Tool/software:
we test the LM5160 EVM board. we can find the PWM signal of chip is stop during power supply start-up; stop time is about 40us；
the peak current value of transformer primary side is about 430mA；
why dose the chip output stop？
Hi Xiaobo,
Hi Xiaaobo,
Please go through the datasheet section 7.3.7
The length of the off time is controlled by the FB voltage and the input voltage VIN. As an example, when VFB = 0 V and VIN = 24 V, the off time is set to 10 µs. This condition occurs if the output is shorted or during the initial phase of start-up.
Here is the EVM test results with CSS=1.5nF.
I had tested with Iout1=20mA, Css=1.5nF Cout being EVM default. I observed off time ~5.15us. I believe very less Tss and higher Co can cause high inrush current.
In your case, the off-time is ~47us. To explain this, we need to have details if there is change on the EVM schematic (if any). Also please confirm the operating conditions.
Regards
Arpita
Hi Xiaobo,
No, the current limit is always same as what is listed in the EVM.
Let's understand that during start-up, there is inrush current drawn from the supply. The output capacitor requires current to charge them from their initial or zero state to their final steady state voltage. This current can have a high peak depending upon voltage slew rate and source impedance and is referred as inrush current.
You can calculate inrush current approximately using 
where C_LOAD=output capacitance, dv~ Vout, dt=Tss.
Large output capacitance and very fast start-up [i.e. very less Tss as in customer case] can lead to very high inrush current resulting in undesirable operation.
In your case, the off time is ~47us. To comment on this, we need to have details if there is change on the EVM schematic (if any).
Hope this explains the inrush current phenomena during start-up.
Regards
Arpita
Any update on this?
It is very difficult to comment without having knowledge of Cout values and EVM modifications.
Where are you exactly measuring current? please confirm if you have checked between IC pin11/12 and T1 pin 1 like this. Also, can you please attach
waveform of iL with good resolution with A/div, BW info etc for proper understanding? Sorry I cannot see iL_peak level due to poor resolution.
Hi Xiaobo,
It looks like inrush current would be much higher than 0.43A for the Css=2nF. As from the shared waveforms current values cannot be read, would advise to reattach the waveforms to confirm that the current amplitude is ~0.43A.
Also, for further debugging, please share the start-up behavior with resistive load in EVM as suggested earlier.
Regards
Arpita
