LM5155: Start-up behavior

Hi Conor,

Thanks for reaching out.

I have some questions:

Can you explain more details avout the test you are doing? What exactly do you mean with hot- and coldstart?

Why is the softstart voltage dropping first when powering up? I can also not see there zero references in the scopeshots.

And why is there a 33k resistor in series with the softstart cap?

Best regards

Moritz

Hi Moritz,

Moritz said:

Can you explain more details avout the test you are doing? What exactly do you mean with hot- and coldstart?

Please see the waveforms in the first post. The LM5155 uses the soft start (SS) pin to control the current during startup. We are concerned about the variation in discharge time due to the difference between hot start and cold start.

Initial condition of Vbias voltage:
In a hot start, Vbias starts with the device already charged, which tends to shorten the SS discharge operation.
On the other hand, in a cold start, Vbias starts from zero, which lengthens the SS discharge time.

Moritz said:

Why is the softstart voltage dropping first when powering up? I can also not see there zero references in the scopeshots.

It is my interpretation that if any charge remains in the capacitor (C1) connected to the SS terminal, the voltage will fall due to the discharging operation of the LM5155 terminal (short-circuited to AGND by an internal switch).

Moritz said:

And why is there a 33k resistor in series with the softstart cap?

This is to minimize the operation delay time until the COMP pin voltage reaches the active operation region (the control voltage at which the GATE pin operates).

Thanks,

Conor

Hi Conor,

The condition for the softstart to rise, is the VCC voltage. So this should be also in the plots.

50us after VCC exceeds the UVLO threshold or after reaching 4.5V, SS starts to rise.

Conor said:

This is to minimize the operation delay time until the COMP pin voltage reaches the active operation region

You mean to start the SS ramp with an offset?

Can you please try to remove this resistor?

Best regards

Moritz

Hi Moritz,

If I understand correctly, R9 has the following role.
- Adjusts the speed at which the voltage rise at the SS pin is transmitted to the COMP pin, indirectly assisting the operation of the COMP pin.
- Shortens the delay at startup, helping the PWM comparator operate quickly.
- Improves the stability of the control loop, achieving a smooth startup.

Based on this, I believe that by placing R9 it is possible to minimize the operation delay time until the COMP pin voltage reaches the active operating region. The datasheet does not specifically mention the placement of R9, but is this understanding correct? Also, could you explain in more detail, including the operating sequence, why there is a difference in discharge time between cold start and hot start?

Thanks,

Conor

Hi Conor,

R9 is not recommended, and also not mentioned in the datasheet. I am not sure about the effects it has on the general functionality of the SS. I recommend to remove this resistor and check how your hot and coldstart bahavior is then.

Normally SS is already discharged when you turn it on. The behavior you see is strange. There should be no difference in the delaytime, when the device was turned off before.

Best regards

Moritz

If you want to have a shorter softstart time, just use a very small capacitor.

Best regards

Moritz