TPS63060: Extend soft start time

Hi,

I wouldn't recommend using the soft-start circuit shown in SLVA553 because the diode affects the device operation after the startup is finished. Adding R3, the diode remains in conductive state, and may cause instability during load transients when the output voltage changes. Instead, please use the circuit shown here:

http://www.ti.com/lit/slva307

After the startup is finished, the diode is reverse polarized and does not affect the circuit significantly, only via its small intrinsic junction capacitance.

Best regards,
Milos

Hi, Milos

I have 2 questions.

1.  The circuit of SLVA307 is better than that of SLVA533 in all ?

If no, can you tell me the merit and demerit of those circuits?

2. If I use the circuit of SLVA307, 

The Cathode voltage is about 0.5V (equal to VFB of TPS63060)

The Anode voltage is about 0V (C6 charges up and circuit open)

So, I understood the diode is reverse polarized and does not affect the circuit significantly.

However, I think the additional circuit affect the control loop If the output voltage changes are big enough to flow  the current through C6 and D1.

My understanding is correct ?

If so, can you tell me the criterion to choose components (D1, C6 and R3 ) to decrease the risk ?

Best regards,

Hi,

Both circuit behave the same during the startup, but in the SLVA533 circuit the diode remains directly polarized if R3 is used. This means you have to take Vd into account when calculating the output voltage. Vd is not very precise, so your output voltage can also vary, especially during transients. Moreover, the circuit will also provide a feedforward path through C5 and D1, which can cause instabilities. If you omit R3, than you have a problem if you need to quickly turn the device off than on, because C5 cannot be discharged quickly. In the SLVA307, the diode is reverse polarized, and only a small junction capacitance remains (for example around 1 pF for 1N4148) which act as a small feedforward capacitor.

Also, your understanding is correct, if there is a large change in the output voltage, both circuits will affect the control, but:

1. In the SLVA533 with R3, the diode always affects the output voltage.
2. In the SLVA533 without R3, if the output voltage has a positive change larger than Vd, the diode will turn on and affect the output voltage and control.
3. In the SLVA307 the output needs to rise by Vfb + Vd, which is higher than in the case 2. Therefore there is a smaller chance to turn on the diode.

There is not much you can do to change the soft start circuit. Again, I recommend SLVA307 circuit because you have less risk of affecting the operation when there is a fast large positive change. Also I recommend using a silicon diode and not a schottky diode. Schottky diodes have somewhat larger junction capacitance and much larger reverse leakage current, so they can affect the control and the output voltage. 

Best regards,
Milos

Hi, Milos

In the SLVA307, how the output voltage is affected by R3 ?

If the reverse leakage current of the diode is small, the effect of R3 is also small ?

Best Regards,