CSD88539ND: Dual MOSFET for Boost Converter Application

Hello Julien,

Thanks for your interest in TI FETs. I am assuming the boost converter is non-synchronous. Is that correct? The CSD88539ND requires VGS = 6V minimum gate drive voltage. This is where the device is tested and on resistance is guaranteed. We do not recommend operating with 5V gate drive as the performance will not be very good or consistent. As shown in the Rds(on) vs. VGS plot on page 1 of the datasheet, the on resistance is in the very steep part of the curve at VGS = 5V. A small variation in threshold voltage can cause an exponential increase in on resistance. Unless you can provide at least 6V gate drive, I would not recommend this device for your application. You might consider using 2 discrete devices such as the CSD18543Q3A which has Rds(on) rated down to VGS = 4.5V and will work with 5V gate drive.

I cannot tell you if there will be any disturbance between signals as we have not tested the device in this type of application. There may be some interaction since both die are in very close proximity within the same package on the same lead frame. The switching frequencies are very far apart but there is a possibility of cross talk and there may be a beat frequency in the noise characteristics of the boost converter and motor drive. We have developed FET selection tools for estimating power loss in the FETs for various topologies including non-synchronous boost converter. However, it does not include the dual device as it assumes a single FET in the boost converter. Please see the app note at the link below. It includes links to all of TI's web based MOSFET technical information including FET selection tools.

https://www.ti.com/lit/an/slvafg3b/slvafg3b.pdf

Please let me know what else I can do to help you.

Best Regards,

John Wallace

TI FET Applications

Hello John,

Many thanks for your detailed reply. It is a non-synchronous boost converter yes. Indeed VGS should be superior to 6V on this IC to ensure low on resistance but we control them from 5V PWM VGS1 and 3,7V(battery level voltage) PWM VGS2 signals. Perhaps there is an existing Dual MOSFET reference suitable for these low VGS voltages?

Our main interest to use a dual MOSFET, is to save space on our PCB layout because we are very limited. I will check your links to study the MOSFET selection and estimate power losses.

Best Regards,

Julien Galtié

Hi Julien,

Our selection of dual FETs is very limited and we don't really have a dual FET solution that would work. I used the non-synchronous boost converter FET selection tool to run some FET loss estimates. The rms current in the FET is around 14.5A - 14.8A (depending on the diode drop). For the CSD88539ND, assuming VGS = 6V, the conduction loss at 3Vin to 12Vo at 4A is about 7.4W which is way more than the SOIC package can safely dissipate (~2W max). This does not include switching losses or the loss in the other FET used to drive the motor. You're going to need to use a FET with much lower on resistance to keep the power loss reasonable. I'd recommend the CSD17577Q3A in 3.3x3.3mm SON for the boost converter. The total estimated FET loss is about 2W which is within the capability of the 3.3x3.3mm SON package. This FET might also work for the motor drive but I don't know enough about the application. Please let me know what else I can do to assist you.

Thanks,

John

Hi Julien,

Following up to see if your issue has been resolved. Please let me know.

Thanks,

John

Hello John,

I selected the CSD17308Q3 MOSFET for both and it works fine for PWM motor control but for boost converter I am still investigating on strange behavior on the boost converter overloading.

Best Regards,

Julien

Hi Julien,

Can you provide additional details of the strange behavior for the boost converter? Schematic and waveforms along with a description of the behavior would be helpful.

Thanks,

John