Part Number:LM5141
I have built a circuit based on a WebBench design for 24-48 V input to 5 V / 5 A output using LM5141.
However, when I test it, I get no output.
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Some observations:
* The low-side FET gate is driven to 5 V. No switching activity is seen. 0 V on the high-side FET's gate.
* When the EN input is asserted, the SS pin initially ramps up to 5 V (takes about 25 ms), but then drops very quickly to ~0.19 V.
* I verified 3.3 V is present on EN pin.
* I verified 5 V is present on VCC pin.
* ~4.9 V present on the boost diode (D8) cathode.
* So far I'm testing with 24 V at the VCC_48 node, and light load (no load or 56 ohms load).
I noticed this text in the LM5141 datasheet and wonder if it could be related (based on the behavior at the SS pin):
At the beginning of the soft-start sequence when SS = 0 V, the internal 20 μA softstart
current source gradually increases the voltage on an external soft-start capacitor connected to the SS pin,
resulting in a gradual rise of the FB and output voltages. The controller is in the forced PWM (FPWM) mode
when the DEMB pin is connected to VDDA. In this mode, the SS pin is clamped at 200 mV above the feedback
voltage. This ensures that SS will be pulled low quickly when FB falls during brief over-current events to prevent
overshoot of VOUT during recovery. SS can be pulled low with an external circuit to stop switching, but this is not
recommended. Pulling SS low will result in COMP being pulled down internally as well. If the controller is
operating in FPWM mode (DEMB = VDDA), LO will remain on and the low-side MOSFET will discharge the
VOUT capacitor resulting in large negative inductor current. In contrast when the LM5141 pulls SS low internally
due to a fault condition, the LO gate driver is disabled.
Any thoughts on possible root cause or next debugging steps is appreciated.
Thanks
Matt
