CSD95373AQ5M: Power loss (Ploss) verses output voltage data question.

Hi Chris,

Could you please be more specific about "voltage approaching the power supply rail" in "how the power loss of the part varies at voltages approaching the power supply rail"? It's not very clear to me what power supply rail are you referring to. Do you mean  "power loss vs .output voltage" when duty cycle gets close to 100%? Thanks!

Best,

Qingquan

Hi Qingquan, yes that's what I mean.  If I have logic high to the PWM input for several seconds for example and during that time the duty cycle is 100% will the losses therefore be high?  It seems odd given that switching losses are not occuring during this period.

Thanks!

Chris.

"Pulse width modulated tri-state input from external controller. Logic LOW sets Control FET gate low and Sync FET gate high. Logic HIGH sets Control FET gate high and Sync FET gate low. Open or High Z sets both MOSFET gates low if greater than the tri-State Shutdown Hold-off Time (t3HT)"

Hi Chris,

This is not a typical application for our power stage product, and that's why you don't see it in datasheet. In most of our applications, Vout is much lower than Vin (D<0.5).

Due to the fact that our power stages are optimized for applications where duty is small (less than 0.5), the low side FET is  designed to be larger than the high side FET, hence less conduction losses if more current flow through the low side FET.

Theoretically if you have 100% duty the power loss might be less as no switching losses in this case, but  you may have some technical issue to implement, as the high side FET driver receives power from bootstrap cap. The bootsrap cap will be drained with long logic high PWM pulse.

Then if you have large duty cycle but not 100% (such as 95%), I would expect the power loss to increase with Vout, as we still have switching losses plus more conduction losses from HS FET.

Please let me know if this answers your question.

Best,

Qingquan

Hi, yes I see.  What I want is a voltage follower which can supply higher currents (10A or more at ~12V) from a PWM source of variable duty cycle.  Can you suggest a TI product?

Thanks!

Chris.

Hi Chris,

Do you have more specific requirement? Such as Vin range, Vout range, Iout range, and do you really need 100% duty?

I will try to see if my group has product meets your requirement.

Best,

Qingquan 

Hi Qingquan

Because I am driving a TEC there are times when it will need to be driven near 100% duty cycle (heating and cooling).  Currently I use the DRV595 at 500 kHz with DAC input to control set temperature but it is not powerful enough. Vin is 12-14vdc. TEC resistance is about 1 ohm so system needs to provide at least 10A.

Alternatively what voltage does the bootstrap circuit of the CSD95373AQ5M produce and require. Could I generate the required potential using a separate boost converter to circumvent the 100% duty cycle boot strap voltage you pointed out?

Thanks Chris.

Hi Chris,

I just checked the DRV595 you are using, which also requires Bootstrap cap to supply gate driver for HS FET. I am wondering if you drive DRV595 with 100% duty or lower duty since there are four switching FETs (with phase shifting)? If you were able to use DRV595 with high duty (close to 100% but less than 100%), then you probably can also use other power management product with bootstrap caps.

CSD95373AQ5M use 5V typical voltage for boostrap circuit, which is internally connected to VDD pin (5V will be built up on Cboot between BOOT and BOOTR pins during switching operation).  You can find that in "Functional Block Diagram" in datasheet.

Best,

Qingquan

Chris Rawle said:

Hi Qingquan

Because I am driving a TEC there are times when it will need to be driven near 100% duty cycle (heating and cooling).  Currently I use the DRV595 at 500 kHz with DAC input to control set temperature but it is not powerful enough. Vin is 12-14vdc. TEC resistance is about 1 ohm so system needs to provide at least 10A.

Altenatively what voltage does the bootstrap circuit of the CSD95373AQ5M produce and require. Could I generate the required potential using a separate boost converter to circumvent the 100% duty cycle boot strap voltage you pointed out?

Thanks Chris.

Thanks Qingquan.  Theoretically I could also provide ~5V (VDD) + 12V (VIN) to the BOOT PIN via a boost circuit and that would allow for function at 100% duty cycle?  I thinking of including a small boost circuit on board as an optional source which can be connected to BOOT as the full board is expensive to make and additional BOOT voltage source as a fall back options might be a good idea?  Thats my last question :).

Thanks, Chris.

Hi Qingquan,

OK thank you. I can't think of an immediate solution to providing 5V to BOOT relative to BOOTR using an external circuit.  This would probably be necessary as I will likely need 100% duty cycle at times.  Perhaps I can use a P-MOS N-MOS configured half bridge in place of CSD95373AQ5M instead?

Hi Chris,

Actually you could find a lot of small isolated DCDC modules from some online distributors,  250mW should be enough to drive the boostrap circuit.

Or if you want to drive with P-N MOS half bridge, you will need to use gate voltage same as your rail voltage (VIN, 0V), and make sure your MOS gate can be drove with such voltage.

Another option is to use full bridge (two power stages) to drive, and control the phase between two half bridges, you could use 50% duty for each half bridge in this case.

Best,

Qingquan