LTC1709_15 datasheet(12/28 Pages) LINER

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LTC1709

Kool M

µ is a registered trademark of Magnetics, Inc.

APPLICATIO S I FOR ATIO

A graph for the voltage applied to the PLLFLTR pin vs

frequency is given in Figure 2. As the operating frequency

is increased the gate charge losses will be higher, reducing

efficiency (see Efficiency Considerations). The maximum

switching frequency is approximately 310kHz.

In a 2-phase converter, the net ripple current seen by the

output capacitor is much smaller than the individual

inductor ripple currents due to ripple cancellation. The

details on how to calculate the net output ripple current

can be found in Application Note 77.

Figure 3 shows the net ripple current seen by the output

capacitors for the 1- and 2- phase configurations. The

output ripple current is plotted for a fixed output voltage as

the duty factor is varied between 10% and 90% on the

x-axis. The output ripple current is normalized against the

inductor ripple current at zero duty factor. The graph can

be used in place of tedious calculations, simplifying the

design process.

Accepting larger values of

∆IL allows the use of low

inductances, but can result in higher output voltage ripple.

A reasonable starting point for setting ripple current is

∆IL

= 0.4(IOUT)/2, where IOUT is the total load current. Remem-

ber, the maximum

∆IL occurs at the maximum input

voltage. The individual inductor ripple currents are deter-

mined by the inductor, input and output voltages.

Figure 2. Operating Frequency vs VPLLFLTR

Inductor Value Calculation and Output Ripple Current

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because

MOSFET gate charge and transition losses increase di-

rectly with frequency. In addition to this basic tradeoff, the

effect of inductor value on ripple current and low current

operation must also be considered. The PolyPhase ap-

proach reduces both input and output ripple currents

while optimizing individual output stages to run at a lower

fundamental frequency, enhancing efficiency.

The inductor value has a direct effect on ripple current. The

inductor ripple current

∆IL per individual section, N,

decreases with higher inductance or frequency and in-

creases with higher VIN or VOUT:

∆I

OUT

=−









where f is the individual output stage operating frequency.

Figure 3. Normalized Output Ripple Current vs

Duty Factor [IRMS ≈ 0.3 (∆IO(P–P))]

OPERATING FREQUENCY (kHz)

120

170

220

270

320

1709 F02

2.5

2.0

1.5

1.0

0.5

DUTY FACTOR (VOUT/VIN)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

1709 F03

2-PHASE

1-PHASE

Inductor Core Selection

Once the values for L1 and L2 are known, the type of

inductor must be selected. High efficiency converters

generally cannot afford the core loss found in low cost

powdered iron cores, forcing the use of more expensive

ferrite, molypermalloy, or Kool M

µ® cores. Actual core

loss is independent of core size for a fixed inductor value,

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