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# LM76005: Output capacitance required differs between Datasheet calcs and T.I. WeBench tool.

Part Number: LM76005

Hi,

Value of output capacitor to use differs between datasheet calculations and WeBench tool. (ambient of 80C max.)

Vin = 11 to 52 V

Vout = 5.667 to 5.881 V (using Rfb of 100k + 21k 0.1%). [Require Vout(min.) = 5.5 V and Vout(max.) = 6.0 V]

Iout(max.) = 4.163 A

f(sw) = 500 kHz

Inductor = 8.2µH (XAL6060-822MEB)

Output capacitor value using formulas in datasheet yields Cap(min.) of 445 µF

Workings as follows:

r = (ΔiL / IOUT) = 0.833 A / 4.163 A = 0.200096

ΔVOUT = 100 mV max. (to keep VOUT within the limits of the downstream regulators)

D’ = 1 – duty cycle = 1 – 0.113 = 0.887 (for 52 V input) and 1 – 0.535 = 0.465 (for 11 V input)

fSW = 500 kHz

IOUT =4.163 A (max.)

D’ worst case is 0.887, so this will be used to work out the minimum capacitance required, as follows:

COUT              > (1 / (500 kHz x 0.200096 x 100 mV / 4.163 A)) x [((0.2000962 / 12) x (1 + 0.887)) + (0.887 x (1 + 0.200096))]

> (1 / 2403.267) x [((0.003336) x (1.887)) + (1.064485)]

> 445.55 µF minimum

TI WeBench output capacitor value recommends 4x 47 µF, 16V, 20%, X7R, 2220 (TDK CGA9N3X7R1C476M230KB), so after derating comes out to an effective capacitance of 109.64 µF minimum!?

What output capacitor choice should I use here as something seems to be wrong by a factor of 4.

Regards,

Richard.

• Hi Richard,

Sorry for the confusion.

To clarify, WB specifies the capacitance needed for stability (by default).

The d/s calculation you are performing is for determining capacitance needed for a load transient requirement.

This calculation is an approximation and it does seem to be yielding a much larger value than I would expect.

I recommend starting with the webench value.

It closely follows our datasheet component values specified for stability.

If you would like to see the transient performance of the device, you can experiment in WB with different capacitor values as below and/or evaluate on the EVM. Hopefully this is helpful for you. Have a good day, Marshall.