TPS23523: About the starting sequence

Hi Pedro,

Thanks for reaching out. I will get back by this Friday.

BR,

Rakesh

Hi Pedro,

It looks like the inrush time period is expiring with large Cout. 

Can you change the Cout from 2mF to 100uF and test again.

This helps to root-cause the problem.

BR,

Rakesh

Hi Rakesh,

OK, I will checked reducing Cout as proposed next Monday and it will send the results.

I have checked to increase the Css (Capacitor soft start) from initial value 110nF to 210nF (added a paralllel 100nF) and the number of iterations in the starting process is reduced to 1 or 2 instead of 4 or 5. 

Thanks in advance.

Best Regards,

Pedro

Hi Rakesh,

I have tested the output with Cload= 3 x 220uF =660uF y the results are OK, the iterations have disappeared and only a ramp up voltage waveform appears.

Please, see attached figure below:

I need Cload about 2mF because the output of TPS23523 supplies the inputs of several DC-DC converters which require high capacitance for EMC filter.

How can you solve this problem?

Thanks in advance.

Best Regards,

Pedro

Hi Rakest,

Please, find below the total circuit.  Cout = 660uF are directly connected to the output of TPS23523 but other capacitors (1320uF) are connected using a switch (P channel transistor). When TPS23253 have been started and +48V is in its output and close the switch, several iterations occur again and when DC-DC are activated and have a consumption, the TPS23523 is constantly re-started. I think that there is a problem when the step of current (inrush current of capacitor or consumption from the starting of DC-DC converter.)

The next figure occurs when the switch (P channel mosfet ) is closed and the DC-DC converter starts to require current, then the TPS is restarted.

Thanks in advance.

Brst Regards,

Pedro

Hi Pedro,

Can you probe the input current and TMR voltage. we need to check whether the startup current is below Ilim2 value 1.5A or not. 

Please enable downstream DC-DC with PG output as shown below.

Best Regards,

Rakesh

Hi Rakesh,

The problem of continuous restart, when current from DC-DC is required, is solved delaying the enable signal for DC-DC converter.

A circuit similar to the indicated was already used, PG signal from TPS23523 was used to enable the DC_DC converters. But an adittional delay was necessary. I have added several capacitors in the enable circuits to delay the enable of DC-DC converters and then the problem is solved.

On the other hand , I have tested some points.

a) I have checked the SS pin to try to reduce Inrush current and to avoid to reach the current limit to trip(0.75A)

Figure 1 : Css=100nF Css,vee=330nF  --> Inrush current = 0.5A (Failute in starting process)

Figure 2: Css=300nF Css,VEE=990nF --> Inrush current = 0.5A (Failute in starting process)

b) We have checked the V TMR. it seems that reach the 1.5V and times out during 64 cycles.

If the problem is current limit (0.75A), PROG pin can be in float (Now is connected to VEE), the the current limit increases from 1.5A to 3A with 1mohms for sense resistor. High limit is increased from 20A to 25A.

Another proposal could be increase the VD when the two limit currents are switched, currenlt is 20V but this could be increased if required.

What do you think about this?

Thanks in advance.

Best Regards,

Pedro

Hi Pedro,

Increasing lower current limit threshold or reducing VD should help. For any change, please plug-in corresponding values in design calculator and verify SOA.

Best Regards,

Rakesh