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Part Number: TPS7A49
Customer is using TPS7A4901 on their design. the input voltage is 24V and output voltage is 15V, output current is lower than 100mA in normal operation condition. Besides the 10uF output capacitor, another 100uF is added at the output. The customer found several failure chips. We found different phenomenon on these failure chips. For #1, #2, the ref voltage is obviously abnormal. For #3 and #4, the ref voltage looks OK but it couldn't start up successfully. Could you share your comment on in what condition will the customer get wrong voltage on the reference pin? And what may cause the device not be able to start up successfully?
#1: no output, the voltage on pin 6 is 7.5V
#2: no output , the voltage on pin 6 is 11V
#3: output voltage start up successfully and then drop down to zero, pin 6 voltage is 1.15V
#4: output voltage start up successfully and then drop down to zero, pin 6 voltage is 1.15V
Start up current in no load condition:
Since the application has a 100uF + 10uF output capacitors, when Vin has been removed abruptly, there could be a reverse current event that could damage the LDO. Please advise customers to check the power-down sequence to see if Vout is higher than Vin.
Please also verify if there are any high voltage spikes that could happen on Vin that could exceed the maximum 36V allowed on the input pin.
In addition, please try to remove the feedforward cap C6382 and rerun the tests to see if the LDO is still being damaged.
Regards, Jason Song
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In reply to Jason Song:
Thanks so much for your reply. After checking the power down sequence. I found the output voltage will be higher than the input voltage. Out to In voltage is about 0.55V. From the datasheet, I see the out to in voltage should be within -36~0.3V. Testing the pin resistance of the damaged device and normal device, no obvious difference can be found. Could you tell what function block may be damaged if the out to in voltage is higher than 0.3V?
In reply to Lenna Yan:
Please see this post for more details.
The reverse current may damage the body diode of the pass-FET.
From the TPS7A4901 block diagram, we can see there is a PNP triode between the input and output, instead of a FET. Is there a body diode in parallel with the PNP triode? The input resistor is 200kOhm, the reverse current wouldn't be very larger. Is it possible to damage the diode?
The datasheet clearly listed that Vout cannot be 0.3V higher than the Vin. It's not just the PNP or MOSFET, there are protected ESD circuits that can also be damaged when an absolute max is violated. Each LDO may have different PNP or Mosfet, and we don't have quantitative value to see how much reverse current it can take without causing a damage. The best practice would be that if there are large output caps, we will need to have a external protected diode if the LDO does not specifically list a built-in reverse current protection.
Does it make sense?
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