Datasheet

LM25119/25119Q

www.ti.com

SNVS680G –AUGUST 2010–REVISED JANUARY 2014

Figure 13. Cancellation Factor vs. Duty Cycle for Output Capacitor

To begin an interleaved design, use the previous equations in this datasheet to first calculate the required value

of components using one-half the current in the output power path. The Attenuation Factor in Figure 13 is the

ratio of the output capacitor ripple to the inductor ripple vs. duty cycle. The inductor ripple used in this calculation

is the ripple in either inductor in a two phase design, not the ripple calculated for a single phase design of the

same output power. It can be observed that operation around 50% duty cycle results in almost complete ripple

attenuation in the output capacitor. Figure 13 can be used to calculate the amount of ripple attenuation in the

output capacitors.

Figure 14. Normalized Input Capacitor RMS Ripple Current vs. Duty Cycle

Figure 14 illustrates the ripple current reduction in the input capacitors due to interleaving. As with the output

capacitors, there is near perfect ripple reduction near 50% duty cycle. This plot can be used to calculate the

ripple in the input capacitors at any duty cycle. In designs with large duty cycle swings, use the worst case ripple

reduction for the design.

To configure the LM25119 for interleaved operation, connect COMP1 and COMP2 pins together at the IC.

Connecting the FB2 pin to VCC2 pin will disable the channel2 error amplifier with a high output impedance at

COMP2. Connect the compensation network between FB1 and the common COMP pins. Connect the two power

stages together at the output capacitors. Finally use the plots in Figure 13 and Figure 14 along with the duty

cycle range to determine the amount of output and input capacitor ripple reduction. Frequently more capacitance

than necessary is used in a design just to meet ESR requirements. Reducing the capacitance based solely on

ripple reduction graphs alone may violate this requirement.

In the LM25119 evaluation board (schematic shown in Figure 15) interleaved operation can be enabled by

shorting both outputs together (with identical components in the power train), and using zero ohm resistors for

R22 and R21. This shorts VCC2 to FB2 and COMP2 to COMP1 respectively. Also the channel2 feedback

network C14, R6, and C15 should be removed. The easy re-configuration between two channel and single

channel operation will allow insight into the benefits of interleaved operation.

Product Folder Links: LM25119/25119Q