ISL6566 datasheet(25/30 Pages) RENESAS

25 / 30 page

ISL6566

FN9178 Rev 4.00

Page 25 of 30

Mar 9, 2006

Since the capacitors are supplying a decreasing portion of the

load current while the regulator recovers from the transient, the

capacitor voltage becomes slightly depleted. The output

inductors must be capable of assuming the entire load current

before the output voltage decreases more than

VMAX. This

places an upper limit on inductance.

Equation 32 gives the upper limit on L for the cases when the

trailing edge of the current transient causes a greater output-

voltage deviation than the leading edge. Equation 33

addresses the leading edge. Normally, the trailing edge dictates

the selection of L because duty cycles are usually less than

50%. Nevertheless, both inequalities should be evaluated, and

L should be selected based on the lower of the two results. In

each equation, L is the per-channel inductance, C is the total

output capacitance, and N is the number of active channels.

Switching Frequency

There are a number of variables to consider when choosing the

switching frequency, as there are considerable effects on the

upper MOSFET loss calculation. These effects are outlined in

MOSFETs

, and they establish the upper limit for the switching

frequency. The lower limit is established by the requirement for

fast transient response and small output-voltage ripple as

outlined in Output Filter Design. Choose the lowest switching

frequency that allows the regulator to meet the transient-

response requirements.

Switching frequency is determined by the selection of the

frequency-setting resistor, RT. Figure 21 and Equation 34 are

provided to assist in selecting the correct value for RT.

Input Capacitor Selection

The input capacitors are responsible for sourcing the ac

component of the input current flowing into the upper

MOSFETs. Their RMS current capacity must be sufficient to

handle the ac component of the current drawn by the upper

MOSFETs which is related to duty cycle and the number of

active phases.

For a three-phase design, use Figure 22 to determine the input-

capacitor RMS current requirement set by the duty cycle,

maximum sustained output current (IO), and the ratio of the

peak-to-peak inductor current (IL,PP) to IO. Select a bulk

capacitor with a ripple current rating which will minimize the

total number of input capacitors required to support the RMS

current calculated. The voltage rating of the capacitors should

also be at least 1.25 times greater than the maximum input

voltage. Figures 23 and 24 provide the same input RMS current

information for two-phase and single-phase designs

respectively. Use the same approach for selecting the bulk

capacitor type and number.

2NCVO



I

2

---------------------------------

V

MAX

I ESR





–



(EQ. 32)

1.25

 NC



I

2

----------------------------------

V

MAX

I ESR





–

VIN VO

–







(EQ. 33)

10.61 1.035



log

–



(EQ. 34)

100

1000

100

1000

10000

FIGURE 21. RT vs SWITCHING FREQUENCY

SWITCHING FREQUENCY (kHz)

FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS

CURRENT FOR 3-PHASE CONVERTER

DUTY CYCLE (VIN/VO)

00.4

1.0

0.2

0.6

0.8

0.3

0.1

0.2

IL,PP = 0

IL,PP = 0.25 IO

IL,PP = 0.5 IO

IL,PP = 0.75 IO

FIGURE 23. NORMALIZED INPUT-CAPACITOR RMS

CURRENT FOR 2-PHASE CONVERTER

0.3

0.1

0.2

00.4

1.0

0.2

0.6

0.8

DUTY CYCLE (VIN/VO)

IL,PP = 0

IL,PP = 0.5 IO

IL,PP = 0.75 IO

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