The RC time constant (delay) for enable should be less than 10uS. But, the circuit has a 100 ohms R and a 0.1uF, two 4.7uF 'C' values. The achieved delay is 950uS.
This delays the gate control of both Boost-converter & Boost-inverter circuit. Since the Boost-converter gate control has an effect of feedback voltage only, whereas boost-inverter gate control has an effect of Input voltage.(As referred from internal block diagram of IC).
After removing the additional two no.of 4.7uF capacitors in parallel with 0.1uF in VIN pin, the -15V was settled at correctly.
Regards,
Devendhiran K