ISL6563CRZ datasheet(18/20 Pages) RENESAS

18 / 20 page

ISL6563

FN9126 Rev 8.00

Page 18 of 20

Jun 10, 2010

The output inductor of each power channel controls the ripple

current. The control IC is stable for channel ripple current (peak-

to-peak) up to twice the average current. A single channel’s ripple

current is approximated using Equation 25:

The current from multiple channels tend to cancel each other

and reduce the total ripple current. The total output ripple

current can be determined using the curve in Figure 10; it

provides the total ripple current as a function of duty cycle and

number of active channels, normalized to the parameter

KNORM at zero duty cycle.

where L is the channel inductor value.

Find the intersection of the active channel curve and duty cycle

for your particular application. The resulting ripple current

multiplier from the y-axis is then multiplied by the normalization

factor, KNORM, to determine the total output ripple current for

the given application.

Input Capacitor Selection

The important parameters for the bulk input capacitors are

the voltage rating and the RMS current rating. For reliable

operation, select bulk input capacitors with voltage and

current ratings above the maximum input voltage and largest

RMS current required by the circuit. The capacitor voltage

rating should be at least 1.25 times greater than the

maximum input voltage. The input RMS current required for a

multiphase converter can be approximated with the aid of

Figure 11.

As 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. Figure

11 can be used to determine the input capacitor RMS current

function of duty cycle, maximum sustained output current (IO),

and the ratio of the peak-to-peak inductor current (IL,PP) to the

maximum sustained load current, IO. Figure 11 can also be used

as a reference demonstrating the dramatic reduction in input

capacitor RMS current in a 2-phase DC/DC converter, as

compared to a single-phase regulator.

Use a mix of input bypass capacitors to control the input

voltage ripple. Use ceramic capacitance for the high

frequency decoupling and bulk capacitors to supply the RMS

current. Minimize the connection path inductance of the high

frequency decoupling ceramic capacitors (from drain of upper

MOSFET to source of lower MOSFET).

For bulk capacitance, several electrolytic or high-capacity MLC

capacitors may be needed. For surface mount designs, solid

tantalum capacitors can be used, but caution must be exercised

with regard to the capacitor surge current rating. These capacitors

must be capable of handling the surge-current at power-up.

ILPP



VIN VOUT

–

FSW L



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

VOUT

VIN

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



(EQ. 25)

KNORM

VOUT

LFSW



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

(EQ. 26)

ITOTAL



KNORM KCM



(EQ. 27)

1.0

0.8

0.6

0.4

0.2

0.1

0.2

0.3

0.4

0.5

DUTY CYCLE (VO/VIN)

FIGURE 10. RIPPLE CURRENT vs DUTY CYCLE

0.3

0.1

0.2

FIGURE 11. NORMALIZED INPUT RMS CURRENT vs DUTY

CYCLE FOR A 2-PHASE CONVERTER

00.2

0.5

0.1

0.3

0.4

DUTY CYCLE (VO /VIN)

IL,PP = 0

IL,PP = 0.5 x IO

IL,PP = 0.75 x IO

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