TPS3840-Q1: Question about input current

Chase Jeong

Mastermind 7750 points

Part Number: TPS3840-Q1
Other Parts Discussed in Thread: TPS3840

Dear TI experts,

My customer now test TPS3840DL30DBVRQ1 on their own PCB.

Could you check the schematic below first?

While testing they found that the waveform below.

(Green:BATT_IN, Red:VDD(P2), Yellow:\RESET(P1))

As you see IC cannot detect VBATT_IN rising signal properly.

And my customer have 2 solutions to solve this problem.

1.Change C2876 from 10nF to 100nF

2.Change R7325,R7326 to lower value maintaing same resistor ratio

Anyway my question is, these 2 solutions are valid for fixing this problem.

And one more thing. Could you check the minimum operating current for startup TPS3840DL30DBVRQ1?

I am asking it because I found that supply current into VDD pin is maximum 700nA, but they found that they had to apply minimum 1mA current by adjusting resistor divider(R7325,R7326).

Please check these issues. Thanks.

Best regards,

Chase

9 days ago

0 Chase Jeong 6 days ago

Mastermind 7750 points

Dear TI experts,

Please let me know if there is an update about the issue above.

Best regards,

Chase

0 Jesse Villanueva 6 days ago in reply to Chase Jeong

TI__Mastermind 19670 points

Hi Chase,

Increasing the capacitor to 100nF should be the better solution. The tps3840 utilizes a capacitor ladder compared to a traditional resistor ladder. With elevated input impedance the cap ladder oscillation is observable. Increasing the bypass capacitor will help filter the oscillation.

Jesse

0 Chase Jeong 5 days ago in reply to Jesse Villanueva

Mastermind 7750 points

Dear Jesse,

Thank you for your support.

I found the E2E link below. Another expert said that R1 should not be larger than 20Kohm.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/994528/faq-tps3840-resistor-divider-for-sensing-wide-input-voltages?keyMatch=TPS3840

Can you give me an exact suggestion about resistor divider? My customer tested several scenarios about input resistor divider, and they found that it should be more than about 800uA (approximately 1mA) for input current to VDD when startup.

And the value of supply current into VDD pin is maximum 700nA, the first page of datasheet, I think it means the supply current of steady state after startup.

Could you give me your opinion about it?

Best regards,

Chase

0 Masoud Beheshti61 5 days ago in reply to Chase Jeong

TI__Mastermind 18351 points

Hello Chase - our team will respond back to you in the next 2 days.

0 Henry Naguski 2 days ago in reply to Chase Jeong

TI__Intellectual 2410 points

Hi Chase,

As the FAQ you linked states, there will be a short inrush current before the supply current settles down to what is given in the datasheet. Therefore, reducing the resistor values along with increasing the capacitor will both be effective in mitigating the issue

Section 9.2.3 of the datasheet has recommendations on selecting resistor and capacitor values:

-Henry

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TPS3840-Q1: Question about input current