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TPS22918-Q1: Technical questions on the IC application

Part Number: TPS22918-Q1
Other Parts Discussed in Thread: ADS7138, TPS22918

The switch device of TPS22918 is used for switching of AVDD power for ADS7138 ADC IC, in the signal measurement block circuit of a control PCBA project, simplified circuit diagram of which is shown below.

The VOUT pin of TPS22918, which is connected to QOD pin and AVDD pin of ADS7138 provides switching for AVDD power for four devices of ADS7138.

One question raised by one of design reviewers.

Is worst case max QOD pin current during RESET state (while QOD is pulled to GND) compatible with the limit for stable operation?

I have calculated worst case max steady state current of QOD pin to find it is lower than 35mA.

Is it OK?

What is limit of QOD current for Ta = -40C to 85C?

AVDD trace has 4uF capacitance (4 IC x 1uF / IC).

 

  • Hi Hyeong, 

    Pleasure to meet you and thanks for posting your first question on E2E! 

    When shorting the QOD pin to the VOUT pin, special care is needed to be taken to ensure excessive current does not flow through Rpd during discharge such that the maximum Tj of 150C is exceeded. To ensure this temperature is not exceeded, when only using the internal Rpd to discharge a load, the total capacitance must not exceed 200uF. Since your output capacitance is significantly less, you shouldn't have an issue. To be certain, the max power dissipation can be calculated as: 

    Pd,max = (Tj,max - Ta,max) / Θja

    Pd,max = (150C - 85C) / 183.2 C/W

    Pd,max = 355mW

    As long as the power dissipated by the switch does not exceed 355mW, there shouldn't be an issue. 

    Best Regards, 

    Elizabeth 

  • Thank you very much.