Other Parts Discussed in Thread: LM3409
I'm trying to apply the design example in the users guide for the LM3409HV Evaluation Board (SNVA390D/AN-1953). I must be doing something wrong, so have a couple of questions.
In the application note's example design procedure, section 6.2 determines some things. Notably:
R6 = 16.5K
FSW is 400Khz or about 2.5uS in period.
tOff is 242nS, about 10% of the period.
For this example, the Coff pin voltage is allowed to rise 10% of the time, and Coff is grounded by the LM3409 about 90% of the time. Given that Vout is 42V, the average Coff pin current is therefore 0.9 * 42/16.5k = 2.3mA.
Further, if one then follows the suggestions in section 7, for this same design example, equation 38 produces an Roff2 value of about 250ohms. Assuming a 5V supply for this section of the circuit and a whole volt for the diode, that would produce average Coff pin current closer to 16ma at low PWM duty cycles. Hmm... that seems like "a lot".
Unfortunately for me, the only thing the datasheet says about Coff current is an absolute maximum rating of "+/- 1mA continuous". Because the average Coff pin current is well in excess of that amount in both of these cases, it would seem something is amiss. But what is it?
Any illumination you could provide would be appreciated. Thanks...
