Texas Instruments TPS40090EVM-002 manual Inductance Value, Input Capacitor Selection

SLUU195 − June 2004

4.2Inductance Value

The output inductor value for each phase can be calculated from the volt-second during off time, shown in equation (2).

	VOUT		VOUT		(2)
L +		￿1 *		￿
	f IRIPPLE		VIN(max)

where

•IRIPPLE is usually chosen to be between 10% and 40% of maximum phase current IPH(max).

With IRIPPLE = 20% of IPH(max), there is a ripple current of 5 A, and the inductance value is found to be 0.63 ∝H. Using SPM12550−R62M300 inductors from TDK, each had inductance of 0.6 ∝H

and resistance of 1.75-mΩ.

In multi-phase high current buck converter design, due to the ripple cancellation factor from interleaving, the inductor value could be smaller than that in a single phase operation. But from conduction loss point of view, the inductor value tends to be big to reduce the ripple current, thus losses.

4.3Input Capacitor Selection

The bulk input capacitor selection is based on the input voltage ripple requirements. Due to the interleaving of multi phase, the input RMS current is reduced. The input ripple current RMS value over load current is calculated in equation (3).

D IIN(nom) ￿NPH, D￿ +

(3)

￿

￿

2

k ￿NPH, D￿

k ￿NPH, D￿ ) 1

NPH

VOUT

(1 * D)

￿￿D *

￿

￿

* D￿￿) ￿

￿

￿

￿

NPH

NPH

12

D2

L f

￿IOUT￿

￿

3

￿

3￿

￿

2

k ￿NPH, D￿

2

k ￿NPH, D￿ ) 1

￿￿k ￿NPH, D￿ ) 1￿

￿D *

￿

) k ￿NPH, D￿

￿

* D￿￿

NPH

NPH

￿

￿

where

• k￿NPH, D￿ + floor ￿NPH

D￿

•floor(x) is the function to return the greatest integer less than NPH ⋅ D

•NPH is the number of active phases

Figure 4 shows the input ripple current RMS value over the load current versus duty cycle with different number of active phases.

8TPS40090 Multi-Phase Buck Converter and TPS2834 Drivers Steps-Down from 12-V to 1.5-V at 100 A