LTC3773EG-PBF LINER [Linear Technology], LTC3773EG-PBF Datasheet - Page 18

• Manufacturer Part Number: LTC3773EG-PBF
• Description: Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3773
This formula has a maximum value at V
I
monly 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 capaci-
tor, 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 LTC3773 multiphase clocking can be
calculated by using the equation above for the highest
power controller and then calculating the loss that would
have resulted if all three channels switched on at the same
time. The total RMS power lost is lower when triple control-
lers are operating due to the interleaving of current pulses
through the input capacitor’s ESR. This is why the input
capacitance requirement calculated above for the worst-
case controller is adequate for the triple 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 multiphase
system. The overall benefi t of a multiphase 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 three top MOSFETs should be placed within
1cm of each other and share a common C
the drains and C
current resonances at V
The selection of C
series resistance (ESR). Typically once the ESR require-
ment is satisfi ed the capacitance is adequate for fi ltering.
The output ripple (ΔV
Where f = operating frequency, C
and ΔI
highest at maximum input voltage since ΔI
input voltage. With ΔI
typically be less than 50mV at maximum V
18
RMS
C
and C
OUT
V
= I
OUT
L
= ripple current in the inductor. The output ripple is
OUT
Recommended ESR < 2R
OUT
/2. This simple worst-case condition is com-
>
I
L
(8 • f • R
ESR +
IN
OUT
may produce undesirable voltage and
1
L
SENSE
OUT
8 • f • C
= 0.3I
is driven by the required effective
IN
) is determined by:
.
)
1
OUT(MAX)
OUT
SENSE
OUT
= output capacitance,
the output ripple will
IN
IN
L
= 2V
(s). Separating
IN
increases with
assuming:
OUT
, where
The fi rst condition relates to the ripple current into
the ESR of the output capacitance while the second
term guarantees that the output capacitance does not
signifi cantly discharge during the operating frequency
period due to ripple current. The choice of using smaller
output capacitance increases the ripple voltage due to the
discharging term but can be compensated for by using
capacitors of very low ESR to maintain the ripple voltage
at or below 50mV. The I
components can be optimized to provide stable, high
performance transient response regardless of the output
capacitors selected.
Manufacturers such as Sanyo, Panasonic and Cornell
Dubilier should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
electrolyte capacitor available from Sanyo has a good
(ESR)(size) product. An additional ceramic capacitor in
parallel with OS-CON capacitors is recommended to offset
the effect of lead inductance.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the relevant ESR or transient
current handling requirements. Aluminum electrolytic
and dry tantalum capacitors are both available in surface
mount confi gurations. New special polymer surface
mount capacitors offer very low ESR also but have much
lower capacitive density per unit volume. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
output capacitor choices are the Sanyo POSCAP TPD, TPE,
TPF, AVX TPS, TPSV, the Kemet T510 series of surface
mount tantalums, Kemet AO-CAPs or the Panasonic SP
series of surface mount special polymer capacitors avail-
able in case heights ranging from 2mm to 4mm. Other
capacitor types include Nichicon PL series and Sprague
595D series. Consult the manufacturers for other specifi c
recommendations.
R
Once the frequency and inductor have been chosen, R
is determined based on the required peak inductor current.
The current comparator has a typical maximum threshold
of 75mV/R
SGND to (1.1) • V
sets the peak inductor current, yielding a maximum aver-
SENSE
Selection for Output Current
SENSE
and an input common mode range of
CC
. The current comparator threshold
TH
pin OPTI-LOOP compensation
SENSE
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