LT8708-1
25
Rev 0
TYPICAL APPLICATIONS
make sure V
IN
voltage ripple is adequately low for the
application.
C
IN
and C
OUT
Selection: V
OUT
Capacitance
Discontinuous V
OUT
current is highest in the boost region
due to the M4 switch toggling on and off. Make sure that
the C
OUT
capacitor network has low enough ESR and
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
– V
IN
) to V
OUT
ratios in boost
operations. The total output RMS ripple current I
(OUT,RMS)
is normalized against the total output current of the mul-
tiphase system (I
OUT
). The graph can be used in place
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
OUT
– V
IN
)/V
OUT
is approximately equal to integers or:
(V
OUT
– V
IN
)/V
OUT
= n/N
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
OUT
capacitor network to handle this RMS current.
C
OUT
is also necessary to reduce the V
OUT
ripple caused
by discontinuities and ripple of I
OUT
. The effects of ESR
and the bulk capacitance must be considered when choos-
ing the right capacitor for a given V
OUT
ripple. A low ESR
input capacitor sized for the maximum RMS current must
be used. Add enough ceramic capacitance to make sure
V
OUT
voltage ripple is adequate for the application.
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
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
Figure 8. Normalized Output RMS Ripple Current vs
(V
OUT
–V
IN
)/V
IN
for One to Six Phases in Boost Operation
1–PHASE
2–PHASE
3–PHASE
4–PHASE
6–PHASE
(V
OUT
– V
IN
)/V
OUT
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
I (
OUT
,RMS)
/I
OUT
87081 F08
