Texas Instruments TPS40090EVM-002 manual Output Ripple Cancellation and Capacitor Selection

SLUU195 − June 2004

IRMS_CIN(nom) − Normalized RMS Input − A

0.6

0.5

0.4

0.3

0.2

0.1

0

0

NPH = 1

NPH = 2

NPH = 3

NPH = 4

NPH = 6

10	20	30	40	50	60	70	80	90	100
			Duty Cycle − %

Figure 4. Input Ripple Current RMS Value Overload Current
The maximum input ripple RMS current can be estimated as shown in (4).
I ^ IOUT D IIN(nom) ￿4, Dmin￿ + 3.18 A	(4)

It is also important to consider a minimum capacitance value which limits the voltage ripple to a specified value if all the current is supplied by the onboard capacitor. For a typical ripple voltage of 150 mV the maximum ESR is calculated in (5) as:

D V		150 mV	+ 47 mW	(5)
ESR + D I	+	3.18 A

Two 68-µF, 20-V Oscon capacitors (20SVP68M) from Sanyo are placed on the input side of the board. The ESR is 40 mΩ for each capacitor.

4.4Output Ripple Cancellation and Capacitor Selection

Due to the interleaving of channels, the total output ripple current is smaller than the ripple current from a single phase. The ripple cancellation factor is expressed in equation (6).

NPH

￿i

N

D￿

P

*

PH

DIOUT

￿NPH, D￿ +

￿i + 1

￿

(6)

￿￿

*1

NPH

￿i

N

D￿

1

P

*

)

￿i + 1 ￿

PH

k ￿NPH, D￿ + if ￿NPH v 1, DIOUT(D), DIOUT ￿NPH, D￿￿

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

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