LTC3856_15 Datasheet, PDF (21/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. A higher frequency
generally results in lower efficiency because of MOSFET
gate charge and transition losses. In addition to this basic
trade-off, the effect of inductor value on ripple current
and low current operation must also be considered. The
PolyPhase approach reduces both input and output ripple
currents while optimizing individual output stages to run
at a lower fundamental frequency, enhancing efficiency.
The inductor value has a direct effect on ripple current.
The inductor ripple current, ∆I_L, per individual section
N, decreases with higher inductance or frequency and
increases with higher V_IN or V_OUT :
∆I_L
=
V_OUT
f_OSC • L

 ^1–
V_OUT
V_IN


where f_OSC is the individual output stage operating fre-
quency.
In a PolyPhase converter, the net ripple current seen by
the output capacitor is much smaller than the individual
inductor ripple currents due to the ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
Figure 8 shows the net ripple current seen by the output
capacitors for the different phase configurations. The
output ripple current is plotted for a fixed output voltage
as the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations. The zero output
ripple current is obtained when:
V_{OUT =} k _{where k = 1, 2,...,N – 1}
V_IN N
Power MOSFET and Schottky Diode
(Optional) Selection
At least two external power MOSFETs must be selected
for each power stage: One N-channel MOSFET for the top
(main) switch and one or more N‑channel MOSFET(s) for
the bottom (synchronous) switch. The number, type and
on-resistance of all MOSFETs selected take into account
the voltage step-down ratio as well as the actual position
(main or synchronous) in which the MOSFET will be used.
A much smaller and much lower input capacitance MOSFET
should be used for the top MOSFET in applications that
have an output voltage that is less than one-third of the input
voltage. In applications where V_IN >> V_OUT , the top MOSFETs’
on-resistance is normally less important for overall efficiency
than its input capacitance at operating frequencies above
300kHz. MOSFET manufacturers have designed special
purpose devices that provide reasonably low on-resistance
with significantly reduced input capacitance for the main
switch application in switching regulators.
1.0
1-PHASE
0.9
2-PHASE
0.8
3-PHASE
4-PHASE
0.7
6-PHASE
12-PHASE
0.6
0.5
0.4
0.3
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)
3856 F08
Figure 8. Normalized Peak Output Current
vs Duty Factor [I_RMS = 0.3(I_OP-P)]
3856f
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