What should be done if the LM2594HV inverting regulator does not start up with relatively large input capacitance?
In this specific situation, the LM2594HV SIMPLE SWITCHER switching regulator was being used in an inverting circuit to generate -12V from +28V. The circuit was based on the -5V output circuit (Figure 25) in the LM2594HV datasheet. The regulator would not start up with 3 paralleled 6.8uF/600mOhm capacitors on the input, although with less capacitance, it started OK. Two things needed to be checked: (1) Make sure the delayed startup circuit is being used as recommended – R1, R2, C1 of Figure 25 in the LM2594HV datasheet. This is intended to allow startup from input voltage sources which cannot deliver adequate current. If it is being used, you might have to tinker with the values. If not, you should try it. (2) Make sure the diode is between the source and the input capacitor (see D1 in Figure 25 of the LM2594HV datasheet). This helps the circuit look like a buck regulator to the IC. An inductor in place of the diode would also work – and it reduces your output ripple.
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