Hi everyone,
I’m testing the LTC4120-4.2 wireless charger IC and I’m observing some unexpected behavior.
I’m currently not connecting any battery to the VBAT pin — just monitoring the output voltage on VBAT while testing the receiver board.
However, VBAT floats up to around 5.4–5.6 V, instead of being regulated at 4.2 V, and the IC gets very hot during operation.
Also, the transmitter current stays at 100% (full power) continuously.
Setup details:
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IC: LTC4120-4.2
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Receiver input (ACIN): ~12 V
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No battery connected (VBAT open)
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Coil: smaller than the one used in the reference design
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Resonant frequency (measured between Cx and GND): ≈109 kHz
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Signal amplitude: ~19 Vpp
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Components (inductor, diode, etc.) follow the datasheet example
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PCB layout similar to the recommended design
Symptoms:
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VBAT floats up to 5.4–5.6 V with no load
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Transmitter stays at full current (100%)
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IC overheats after a short time
Questions:
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Is this behavior expected when no battery is connected to VBAT?
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Could the smaller coil or different resonance frequency cause improper power regulation?
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Should I add a dummy load or resistor on VBAT to stabilize it?
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Any advice on what could cause the transmitter to stay at full power continuously?
Any suggestions or insights would be greatly appreciated.
Thanks!