ISL78227 Datasheet, PDF (27/43 Pages) Intersil Corporation – 2-Phase Boost Controller with Integrated Drivers

ISL78227
I_L1
180°
t
I_L2
t
I_L1+I_L2
t
FIGURE 52. PHASE NODE AND INDUCTOR-CURRENT WAVEFORMS
FOR 2-PHASE CONVERTER
To understand the reduction of the ripple current amplitude in the
multiphase circuit, examine Equation 4 representing an individual
phase’s peak-to-peak inductor current.
In Equation 4, V_IN and V_OUT are the input and the output voltages
respectively, L is the single-phase inductor value and f_SW is the
switching frequency.
I_{PPCH =} ^_---V----^O_-L----^U_-f-S--^T_--W---^–_{----VV----OI--}^N_--U--^_--T--V----^I_--^N_--
(EQ. 4)
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 K_P-P can be found in Figure 53 under
specific duty cycle.
I_PPALL= K_P-P  I_PPCH
(EQ. 5)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.4 5 0.5 0.55 0.6 0.65 0 .7 0.75 0.8 0.85 0.9
DUTY CYCLE
FIGURE 53. K_P-P vs 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 V_RAMPx of each phase are compared to the same error
amplifier’s output at the COMP pin by the PWM comparators to
turn off LGx when V_RAMPx reaches COMP. Thus, the V_RAMPx
peaks are controlled to be the same for each phase. V_RAMPx is
the sum of instantaneous inductor current sense ramp and the
compensating slope. Since 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 to the case when multiple
ISL78227s are configured in parallel for multiphase boost
converter. Basically, the COMP pin of each ISL78227 are tied
together for each phase’s current sense ramp peak to be
compared with the same COMP voltage (V_RAMPx = COMP),
meaning the inductor peak current of all the phases are
controlled to be the same. The “4-Phase Operation” section
describes how to configure two ISL78227 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 4 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.
CLKOUT
COMP
MASTER IC
FB
ISL78227
SS
IMON
EN
FSYNC
COMP
FB
SLAVE IC
SS
ISL78227
IMON
EN
FIGURE 54. CONFIGURATIONS FOR DUAL IC 4-PHASE OPERATION
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FN8808.2
February 24, 2016