TPS65265: Schematic review

Hi Muralidharan

Would you please send me the schematic in PDF format? The picture above is not that clear to view. Thanks.

BR 

Ruby

Hi Ruby,

Please find the attached PDF document.

Regards,

Nandini Muralidharan.

TPS65265_3BUCK_CKT.pdf

Hi Muralidharan

Here are my comments:

1. What is the load current of each channel? 

2. What does Vout2: VDDFD(1.8V/1.2V/3A) mean? Does it mean channel 2 is designed to output 1.8V or 1.2V? If the load current is 3A, then I would recommend to use an inductor with higher saturated current which may be 5A.

3. I see that L9's saturated current is only 1.9A, right? I would recommend to use the inductor with higher saturated current.

Thanks.

BR

Ruby

Hi Ruby,

1. Load currents are these: Vout1 - 0.9V/4A, Vout2 - 1.8V/3A, Vout3 - 1.8V/1A

2. Yes, Vout2 will be 1.8V as per default BOM. In some special BOM, we might adjust the voltage divider to get 1.2V also. o/p current will be 3A for this rail. Can TYA40202R2M-10 inductor be used for this rail? 

3. Can this inductor be used for L9? 74438336068

These inductors have bigger footprint. Can you advise if we can use a different frequency so that smaller footprint inductors can be used? 

Are any other changes required in our design?

Thanks!

Hi Ruby,

We can't use the inductor 74438357022 since it is above 2.8mm which is our height limit. Also, it looks bigger on length and width.

How to choose Isat value for these inductors? Any formula/guidelines is there?

Hi Muralidharan

Isat has to be taken into consideration to avoid magnetic saturation of the inductor. So you should calculate the maximum value of the inductor current, and consider at least 0.2 margin which means that Isat >1.2 ILmax. I think 0.5 margin is much more better considering over current situation. 

BR

Ruby

S0001769A_3buck.pdf

1. Since we want to keep the inductor footprint smaller, I am using 1MHz FSW. The attached schematics has been modified for the same. Please help to review.
2. Are the FB loop and LX loop discretes fine?
3. Is there a scope for reducing the Input or Output capacitors any further?

There is no change in Load current/Output voltage.

Thanks!

Hi Muralidharan

Here are my comments:

1. For CH2, I would recommend to use 22uF * 2 + 0.1uF =44.1uF output capacitor.

2. For CH3, I would recommend to use 22uF * 2 + 0.1uF =44.1uF output capacitor, and R121= 10kohm, C398=10pF, C399=100pF.

3. If the input pins are not close to each other, when choosing the input capacitor, please make sure there is a 22uF capacitor and a 0.1uF capacitor connected close to the input pin. So there should be at least 4*22uF and 4*0.1uF capacitors for 4 input pins. But in this case, pin 13 and pin 12 are input pins which are next to each other, so please connect total 2*22uF +1* 0.1uF capacitors close to these two pins, pin 30 and pin 31 are input pins close to each other, so again connect 2*22uF +1* 0.1uF capacitors close to these two pins.

4. When choosing the output capacitors, I would recommend to use 22uF * 2 + 0.1uF =44.1uF for each channel.

BR

Ruby

Hi Muralidharan

Would you please tell me what is the application and if it is possible, would you please tell me the power rail (where does the input come from and where does the output do to)? Why do you need such small size?

Many thanks!

BR

Ruby

Hi Ruby,

I made the capacitor changes as you mentioned. Hope the compensation and feedback loop values are correct in the circuit for 1MHz.

The application is 2.5" drive. Input 5V comes from the edgefinger and output are 0.9V (controller core power), 1.8V (Nand Flash Power), 1.8V (GPIO of controller)

There are many NAND flashes and other ICs used in the design. So, board real estate is very less. So, we need to use small size inductors.

Thanks!