FN8808 datasheet(27/44 Pages) RENESAS

27 / 44 page

ISL78227

FN8808 Rev.6.02

Page 27 of 43

Feb 28, 2025

To understand the reduction of the ripple current amplitude in the

multiphase circuit, refer to Equation 4, which represents an

individual phase’s peak-to-peak inductor current.

In Equation 4, VIN and VOUT are the input and output voltages

respectively, L is the single-phase inductor value, and fSW is the

switching frequency.

The input capacitors conduct the ripple component of the

inductor current. In the case of a 2-phase boost converter, the

capacitor current is the sum of the ripple currents from each of

the individual phases. Use Equation 5 to calculate the

peak-to-peak ripple of the total input current which goes through

the input capacitors, where KP-P can be found in Figure 53 under

the specific duty cycle.

CURRENT SHARING BETWEEN PHASES

The peak current mode control inherently has current sharing

capability. As shown in Figure 3 on page 7, the current sense

ramp, VRAMPx, of each phase is compared to the same error

amplifier’s output at the COMP pin by the PWM comparators to

turn off LGx when VRAMPx reaches COMP. Thus, the VRAMPx

peaks are controlled to be the same for each phase. VRAMPx is

the sum of instantaneous inductor current sense ramp and the

compensating slope. Because the compensating slopes are the

same for both phases, the inductor peak current of each phase is

controlled to be the same.

The same mechanism applies if multiple ISL78227s are

configured in parallel for a multiphase boost converter. The

COMP pin of each ISL78227 is tied together for each phase’s

current sense ramp peak to be compared with the same COMP

voltage (VRAMPx = COMP), thus, the inductor peak currents of all

the phases are controlled to be the same. The “4-Phase

Operation” section describes how to configure two ISL78227s in

parallel for a 4-phase, interleaved boost converter.

4-PHASE OPERATION

Two ISL78227s can be used in parallel to achieve interleaved

4-phase operation. Figure 54 shows the recommended

configuration. The CLKOUT from the master IC drives FSYNC of

the slave IC to synchronize the switching frequencies. This

achieves a 90° phase shift for the four phases switching and the

respective COMP, FB, SS, EN, and IMON pins of the two ICs are

connected.

CLKOUT is 90°out-of-phase with the rising edge of LG1.

Therefore, the two phases of the second IC are interleaved with

the two phases of the first IC.

Oscillator and Synchronization

The switching frequency is determined by the selection of the

frequency-setting resistor, RFSYNC, connected from the FSYNC

pin to GND. Equation 6 is provided to assist in selecting the

correct resistor value, where fSW is the switching frequency of

each phase.

FIGURE 52. PHASE NODE AND INDUCTOR-CURRENT WAVEFORMS

FOR 2-PHASE CONVERTER

IL1

180°

IL2

IL1+ IL2

IPPCH

VOUT VIN

–





VIN

L fSW VOUT

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

(EQ. 4)

IPPALL KP-P IPPCH



(EQ. 5)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

DUTY CYCLE

FIGURE 53. KP-P vs DUTY CYCLE

FIGURE 54. CONFIGURATIONS FOR DUAL IC, 4-PHASE OPERATION

MASTER IC

ISL78227

SLAVE IC

ISL78227

CLKOUT

FSYNC

COMP

IMON

(EQ. 6)

RFSYNC 2.49x 10



10 0.505

fSW

--------------- 5.5X10 8

–









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