LTC3773 Datasheet, PDF (18/32 Pages) Linear Technology – Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking

LTC3773
APPLICATIONS INFORMATION
This formula has a maximum value at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant 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 benefit 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 benefit of a multiphase design will only
be fully realized when the source impedance of the power
supply/battery is included in the efficiency testing. The
drains of the three 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 undesirable voltage and
current resonances at V_IN.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple (ΔV_OUT) is determined by:

V_OUT


I_L


ESR
+
8
•
f
1
• C_OUT


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. With ΔI_L = 0.3I_OUT(MAX) the output ripple will
typically be less than 50mV at maximum V_IN assuming:
C_OUT Recommended ESR < 2R_SENSE
and
C_OUT
>
(8
•
f
1
• R_SENSE)
18
The first condition relates to the ripple current into
the ESR of the output capacitance while the second
term guarantees that the output capacitance does not
significantly 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_TH pin OPTI-LOOP compensation
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 configurations. 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 specific
recommendations.
R_SENSE Selection for Output Current
Once the frequency and inductor have been chosen, R_SENSE
is determined based on the required peak inductor current.
The current comparator has a typical maximum threshold
of 75mV/R_SENSE and an input common mode range of
SGND to (1.1) • V_CC. The current comparator threshold
sets the peak inductor current, yielding a maximum aver-
3773fb