24V, 15A Buck converter Design

Dear Expert,

I have made a circuit that meets my requirements using the LTC7801 specification page 34 and the LTPower II V2. Would you please review this circuit?

Sincerely,

Eui-heon

Looks like it is mostly ok from a design standpoint.  R5 needs to be 0Ohms and you show no input capacitance.   Heat is going to be your enemy.  You may end up having to double up your Mosfets or go to two phases for this kind of power.  

Dear Seth_S

Thank you for your reply. I agree with your advices. The FDMS86200 recommended by this tool did not seem to be a FET optimized for my needs, so I contacted you. If so,would you please reply IPB065N15N3G(www.infineon.com/.../Infineon-IPB065N15N3G-DS-v02_01-en.pdf is appropriate or not? And I have two more questions,
1. Is there any reason apply the LTC3639 to EXTVCC in the DC2641A (www.linear.com/.../DC2641A) that is a demo circuit of the LTC7801?
2. Is there any reason using two or more in parallel instead of a single type when selecting the FET of a converter with large current capacity?

Sincerely,
Eui-heon

Hi Eui-heon

1. Is there any reason apply the LTC3639 to EXTVCC in the DC2641A (www.linear.com/.../DC2641A) that is a demo circuit of the LTC7801? 

It is used to supply the DRVCC Regulators instead of the  internal LDO to supply it.

2. Is there any reason using two or more in parallel instead of a single type when selecting the FET of a converter with large current capacity?

as Seth said, it is a way to dissipate the heat generated by the switches. Another way is connecting two or more regulators in parallel with their switching shifted out of phase of each other.

To hasten your development time I would suggest using LTpowerCAD to verify your design. 

Regards,

Mikee 

Hi Mikee

1. Is there any reason apply the LTC3639 to EXTVCC in the DC2641A (www.linear.com/.../DC2641A) that is a demo circuit of the LTC7801? 

It is used to supply the DRVCC Regulators instead of the  internal LDO to supply it.

With the current configuration of the DC2641A, with  LTC3639 Vprg1 to SS and VPRG2 to ground so ETXVcc=5V, and DRVSET to 80K resistor (8V in DRVCC), and NDRV tied to DRVCC, wouldn't that still be supplying the 8 volts of the gate driver from the internal LDO?

If this is the case, what would be best, lowering DRVcc to 5V? or increasing ExtVcc to 8V?

Regads,

Frank

Hi Mikee

1. Is there any reason apply the LTC3639 to EXTVCC in the DC2641A (www.linear.com/.../DC2641A) that is a demo circuit of the LTC7801? 

It is used to supply the DRVCC Regulators instead of the  internal LDO to supply it.

With the current configuration of the DC2641A, with  LTC3639 Vprg1 to SS and VPRG2 to ground so ETXVcc=5V, and DRVSET to 80K resistor (8V in DRVCC), and NDRV tied to DRVCC, wouldn't that still be supplying the 8 volts of the gate driver from the internal LDO?

If this is the case, what would be best, lowering DRVcc to 5V? or increasing ExtVcc to 8V?

Regads,

Frank

Hi Frank

Yes, DRVcc is used as the gate drive source for the bottom FET but for the top a charge pump can be used instead. Please see block diagram on page 11 of LTC7801 datasheet

Please use table on page 23 of LTC7801 datasheet as reference to set the desired DRVcc

To increase ExtVcc, you would need to modify the feedback resistor/s in the case of DC2641A R32 and/or R34 .

Regards,

mikee