Inverting voltage regulator with the NCP3063 - Page 1
Forum section: electronics / projects

author: Zeyneb
author_rank: Regular Contributor
timestamp: June 29, 2022, 03:51:32 pm
content: Hi there,The objective is to make a voltage regulator that makes -10V at 150mA from 12.6V input voltage. I do have the NCP3063 at home and I would like to make it work with what I have. Even if I’d need many components surrounding it. I made a design on a breadboard based on the example from onsemi and it gets to -10V but with many oscillations on the Timing Capacitors pin and the Ipk sense pin. I would like to redesign this with an external pnp transistor and having the chip referenced to ground instead of the negative output voltage. I can use some help with that.Here is the datasheet:https://www.onsemi.com/pdf/datasheet/ncp3063-d.pdfThis is inside.I use Fsw = 120 kHz set with a 2.2nF and 1nF timing capacitor parallel.So here is the circuit idea.I do not have a simulation model for the NCP3063. I just made a symbol to show you this. On the NCP3063 only the comparator inverting input is pinned out. But to increase negative voltage it has to increase the duty-cycle. So, I’d think I do need to additionally invert the signal to have the regulator control in the right direction. But what is the best place to do that inversion? The switch driver circuit or the feedback path. The above is showing in the driver circuit. Have I got this correct so the regulator controls in the right direction?I cannot properly design a driver circuit. I’ve done a separate simulation for just the pnp driver circuit and there is looks quite well. Any comments or suggestions to improve this or reduce the number of components?As for the feedback. When Vo = -10 the voltage at Vcomp should be 1.25V. The current source around Q1 sends 5.1 mA down, so 11.25 / 5.1e-3 = 2205 ohm, so Rshift = 2k2 in practice. Alright?Thanks for your time!

author: mariush
author_rank: Super Contributor
timestamp: June 29, 2022, 07:17:43 pm
content: There's a bunch of online calculators for the parameters and there's even a calculator on sourceforgefor examplehttp://www.nomad.ee/micros/mc34063a/index.shtmlcalculates this for 12v in, -10v out 150mA , 50mV ripple, 100kHzCt=194 pFIpk=584 mARsc=0.514 OhmLmin=92 uHCo=131 uFR1=13k R2=91k (10V)I'd aim for 80-100kHz as would be more compatible with various brands of mc34063 and you can tweak those ex Rsc can be 2 1 ohm in parallel (0.5 ohm) , you can use 200 pF , 100uH inductor, 150uF polymer on the output and you can tweak the resistors to maintain the ratio ex 10k and 75k to get a bit more than -10vConsider using a pi filter to smooth out the output ... or just output -12v and use a cheap ldo for an extra smoothing step (ex lm337)If you can manage to get your power consumption down, for example down to 100mA or less, you may want to look into charge pump regulators.For example LT1054 can do -Vin (adjustable with 2 resistors) at up to 100mA and all it needs is a couple ceramic capacitors. It is a bit expensive, but if you add the mc34063 plus inductor plus electrolytics plus various ceramic capacitors and resistors and pcb space, it could cost about the same.

author: Zeyneb
author_rank: Regular Contributor
timestamp: June 29, 2022, 08:13:16 pm
content: Sure but how about something I actually wrote about..."it gets to -10V but with many oscillations on the Timing Capacitors pin and the Ipk sense pin. I would like to redesign this with an external pnp transistor and having the chip referenced to ground instead of the negative output voltage. I can use some help with that."For the main power switch I plan to use the BD140. I have these at home.https://www.onsemi.com/pdf/datasheet/bd136g-d.pdf