LM5176: average current limit

Hi Fred,

Thank you for reaching out.

You can determine the average current limit is trigger when the SS voltage decreases.

The delay depends on 1.RC filter delay 2.Time to discharge Css from 1.21V to 0.8V.

Best Regards,

Feng Ji

HI ,

how should we choose the right RC value we need , I think this is not mentioned in the datasheet, thanks

neither is the discharge time for soft start 

Hi Fred,

let me dig into that and provide a summary till tomorrow.

Best regards,

 Stefan

Hi Stefan,

ok, looking forward to your feedback

Hi Fred,

the average current limit works on the softstart cap and reduces the reference voltage level (as the softstart does)

So it basically follows the system for the Softstart with that formula

Assuming the system was running without limitation so softstart cap has charged up to the max of 1.21V (see datasheet)

t(avg limit) = Css * (SS_clamp_voltage - Vref ) / [ (Vrisns - Vsns ) * Gm  - Iss]

with:

• t(avg_limit): time till average current limit does start limiting
• Css : cap on SS pin
• SS_clamp_voltage: SS_clamp_voltage    = 1.21V (see datasheet)
• Vref : 0.8V (see datasheet)
• Iss : 5uA
• Vrisns : Voltage over Risns
• Vsns :45mV  (see datasheet)
• Gm : 1mS  (see datasheet)

Note: needs to be positive otherwise current limit is not active.

The RC filter on the ISNS+ and ISNS- are mainly to limit noise and switching spikes to get injected into the sensing circuit.

Best regards,

 Stefan

Hi Stefan,

is average current and peak current limit  independent?

if yes, what's average current limit aim for ?

I mean , normally we only have peak current limit