adding TPS61220 input voltage limiting resistor

Can you post your schematic?

Dear Jim,

Thanks for help!

Please find the schematic and layout below:

Tony

It was mentioned" The TPS61220 is not enabled until the Vdd reaches almost 9V."

What's the Vin voltage when TPS61220 is not enalbed before VDD reached 9V?Can you test it?

Vin is around 0.6-0.7V when VDD < 9V.

When VDD>9V Vin = Vdd

After the circuit is enabled,  Vout stays at 5V until VDD < 0.7V.

Please note that the load (Vout pin) is currently open circuit

So it looks like it's the problem of the Zener rather than the chip.

May I know the Zener's part number ?Do you have a datasheet?

The problem persists even when the ZENER is removed. So it seems that the zener isn't the cause of this problem.

The part num of the zener is : BZV55C5V1-TPMSCT-ND

It was mentioned" If I connect the power supply directly to the Vin of the TPS 61220 it turns on immediately"

It's not in line with "The problem persists even when the ZENER is removed".How should I understand it?

What's you load currnet when TPS61220 is not enabled?

Vout is currently open circuit. So load current is 0

Also input current is nearly 0 when not enabled.

In order to make the chip work.The Vin voltage should be always above 0.7V.

You need to verify your Zener is working correctly.Disconnect the chip to the Zener, the voltage on the Zener should follow VDD when VDD is less than 5.1V and stay at 5.1V when VDD goes high.

 You may connect your indurctor directly to VDD and see the results.It might be help.

I removed the Zener already. The same problem still remain. But when I connect VDD directly to Vin i.e. shorting the 200 ohm resistor, the chip function properly. So I wonder why the problem arises only by adding just a 200 ohms resistor at the Vin.

You can see the startup waveform of this device on Figure19,and 20 on Datasheet.

The inductor current can goes up to 300mA in the startup. 300mA goes across 200ohm resistor is a very large Voltage drop.....So the device can not work correctly. It might be helpful if you connect you inductor directly to VDD...

200Ohm is big for normal operation. You can not have a high output current in this configuration (less than 100uA at 0.8V input), but the startup problem might be solved with a significantly bigger input capacitor (C3).

Thank you all of you for your reply!

Do you think it is the issue mentioned in the PDF below that causes the problem?

http://www.ti.com/lit/an/slva387/slva387.pdf

It seems that TPS61020 has precharge feature that might give some light to the solution. What do you think?

Is TPS61020 the only series of DC DC which has precharge feature. Does TPS61202 also have the same feature?

Hi Jim

Thank you for your message.

I tried to paraller 2 more 33uF caps with C3 so that it adds up to 76uF. In order to precharge the cap, the enable pin need to be short to GND until the cap is precharged, then set the enable pin to high afterwards, then it worked. May be a circuit which delays the enable pin from going high for a while would make it work.

Since the VDD ranges from 0.8-10V, I worried connecting the inductor to VDD would kill the TPS61220 when VDD is high.

Thank you and best regards,

Tony