LTC3707-SYNC
16
3707sfa
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 LTC3707-SYNC multiphase can be
calculated by using the equation above for the higher
power controller 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
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 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 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 satisfi ed the capacitance is adequate for fi ltering.
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. With ΔI
L
= 0.3I
OUT(MAX)
the output ripple will
typically be less than 50mV at max V
IN
assuming:
C
OUT
Recommended ESR < 2 R
SENSE
and C
OUT
> 1/(8fR
SENSE
)
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.
APPLICATIONS INFORMATION
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 Nichicon, United Chemicon
and Sanyo can be considered for high performance
through-hole capacitors. The OS-CON semiconductor
dielectric capacitor available from Sanyo has the lowest
(ESR)(size) product of any aluminum electrolytic at a
somewhat higher price. An additional ceramic capacitor
in parallel with OS-CON capacitors is recommended to
reduce the inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages.
Special polymer surface mount capacitors offer very low
ESR but have lower storage capacity per unit volume
than other capacitor types. These capacitors offer a very
cost-effective output capacitor solution and are an ideal
choice when combined with a controller having high
loop bandwidth. Tantalum capacitors offer the highest
capacitance density and are often used as output capacitors
for switching regulators having controlled soft-start.
Several excellent surge-tested choices are the AVX TPS,
AVX TPSV or the KEMET T510 series of surface mount
tantalums, available in case heights ranging from 2mm
to 4mm. Aluminum electrolytic capacitors can be used
in cost-driven applications providing that consideration
is given to ripple current ratings, temperature and long
term reliability. A typical application will require several
to many aluminum electrolytic capacitors in parallel.
A combination of the above mentioned capacitors will
often result in maximizing performance and minimizing
overall cost. Other capacitor types include Nichicon PL
series, NEC Neocap, Cornell Dubilier ESRE and Sprague
595D series. Consult manufacturers for other specifi c
recommendations.