UA78: +15V voltage regulator delay when power on

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 9.2.2.1 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.

Kai

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.