TPS62085: TPS 62085 switching frequency incorrect

Jim Tsai

Expert 4470 points

Part Number: TPS62085

Based on datasheet, I can find fsw curve on Figure 1.

fsw is variable on different output current due to power saving mode.

But now, I apply 0A/0.3A/0.6A even 1~2A to load, the fsw are all the same which is 1.5Mhz.

fsw should be 2.4Mhz@over 0.4A.

Could you please help to check the issue?

over 4 years ago

0 Excel Regidor over 4 years ago

TI__Mastermind 30356 points

Hi Jim,

TPS62085 does not have Forced PWM mode so switching will vary depending on the load current especially at light load conditions.

Anyway, please share the waveforms (Vout, Iout, Inductor current, voltage across SW pin) at different output current conditions and schematic diagram as well. What is the Vin and Vout?

Best regards,

Excel

0 Jim Tsai over 4 years ago in reply to Excel Regidor

TI__Expert 4470 points

Hi,

I can understand the fsw will vary depending on different load current.

But I measured fsw on different load current 0A/0.3A/0.6A/1A/2A and got all the same fsw is around 1.5Mhz.

According to equation in datasheet, fsw can be calculated and should not be the same.

Test condition:

Vin=3.3V Vout=1.0V Imax=0.6A L=1uF

Waveform as attached!!

0 Excel Regidor over 4 years ago in reply to Jim Tsai

TI__Mastermind 30356 points

Hi Jim,

Thanks for the quick response.

Please share schematic diagram and the device marking because it should be PD5Q.

Best regards,

Excel

0 Jim Tsai over 4 years ago in reply to Excel Regidor

TI__Expert 4470 points

Hi,

I haven't sample on hand, the complete PN is TPS62085RLTR.

Below is schematic:

0 Excel Regidor over 4 years ago in reply to Jim Tsai

TI__Mastermind 30356 points

Hi Jim,

The VOS pin connection is wrong and it should be connected directly to the output inductor. See figure below for reference.

Best regards,

Excel

0 Jim Tsai over 4 years ago in reply to Excel Regidor

TI__Expert 4470 points

Hi,

Is that the root cause? I'll rework and see if the problem fix, thanks.

0 Excel Regidor over 4 years ago in reply to Jim Tsai

TI__Mastermind 30356 points

Hi Jim,

Did the changing of VOS pin connection resolved the issue?

Best regards,

Excel

0 Excel Regidor over 4 years ago in reply to Excel Regidor

TI__Mastermind 30356 points

Hi Jim,

If the suggestion resolved the issue, then I will close the thread.

Best regards,

Excel

0 Jim Tsai over 4 years ago in reply to Excel Regidor

TI__Expert 4470 points

Hi,

fsw is 2.xMHZ after correct the schematic.

I have 2 question as below:

1. We measured almost the same fsw in different load condition.

0A/0.35A/0.7A/1A/2A/3A, the fsw is all 2.1~2.3Mhz

fsw should be matched with below picture?

2. Can I change below resister 10ohm to 0ohm?

Or I have to modify layout and connect VOS to CHOKE directly?

What's different for VOS connect to CHOKE and go through 10ohm? Should no be big voltage drop on the 10ohm resister.

0 Excel Regidor over 4 years ago in reply to Jim Tsai

TI__Mastermind 30356 points

Hi Jim,

Please see response below.

1. If schematic diagram and layout were correct, then the switching frequency will be similar to the graph below.

2. The VOS pin is used to replicate the inductor current hence should be connected directly to the inductor as short as possible. It (VOS pin) is required to have a separate connection with the FB resistor dividers so replacing R125 to 0R might not work.

Can you share the layout?

Best regards,

Excel