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 BoardRX: Custom Board based on DC2445A (LTC4120) Demo BoardImprovement Measures- Reduce switching frequency fronm 1.5MHz to 750kHz- Increase Vin voltage by increassing zener voltage of R1 in clamping circuit
SchematicFig 2: Schematic of LTC4120-4.2 section.
Fig 3: 3D Top View Fig 4: GND-Plane highlighted
Any ideas why fault conditions occur in this setup?
Could you probe the voltage on BAT pin. From the waveform, it seems like the battery resistance is relatively high. Did you connect a current meter in series with the battery?
Another way to debug is to power the receiver with a 15V DC power supply from LTC4120 VIN pin to GND, and check whether you see the same problem or not.
Hi Wenwei, thanks for the fast response!The battery resistance is about 0.16 Ohm. The battery voltage plotted:
I checked the receiver with a 15 VC power supply at Vin, the problem is still the same.
Sorry I was on vacation and miss your reply. Could you show how BATSNS pin is connected? Could you also probe BATSNS pin voltage when this happens?
Hi Wenwei, no problem, hope you had a good time.
between BATSNS and the battery are a mosfet for reverse protection and a polyswitch.
The voltage probed at the LTC4120's BATSNS Pin:
For test purposes I replaced the polyswitch and the mosfet with jumper wires, but it didn't solve the problem.
From the BATSNS scope picture, I see that the voltage is really close to 4.2V when nCHRG is low and it suddenly drops as nCHRG goes high. This fast slew rate in BATSNS should not happen as the change of the battery voltage should be really slow. From the previous scope, the battery voltage is only at around 4V, when BATSNS is at 4.2V, and this is really suspicious. The BATSNS voltage should tightly follow battery voltage to provide accurate feedback to the battery charger. It seems as if the charge current flow through certain kind of resistance to bring up BATSNS pin as LTC4120 tries to charge the battery.
I think you should really look into why the BATSNS voltage does not follow the really battery voltage.