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CSD95379Q3M: SKIP pin connection and dead time control

Part Number: CSD95379Q3M
Other Parts Discussed in Thread: SN74LVC2G126, CSD

Hi, 

I'm planning to incorporate CSD95379Q3M in a half bridge push pull design (a half bridge on primary side, and output without inductor). This design has intrinsic ZVS behavior, which saves power.

However, to achieve ZVS, it must operate with considerable amount of dead time between high drive and low drive.

To implement this feature, I need some data from TI:

1. What's the switching node capacitance (Cds_hs+Cds_ls) at 5V Vbus?

2. How can I control the dead time?

So far, my plan was to use the tri-state to trigger a dead time using 2 PWM signals, one for tri-state driver OE, the other for real PWM signal. That should give a 3-level signal.

Another question is on the SKIP pin. I would like to have it floated when I don't need it, so I can save some 122uA of current at idle. However, is there a guideline on how to implement very low power sleep mode? I noticed the datasheet saying do not float SKIP during power up, so how I'm supposed to float it?

I can drive SKIP pin with a tri-state-able MCU pin, but at power up, MCU pins are all tri-stated, so SKIP will be floating. I can only drive SKIP high or low after MCU boot up, which takes time. How should I connect this pin?

To be clear, the behavior I want is that the chip does very low power when I enter sleep, and they chip goes to FCCM when I want it to run.

Thanks, 

B.G.

  • Hello,

    When PWM is tri-state, both high-side and low -side FETs are off. dead time is achieve.
    Regarding the SKIP pin, when SKIP is floating, SKIP pin is in tristate.

    Thanks
    Qian
  • Please read me question carefully. I was asking the output charge of the device, and what happens if I power on the device with SKIP pin floating providing I'm not going to drive PWM pin at that time. Thanks.
  • Please see the answers below:

     

    1. What's the switching node capacitance (Cds_hs+Cds_ls) at 5V Vbus?

    ~1nF

    2. How can I control the dead time?

    The dead time is automatically controlled by the driver IC inside CSD95379. The driver IC utilizes an anti-shoot-through and adaptive dead-time control to obtain proper dead times. The typical numbers are below: ~25ns (rising), ~15ns(falling).

    What is the dead time you target? If you want to use tri-state to add equivalent dead time, here lists two things that you need to consider:

    (1)    Is your tri-state driver driving tri-state(sending out a 1.7V signal) or having an high-Z output when tri-state is enabled? As specified on CSD95379 datasheet, the PWM has high PWM pull-up and pull-down impedance, it will take time for CSD95379 to change PWM to tri-state if the controller/driver outputs high-Z, so the controller/driver needs to drive 1.7V tri-state.

    (2)    CSD95379 has tri-state activation time tHOLD(typical 60ns), this is also specified on datasheet

     

    3. For SKIP# pin, we suggest to connect this pin high (FCCM) or low(DCM) at power on. CSD95379 will enter Ultra-Low Quiescent (ULQ) Current Mode(sleep mode) with SKIP# floating. To exit ULQ mode, you will need to (1) pull SKIP# low, (2) pull SKIP# low first for more than 35us(typical), then pull SKIP# high if FCCM is needed.

    Thanks!

  • Hi,

    The tri-state I'm using is an SN74LVC2G126, and it can drive true tri-state.
    I plan to use 2 PWM signals aligned perfectly, one operating at desired frequency with 50% duty cycle, the other operating at double the frequency at 100% duty cycle minus dead time. The first PWM controls input of 126, and the other PWM controls OE of the same 126. That should give me a good on-tri-off-tri-on waveform on the output of the 126.

    The PWM generator I use is an Analog Devices ADSP-BF706, which has a simultaneous timer start/stop feature.

    For SKIP, my particular question was whether I can get around with floating SKIP at power up while NOT driving PWM pin. I will make sure PWM is not driven until the SKIP pin has been asserted. Is that okay?






    Regards
  • Hi,

    We don't suggest to power on CSD95379 with SKIP# floating. Are you using different power supplies for the control circuit and CSD95379? if yes, proper power-on can be achieved by controlling the power-on sequence.

    Thanks!
  • The CSD will be the first part to power up. It sits on 5V rail which powers up first. However, I can try to drive SKIP with a tri-state buffer with OE high biased by a resistor. Thanks.