LTC4120-4.2 charge interruption at high charge currents

Hello everyone,

I use the LTC4120-4.2 (RX) in conjunction with the LTC4125 (TX) as a wireless power system to charge a LiIon-Battery. My problem is that for higher charge currents (above 300mA) charging is interrupted as indicated by nCHRG Pin going high and nFAULT Pin going low at a rate of ~2.5kHz as shown in Fig 1.


Fig 1: Interruptions during charging. Ch1 (yellow) .. Vin; Ch2 (green) .. nCHRG; Ch3 (blue) .. nFAULT; Ch4 (red) .. INTVCC

Test Setup:
TX: DC2330A (LTC4125) Demo Board
RX: Custom Board based on DC2445A (LTC4120) Demo Board

Improvement Measures
- Reduce switching frequency fronm 1.5MHz to 750kHz
- Increase Vin voltage by increassing zener voltage of R1 in clamping circuit

Schematic

Fig 2: Schematic of LTC4120-4.2 section.

Layout

Fig 3: 3D Top View                                                                          Fig 4: GND-Plane highlighted

Any ideas why fault conditions occur in this setup?