LTC3860_15 Datasheet, PDF (20/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output ca-
pacitors. The RMS input ripple current is divided by, and
the effective ripple frequency is multiplied by, the number
of phases used (assuming that the input voltage is greater
than the number of phases used times the output volt-
age). The output ripple amplitude is also reduced by the
number of phases used. Figure 8 graphically illustrates
the principle.
SW1 V
I_CIN
SINGLE PHASE
I_COUT
SW1 V
SW2 V
I_L1
I_L2
DUAL PHASE
I_CIN
I_COUT
3860 F08
RIPPLE
Figure 8. Single and 2-Phase Current Waveforms
The worst-case RMS ripple current for a single stage de-
sign peaks at an input voltage of twice the output voltage.
The worst case RMS ripple current for a two stage design
results in peak outputs of 1/4 and 3/4 of input voltage.
When the RMS current is calculated, higher effective duty
factor results and the peak current levels are divided as
long as the current in each stage is balanced. Refer to
Application Note 19 for a detailed description of how
to calculate RMS current for the single stage switching
regulator. Figures 9 and 10 illustrate how the input and
output currents are reduced by using an additional phase.
For a 2-phase converter, the input current peaks drop in
half and the frequency is doubled. The input capacitor
requirement is thus reduced theoretically by a factor of
four! Just imagine the possibility of capacitor savings
with even higher number of phases!
Output Current Sharing
When multiple LTC3860s are daisychained to drive a com-
mon load, accurate output current sharing is essential to
achieve optimal performance and efficiency. Otherwise,
if one stage is delivering more current than another, then
the temperature between the two stages will be different,
and that could translate into higher switch R_DS(ON), lower
efficiency, and higher RMS ripple. When the COMP and I_AVG
pins of multiple LTC3860s are tied together, the amount
of output current delivered from each LTC3860 is actively
balanced by the I_AVE loop. The SGND pins of the multiple
LTC3860s must be kelvined to the same point for optimal
current sharing.
1.0
0.9
0.8
1 PHASE
0.7
0.6
0.5
0.4
0.3
2 PHASE
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_OUT/V_IN)
3860 F09
Figure 9. Normalized Output Ripple Current
vs Duty Factor [I_RMS” 0.3 (DI_C(PP))]
0.6
1 PHASE
0.5
0.4
0.3
2 PHASE
0.2
0.1
20
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_OUT/V_IN)
3860 F10
Figure 10. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
3860fc