Datasheet

LTC3708
16
3708fb
APPLICATIONS INFORMATION
highest (V
OUT
)(I
OUT
) product needs to be used in the
formula below to determine the maximum RMS current
requirement. Increasing the output current, drawn from
the other out-of-phase controller, will actually decrease
the input RMS ripple current from this maximum value
(see Figure 2).
The type of input capacitor, value and ESR rating have ef-
fi ciency effects that need to be considered in the selection
process. The capacitance value chosen should be suffi cient
to store adequate charge to keep pulsating input currents
down. 20μF to 40μF is usually suffi cient for a 25W output
supply operating at 200kHz. The ESR of the capacitor
is important for capacitor power dissipation as well as
overall effi ciency. All of the power (RMS ripple current
2
• ESR) not only heats up the capacitor but wastes power
from the battery.
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used
as input capacitors, but each has drawbacks: ceramic
voltage coeffi cients are very high and may have audible
piezoelectric effects; tantalums need to be surge-rated;
OS-CONs suffer from higher inductance, larger case
size and limited surface-mount applicability; electrolyt-
ics’ higher ESR and dryout possibility require several to
be used. 2-phase systems allow the lowest amount of
capacitance overall. As little as one 22μF or two to three
10μF ceramic capacitors are an ideal choice in a 20W to
35W power supply due to their extremely low ESR. Even
though the capacitance at 20V is substantially below their
rating at zero-bias, very low ESR loss makes ceramics
an ideal candidate for highest effi ciency battery operated
systems. Also consider parallel ceramic and high quality
electrolytic capacitors as an effective means of achieving
ESR and bulk capacitance goals.
In continuous mode, the current of the top N-channel
MOSFET is approximately a square wave of duty cycle
V
OUT
/V
IN
. To prevent large voltage transients, a low ESR
input capacitor sized for the maximum RMS current of
one channel must be used. The maximum RMS capacitor
current is given by:
II
VVV
V
RMS MAX
OUT IN OUT
IN
≈
−
()
⎡
⎣
⎤
⎦
12/
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even signifi cant deviations do not
offer much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design. Always consult
the manufacturer if there is any question.
The benefi t of the LTC3708 2-phase operation can be
calculated by using the equation above for the higher
power channel and then calculating the loss that would
have resulted if both controller channels switch on at the
same time. The total RMS power lost is lower when both
controllers are operating due to the interleaving of current
pulses through the input capacitor’s ESR. This is why the
input capacitor’s requirement calculated above for the
worst-case controller is adequate for the dual controller
design. Remember that input protection fuse resistance,
battery resistance and PC board trace resistance losses
are also reduced due to the reduced peak currents in a
2-phase system. The overall benefi t of a 2-phase design
will only be fully realized when the source impedance of
the power supply/battery is included in the effi ciency
testing. The drains of the two top MOSFETS should be
placed within 1cm of each other and share a common
C
IN
(s). Separating the drains and C
IN
may produce un-
desirable voltage and current resonances at V
IN
.
The selection of C
OUT
is driven by the effective series
resistance (ESR) required to minimize voltage ripple
and load step transients. The output ripple (ΔV
OUT
) is
determined by:
ΔΔV I ESR
fC
OUT L
OUT
≈+
⎛
⎝
⎜
⎞
⎠
⎟
1
8
where f = operating frequency, C
OUT
= output capacitance,
and ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI
L
increases
with input voltage. Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering and
has the necessary RMS current rating.