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CSD95378BQ5M: CSD95378BQ5M maximum output voltage

Part Number: CSD95378BQ5M

Hi support team,

Please answer a couple of questions regarding CSD95378BQ5M and other CSD9xxxx ICs:

1. Why is the output voltage restricted to 5.5 V? What happens if it is powered from a 12 V supply, and the duty cycle is set to 80% at 400 kHz to 1 MHz? I need output currents up to 2 A.

2. Can two such power stages be used to deferentially drive a TEC?

Thanks in advance and best regards,

Michael

  • Hi Michael,

    About your first question, the power stages with recommended Vout_max=5.5V are not optimized for applications with high Vout voltage, the current sense accuracy may not be guaranteed in that case.  For the power stages with Vos pin to sense Vout voltage, the max voltage on VOS pin is 7V, so there is risk to damage the Vos pin if Vout goes too high.

    Regarding to your second question, I am not sure what is TEC, and could you be more specific about how you want to drive differentially (with a simple schematic)?

  • Hi Qingquan,

    thank you for your reply.

    1. This specific power stage does not have a Vos pin:

    https://www.ti.com/lit/ds/symlink/csd95378bq5m.pdf?ts=1616614765791&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FCSD95378BQ5M

    Therefore, I'd like to repeat my question: What happens if this power stage is powered from a 12 V supply, and the duty cycle is set to 80% at 400 kHz to 1 MHz, and the output current is up to 2 A. 

    Please note that I would like to get an answer based upon some physical reason (the efficiency will fall to 50 %, the IC will get overheated and can burn, and so on). Words like "recommended" and "not optimized" do not explain anything.

    2. TEC is thermoelectric cooler, differential driving means connecting it between two power stages with different voltages, so that both the magnitude and the polarity of the applied voltage can be controlled. My question was if a TEC can be driven using two such power stages.

    The schematic will be much like that found here on page 22:

    www.ti.com/.../drv8412.pdf

    Best regards,

    Michael

  • And, by the way, even if the Vos voltage is limited to 7 V, while the output voltage is 10 V, the Vos pin can still be supplied using a voltage divider. Is there anything wrong with this?

  • Hi Michael,

    I don't see any issue to use power stage in the schematic you pointed to. However, it's not recommended to use CSD95378 or similar power stage for the HIGH VOUT applications. Potential issues like higher than expected power loss and the current sense function/performance cannot be guaranteed and it may not work properly.  There's always potential risk if you use beyond datasheet range especially in high volume production as the condition may not covered by the Power stage's design or test. If you want to try with one or two device under the condition, you may not see damage or  malfunction, but it could be different case for mass production and use them in the field.

  • The VOS sensed voltage is also used for current sense circuity, which requires high accuracy and fast dynamic response for output voltage snesing. We usually suggest to directly tie to the inductor output pad inner edge with kelvin connection, to prevent effect from parasitics. With said been said, we cannot use voltage divider for VOS pin.

  • Hi Qingquan,

    thank you very much for the explanation. However, I still don't understand how the datasheet restrictions are violated if at higher output voltages. After all, the switching node always switches between 0 and power supply voltage, which can be up to 16 V. Thus I don't see how the device can be damaged if the dissipated power and junction temperature do not exceed their maximum ratings.

    Note that the datasheet says the maximum duty cycle is 85 % at 1 MHz, and this is not conditioned by any specific power supply voltage. Thus, the maximal possible voltage @12V must be 10.2 V. And, as I noticed before, this IC does not have a Vos pin, and the power stage output (after the inductor) is not connected to the IC in any way.

    Therefore, the IC can in principle only have the information about the average voltage at the switching point, but it is not clear from the datasheet if this average voltage is really being tracked by the IC, and if it is subject to any limitation other than Vin*0.85.

    Best regards,

    Michael

  • Hi Qingquan,

    this makes sense, of course. Thank you very much!

    Best regards,

    Michael

    P.S. This is about your answer regarding Vos connection.

  • HI Michael,

    The high Vout condition you mentioned may not cause physical damage to power stage, but it may affect the functionality/performance. As I mentioned before, what I can think of the influence would be the higher power loss (power loss increases with Vout and roughtly 50%~60% increase from Vout=1.8V to Vout=5.5V, it could go higher if you keep increasing Vout) and current sensing circuitry inside the power stage. Although we don't have Vos pin in CSD95378, the VOUT information will be used by the current sense circuitry indirectly through internal emulation. And it may cause issue when the current sense signal is used for close-loop control or protection purpose.

    The max duty at 1MHz is to make sure we have sufficient time to recharge the boostrap capacitor, as that might be possible during some extremely heavy transients, but it should not violate the recommended max Vout spec.

  • Hi Qingquan,

    thank you very much for your explanations!

    Best regards,

    Michael