LTC4121-4.2 charging current issue

Question

Good morning, I made a circuit using LTC4121IUD # PBF as shown in the schematic of the datasheet in figure 12. As a test bench I've connected at the input Vin a DC power supply and a series resistance (Rin = 100R) and a Lithium battery of 3.7V/2000mAh at the output BAT. 
 My measurements do not agree with the datasheet values, in particular despite the maximum charge current has been set to 400mA (Rprog = 3.01k) the battery current was different; it increases with the supply voltage in the initial section but at 14V it is limited at a maximum of 100mA. Further increasing of the input voltage lead to a drop of the current to 90mA. 
 
 The graph in figure 13 of the datasheet shows that the charge current is initially increasing until it becomes constant at 400mA when the input voltage is 28V. 
 My question is the following: why does the battery charge current on my circuit does not increase more than 100mA despite the selected Rprog is 3.01k (Ichrg = 400mA)? What should I do to bring the charging current to its maximum value (400mA)?

Wenwei · Accepted Answer

Hi Nic84, 
 To debug, would you please first disable the MPPT feature (connect MPPT to either INTVCC or IN with a minimum 10k resistor) and check whether the LTC4121 can deliver 400mA ? Also would you please also check PROG pin voltage when it is charging, and see if it is at 1.227V? 
 The LTC4121 is very sensitive to input loop (CIN to VIN pin to GND and back to CIN_GND) parasitic inductance. With larger parasitic inductance, the part could be mal-functioning. So it is good to debug it with a simpler setup to check the basic feature first. 
 Thanks, 
 Wenwei.)