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UA78: +15V voltage regulator delay when power on

Part Number: UA78

I have a problem when powering on with TI UA7815 while applying unregulated 20V DC to the input. The +15V (A21-3) from UA7815 will have about 10ms delay compare to -15V(A22-3) from the scope shot below. Is there anyone know what causes this and how to avoid it? Thanks.

+15V is voltage waveform from A21-3 which use TI U7815C (The is no time delay if Fairchild LM7815C is used.)

-15V is voltage waveform from A22-3 which use ON 7915CT

  • Hi Weichuw,

    this usually happens if you have a load connected between the +15V rail and the -15V rail AND if the circuit draws from the one supply line much more current than from the other. You can see that the +15V takes much longer to settle to the final value. So, from the +20V supply line and/or from the +15V supply line much more current is drawn than from the -15V supply line and from the -20V supply line. So, immediately after power-on the input voltage of +15V regulator is not high enough to make the +15V regulator work. The output is still off. But the input voltage of the -15V regulator is higher and this regulator already works. It's drawing current from the +15V regulator which is not yet working and makes the output of the +15V regulator even go below 0V! This is a dangerous operating condition and can make the regulator go into latch-up!

    Section of datasheet of UA78 recommends a clamp diode for this operating condition. You should add it to your design. We use a 3A Schottky diode for this purpose, by the way.

  • Hi Weichuw,

    I just wanted to confirm Kai's answer. You should use a clamp diode to prevent the output of uA78 from going negative and this should resolve your delayed startup issue.
  • Thanks. This solve our problem.