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LTC4040 backup question

Dear Sir,

Our customer have some question when using LTC4040 backup power as below.

Customer's output had a deep drop(Vsys) when system is change to backup mode(Please see below pic). This drop will cause the output system(watchdog) restart.
Customer's output spec is Vin 5V, Vout(Vsys 5V 1.4~1.5A.)

I have customer's schematic on hand, but I can't post on website. Could you please help provide some advise for us? Thanks.