DC/DC Boost Converter Startup Problem with Very Slowly Rising Input Voltage - Page 1
Forum section: electronics / projects

author: bendras
author_rank: Contributor
timestamp: June 03, 2019, 09:08:08 pm
content: I am trying to use a DC/DC boost converter to increase voltage coming from a super capacitor from 2.5V to 3.3V (see the schematic below). The problem is that since the power source can provide only a few mA of current the supper capacitors takes multiple seconds to charge up and in turn the slow voltage rise makes the boost converter oscillate and then output around 1.2V instead of the expected 3.3V (see the graph below). Anybody has any ideas on how could this be solved?Note: The DC/DC converter enable threshold voltage is 0.8VSchematic:Graph:

author: mikerj
author_rank: Super Contributor
timestamp: June 03, 2019, 09:14:40 pm
content: You will likely need to add some kind of supply supervisor to hold the switcher in it's disabled state until the caps have charged to sufficient voltage. You can get reset controllers in three pin packages (TO92, SOT23 etc) with various voltage thresholds which may be suitable.

author: ogden
author_rank: Super Contributor
timestamp: June 03, 2019, 09:20:32 pm
content: R1 and R2 supposedly shall be KOhms, not Ohms. Also try using capacitor values which charges within 1sec or so. Now your simulation is kinda useless, at least for others - because it covers just input 0.8V to 0.9V range and that's it. I want to see whole 0V to 2.5V Also C6 shall be some real electrolytic capacitor, with some internal ESR.Quote from: mikerj on June 03, 2019, 09:14:40 pmThere is built-in supervisor in form of EN pin. It triggers at0.9V0.8V. So what you do is - add voltage divider. If you add 200K/200K resistors - it supposedly will start at 1.6V

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2019, 09:44:55 am
content: Quote from: ogden on June 03, 2019, 09:20:32 pmNo hysteresis though, so still likely to get instability at switch on threshold.

author: ogden
author_rank: Super Contributor
timestamp: June 04, 2019, 10:09:31 am
content: Quote from: mikerj on June 04, 2019, 09:44:55 amWhich exactly datasheet did you read, if any? I see 0.25/0.8 V hysteresis:

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2019, 12:59:33 pm
content: That's the exact same data sheet I read. I see maximum Vil and minimum Vih values, exactly as you would see for any standard logic input, but no mention of of a Schmitt trigger input. The block diagram doesn't show any kind of Schmitt trigger input either. Did you find something else in the datasheet that mentions hysteresis?

author: ogden
author_rank: Super Contributor
timestamp: June 04, 2019, 01:22:52 pm
content: Quote from: mikerj on June 04, 2019, 12:59:33 pmYou are right. EN input may not be Schmitt trigger - there is no clear mention of such. My prior knowledge let me believe that this IC is no different to some I know (LT switchers).

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 04, 2019, 07:20:18 pm
content: Indeed. But anyway, as ogden said, the main issue is with the values of R1 and R2. Way too low, so they load the boost regulator way too much. I simulated the op's circuit with 172k and 107k instead and it appears to start correctly and get stable in approximately 1.5s with some minor ripple. So the feedback resistors were the main issue.Still, the output current it can deliver will be pretty limited (as the low-value feedback resistors showed).

author: bendras
author_rank: Contributor
timestamp: June 04, 2019, 10:39:17 pm
content: I have done the following changes to the model:1. As suggested I have set the R1 and R2 resistors to appropriate values2. Added realistic input and output capacitor models (Taiyo Yuden JMK107ABJ106MA-T 10uF 6.3V ceramic caps.)3. Added RT, RB and RHYS resistors to shift the turn-on voltage to 2V while keeping the turnoff voltage at 0.8V.Now the startup seems to be reasonable.New schematic:Zoomed out graph:Zoomed in graph: