LT8334 for -23V 1A

Question

Hello, 
 I need to supply my circuit with -23V/1A and I'm trying to use the LT8334 to generate the negative voltage. 
 I have two questions regarding this part, (this is the datasheet to the part): 
 1. In page 18, equation 35 and 37 to calculate Maximum Duty Cycle: I don't understand how Dmax can be smaller than 1? 
 2. I'm using the schematic at page 21 of the datasheet to generate -23V/1A (max). This is my schematic (the Inductor is DRQ74-2R2-R as suggested) 
 I'm able to generate -23V; however, I don't know why but the output current seems to be too low: 
 whenever I connect the output to my circuitry, the -23V drops to -2V immediately even though there's no (or little) load at all. 
 
 Can you advise me on these 2 questions? Thank you very much in advance. 
 Best Regards, 
 Thai.

FormerMember · Accepted Answer

Tekemachi said: 1. If I use the equation 35 to calculate Dmax: (my Vin is 12V) 
 Dmax = (|23| - Vd) / (|23|-Vd - 12V) 
 The result of this equation is always greater than 1 regardless of Vd (which is about 0,47V) -> If then, how can it obey the equation 37 where Dmax has to be smaller than 1? 
 I suspect that the correct equation should be Dmax = (|23| + Vd) / (|23| + Vd + 12V) 
 The sign should be positive in Vd and Vin. Mistake on the datasheet. 
 
 Tekemachi said: 2. I'm trying to use one today. 
 Howevery, can you spot any mistake in the design? 
 your output ceramic capacitor should be at least rated twice your output voltage because of DC biasing and fluctuations on the output during transient might go over the rated output voltage. Make sure that components derated at 80% can still satisfy your needs. Same goes for your input ceramic capacitors. 
 Example: 
 If your output voltage is 23V, the bulk capacitors should at least be: 28.75V 
 0.8*28.75= 23V. 
 
 Regards, 
 Jhun)

FormerMember · Answer

Tekemachi said: 1. In page 18, equation 35 and 37 to calculate Maximum Duty Cycle: I don't understand how Dmax can be smaller than 1? 
 Theoretical maximum of duty cycle is 100%. The controller does not support 100% (which is the 1 you are pertaining to) duty cycle. 
 
 Tekemachi said: 2. I'm using the schematic at page 21 of the datasheet to generate -23V/1A (max). This is my schematic (the Inductor is DRQ74-2R2-R as suggested) 
 I'm able to generate -23V; however, I don't know why but the output current seems to be too low: 
 whenever I connect the output to my circuitry, the -23V drops to -2V immediately even though there's no (or little) load at all. 
 Have you tried using an electronic load for now to test your design? 
 
 Regards, 
 Jhun

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LT8334 for -23V 1A