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LM3480: LM3480 turns off output unexcpectedly at startup

Part Number: LM3480


When I apply voltage to the LM3480IM3-15, the output from the regulator is turned on and off a couple of times before it stays at the wanted 15V.

Vin ( and also Vout from regulator) ramps from 0V to 15V for approx. 20ms. Vin continue to increase until it reaches 24V. Just about when the output from the regulator reach 15V it makes a number of fast on/offs, Period about 15ms. The number of on/offs varies from 2 to maybe 10 times. In some few cases it does not clip at all.

FYI, when I pull the DC plug from the power supply and inserts it rapidly, i.e. the ramp-up of the voltage to Uin on the regulator is very fast, 10's of µs, the output from the regulator does not clip. This indicates that it is not the inrush current to the load that is the problem since the inrush must be much higher in the fast ramp-up compared to the 20ms ramp so the clipping of the output must be due to another reason.

When I reduce the current limiter on the power source and making the ramp even slower than 20ms, then there is no clipping.

Can You also shortly describe the intended behavior at over current? Will it turn off the output or will it limit the voltage?

Thank You

  • Hi Stefan,

    Let's address your last question first because it is the most straight forward. Current limit does not turn off the output it simply limits the output voltage such that the load current does not exceed a certain limit. 

    As for what is happening in your application there are a two things I can think of from your description which might cause the output to turn on and off.

    1. In-rush current: During startup the LDO needs to charge the output capacitors. Vin could be drooping due to the current needed to charge the caps (plus any load current)and since the LDO is still in dropout (for Vin<15V) any movement in the input supply will be seen on the output as well. 
      1. Slowing down the input voltage ramp (using your current limiter) could help as you saw by slowing down the dV/dt on the output reducing the current needed to charge the caps (i=C*dV/dt).
      2. It is possible that a very fast ramp might get Vin to a high enough voltage that the LDO is not in dropout and therefore a droop in Vin would not be seen at the output of the LDO.
    2. Thermal shutdown: when the junction temperature of the LDO gets too hot it will shutdown to protect itself.
      1. Slowing down the input voltage ramp would help since less current is drawn to charge the output caps. 
      2. However, I'd expect it to be worse in the very fast startup (hot plugging the DC plug) so this seems less likely. 

    Can you provide scope shots of these three scenarios showing both Vin and Vout on the same plot? This would help me to look for any other clues as to what might be happening in your application.