Hello Nordic Support Team,
I am experiencing a startup issue with the nPM1300 PMIC. Below is the summary of my hardware configuration and the behavior I am observing:
Hardware Setup:
VBUS: Supplied with a constant 5.0 V from a bench power supply. C2 was tried with 1uF, 10uF and 100nF.
VSYS Capacitors: C1 and C3 are 10 µF, following the datasheet recommendations. No external active components are attached directly to VSYS during initialization.
VOUT2 (BUCK2): Configured as the main power rail for the system, which has a significant capacitive and IC load attached.
VOUT1 (BUCK1): Supplies a unique IC with a typical light load of ~5 mA.
The Problem: Even though 5.0 V is constantly applied to VBUS , VSYS drops and stabilizes at 2.5 V. Because the minimum VSYS voltage required to enable the BUCK regulators is 2.7 V, the BUCKs do not start up.
The only workaround I found to make the system boot is to perform a momentary short-circuit between VBUS and VSYS at startup. When I do this, VSYS stays at 5.0 V, and both VOUT1 and VOUT2 regulate to their expected targets correctly.
I initially suspected that the default 100 mA VBUS input current limit was causing a voltage collapse due to the high inrush current demanded by the VOUT2 rail capacitors. To isolate this, I performed the following tests:
Test 1: Both VSET1 and VSET2 connected to GND
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Goal: Disable both BUCKs at startup to eliminate their inrush current.
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Result: Even if I apply the initial manual short between VBUS and VSYS, VSYS drops back to 0 V immediately after the short is removed.
Test 2: VSET1 connected to GND and VSET2 configured for 3.0 V
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Goal: Keep BUCK1 disabled and let BUCK2 boot up automatically.
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Result: Applying the initial manual short between VBUS and VSYS allows VSYS to stabilize at 5.0 V, and BUCK2 regulation works perfectly even after the short is removed.
Conclusion & Questions:
Based on Test 1, inrush current from the BUCKs does not seem to be the only issue, because even with both BUCKs disabled—meaning VSYS only needs to charge its own two 10 µF capacitors—the chip still fails to maintain VSYS without the manual short.
Could this behavior be related to the digital core or the VDDIO rail not initializing properly when BUCK1 is disabled? Since VDDIO must be present for the TWI and digital logic , does disabling the BUCKs trap the PMIC in an internal brownout or Power-Fail (POF) reset loop?
How can I ensure the nPM1300 successfully powers up VSYS autonomously under the default 100 mA startup limit?
Thank you for your help.