TPS54527: Soft start

Hello Kura,

The soft start function is adjustable. When the EN pin becomes high, 6-µA current begins charging the capacitor which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start up. The equation for the slow start time is shown below. VFB voltage is 0.765 V and SS pin source current is 6-µA, these are typical values.

Where tss is the desired start-up time for your application. 

For a soft-start an external capacitor must be connected to GND as shown below:

The value of the capacitor C6 will depend on the desired delay-time you would like to give to your device depending on the application.

Hope this helps.

Best,

Jose

Hello Kura,

Your understanding is correct. The soft-start sequence defines the time it takes the output to achieve its final target.

Please illustrate with the below snapshot.

The green circle shows the time it takes the output to go from low (let's say it's zero) to high (its final target value).

Hope this helps.

Best,

Jose

Hello Kura,

I read some number out from circuit design.

This delay time depends on SS cap. Simple equation could be used.

T= C*V/I

C = SS Cap,

V ~= 0.7V, it goes to turn on 1 npn from base to change charging current.

I ~= 18uA.

For example,

C = SS Cap = 8.2nF.

T = C *V/I = 6.8n * 0.7 / 18uA ~= 0.319 ms.

Let me know if it could answer customer’s question. Thanks.

Best,

Jose