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# TPS40210: Excel tool vs equation in datasheet

Part Number: TPS40210

Hi Sir,

My cusotmer use TPS40210 at their PoE boost application.They use excel tool to calculate the setting of components. But there're some mismatch between excel tool and equations at datasheet. For example:

1. Inductor selection: Their Vin max is 5.25V, max output current is 0.2A adn Vout is 51V. So if using equations 32 to 36 at datasheet page 27, the Induct(min) is 24.6uH, but if using excel tool, they'll get 22uH.

2. Rsense selection: If using equation 39 and 40 on datasheet, R(ISNS) might be 49.34m ohm and it'll cause system OCP. But if using the calcuation value on excel tool, it will be 12m ohm and system is stable.

What customer have to take care if they use equation value to build up their sysem in stad of using excel tool? And why there are different result between datasheet's equations and excel tool? Please help to clear it for my customer. Thanks.

Regards,

Gary Teng

TPS4021x_Boost_Quickstart_Calculator -5V.xlsx

• Hi Gary,

Thanks for using the e2e forum.
When it comes to inductor selection, there can be difference depending on CCM and DCM calculation and where the entrance point to DCM operation should be put at light load operation.
The Rsense calculation should be unique, so I have a closer look here.
I will look into the datasheet equations and excel calculation and compare the formula.

I will get back to you with my feedback within by tomorrow.

Best regards,
Niklas

• Hi Gary,

I checked the formula within the calculator.
Inductor calculation:
The excel sheet calculates the inductance for min, typ, and max Vin and then takes the average of these three values for the final inductor value.
The equation used is:
(VIN*D)/(IL_avg*ILrip*Fsw)
D: duty cycle at Vin
IL_avg: inductor current average at VIN
ILrip: inductor ripple ratio (set in Line 32)

The difference in results should come from the averaging over Vin min, typ and max of the calculator.

The calculation of the Rsense resistor is dependent on CCM or DCM operation.
For CCM, the excel uses following equation:
VISNS_OC/(IGateDrive+(IOUT/(1-Dc_VIN_min))+(Dc_VIN_min*VIN_min/(2*Fsw*Lm)))
Dc_VIN_min: duty cycle at minimum Vin

The calculator equations should be valid for the Rsense of a boost design.

Best regards,
Niklas