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TYPICAL APPLICATIONS
make sure V
voltage ripple is adequately low for the
IN
application.
C
and C
Selection: V
IN
OUT
Discontinuous V
current is highest in the boost region
OUT
due to the M4 switch toggling on and off. Make sure that
the C
capacitor network has low enough ESR and
OUT
is sized to handle the maximum RMS current. Figure 8
shows the output capacitor RMS ripple current for one to
six phases with the (V
OUT
operations. The total output RMS ripple current I
is normalized against the total output current of the mul-
tiphase system (I
). The graph can be used in place
OUT
of tedious calculations. From the graph, the minimum
total output RMS ripple current can be achieved when
the product of the number of phases (N) and duty cycle
(V
– V
)/V
is approximately equal to integers or:
OUT
IN
OUT
(V
– V
)/V
= n/N
OUT
IN
OUT
where n = 1,2,..., N-1
Therefore, the number of phases be chosen to minimize
the output capacitance for given input and output voltages.
Figure 8 also shows the maximum total normalized output
RMS current for one to six phases. Choose an adequate
C
capacitor network to handle this RMS current.
OUT
C
is also necessary to reduce the V
OUT
by discontinuities and ripple of I
and the bulk capacitance must be considered when choos-
ing the right capacitor for a given V
input capacitor sized for the maximum RMS current must
be used. Add enough ceramic capacitance to make sure
V
voltage ripple is adequate for the application.
OUT
Figure 7 and Figure 8 show that the peak total RMS input
current in buck operation and the peak total RMS output
current in boost operation are reduced linearly, inversely
proportional to the number of phases used. It is important
to note that the ESR-related power loss is proportional to
the RMS current squared, and therefore a 3-phase imple-
mentation results in 90% less power loss when compared
to a single-phase design. Battery/input protection fuse
resistance (if used) PCB trace and connector resistance
losses are also reduced by the reduction of the ripple
Capacitance
OUT
– V
) to V
ratios in boost
IN
OUT
(OUT,RMS)
ripple caused
OUT
. The effects of ESR
OUT
ripple. A low ESR
OUT
For more information
3.5
1–PHASE
2–PHASE
3.0
3–PHASE
4–PHASE
6–PHASE
2.5
2.0
1.5
1.0
0.5
0
0.1
0.2
0.3
0.4
Figure 8. Normalized Output RMS Ripple Current vs
(V
–V
)/V
for One to Six Phases in Boost Operation
OUT
IN
IN
current in a multiphase system. The required amount of
input and output capacitance is further reduced by the
factor, N, due to the effective increase in the frequency of
the current pulses.
VINHIMON, VOUTLOMON AND RVSOFF
VINHIMON and VOUTLOMON offer the identical functions
on the LT8708 and LT8708-1(s). See the VINHIMON,
VOUTLOMON and RVSOFF section of the LT8708 data
sheet for more details. If the VINHIMON and VOUTLOMON
functions are used on the LT8708-1(s) as redundant mon-
itoring functions, in general use the same value resis-
tor dividers as on the LT8708. If the VINHIMON and/or
VOUTLOMON functions are not used on the LT8708-1(s),
tie VINHIMON to GND and/or VOUTLOMON to the respec-
tive LT8708-1's LDO33 pin.
The RVSOFF pin has an internal comparator with a ris-
ing threshold of 1.374V (typical) and a falling thresh-
old of 1.209V (typical). A low state on this pin inhibits
reverse current and power flow. It is recommended to
tie the RVSOFF pins of all the synchronized LT8708 and
LT8708-1(s) together. In a multiphase system, if one or
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LT8708-1
0.5
0.6
0.7
0.8
(V
– V
)/V
OUT
OUT
IN
25
0.9
87081 F08
Rev 0
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