VSTORE and LDO2 Voltage Drop When Loading STEVAL-ISV019V1

Welcome @HUANG2, to the community!

Important is:

• the pass transistor remains open as long as VSTOREV does not reach the EOC threshold
• according to your schematic, VEOC with the resistors is about 4.21V
• only above VEOC is the internal path enabled, so that LDO2 can become active and the supply can be switched through accordingly

VSTORE only rises sufficiently if more energy is supplied to the storage capacitor or the STORE node than is drawn from it, for example by:

• The energy source at IN_LV must deliver enough power. If the source is too weak, VSTORE remains below the switching threshold.
• Enough charging time for the STORE capacitor to charge. Small currents or a high load prevent VEOC from being reached.
• If the load on LDO2 or the system is too high, the voltage on STORE is pulled down again.
• Missing or incorrect connection of BATT, STORE, LDO2_EN, external resistor dividers or protection paths can prevent charging. Also important: no unwanted leakage current or short circuit at the STORE node.

Check:

• measure VSTORE directly while the source at IN_LV is active
• vheck whether the STORE capacitor is correctly dimensioned
• reduce the load on LDO2 as a test
• check whether the circuit can really rise to VEOC ~4.21V

As long as VSTORE does not reach VEOC, LDO2 remains off. The battery is not meaningfully enabled in this state. Only when STORE rises above the EOC threshold through sufficient energy input does the internal pass transistor close.

Regards
/Peter