LTC4121EUD charge when Vin<12V

Only at an input voltage >12V PWM occurs and the charging current appears (0.14A input current). Otherwise, there are no signals. The charge status and error are read at <12 volts as well as at >12 volts at the input: nCHG = 0, nFLT = 1.

I need Vfloat = 4.0-4.2V, batery voltage which I try to charge V=3.5V. Vin=6-8.4V (two LiPo cells), Vout=4V (single cell LiFePO4).

May be only LTC4121-4.2 can work with <12Vin? But I need Adj Vfloat.

What should I do to start PWM on low voltage with LTC4121EUD?

PS. When Vin < 12V Vfloat avg = 2.74V, voltage ripple with a frequency of 1/280 ms, Vp-p=2.7-2.8V looks like a saw.