cycle and starts switching again from the next cycle. If overload exists in for the next cycle, then it again detects ILIM and turns off the switch. Due to this the input current should be a pulsating current if monitored at the drain pins. The input current shown in the waveform is most likely the current before bulk cap to SN6505 or transformer and that's why it doesn't see the switching currents. The waveform is also zoomed out to 300ms/div and this could be another reason for seeing a flat input current.
Following that question, what specific factors determine that current limit? The datasheets lists a wide range for the Current Clamp Limit, I_Lim of 1.42 – 2.15 A with a nominal value of 1.75 A for our operating Vcc as seen below.
The variation in current limit thresholds is due to internal references which vary part-to-part and with temperature.
I helps you with the clarification you were looking for, thanks.
Regards,
Koteshwar Rao
