ltc3728legn-1-tr Linear Technology Corporation, ltc3728legn-1-tr Datasheet - Page 16

• Manufacturer Part Number: ltc3728legn-1-tr
• Description: Dual, 550khz, 2-phase Synchronous Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3728L-1
This formula has a maximum at V
= I
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 LTC3728L-1 multiphase clocking 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 switched 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 multi-
phase 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 test-
ing. The drains of the two top MOSFETS should be placed
within 1cm of each other and share a common C
Separating the drains and C
voltage and 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
is highest at maximum input voltage since ΔI
with input voltage. With ΔI
16
OUT
V
OUT
/2. This simple worst case condition is commonly
L
= ripple current in the inductor. The output ripple
I
L
ESR +
OUT
OUT
is driven by the required effective
8fC
) is determined by:
1
OUT
L
IN
= 0.3I
may produce undesirable
OUT
IN
= output capacitance,
= 2V
IN
OUT(MAX)
.
OUT
, where I
L
the output
increases
IN
RMS
(s).
ripple will typically be less than 50mV at the maximum
V
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
OPTI-LOOP compensation components can be optimized
to provide stable, high performance transient response
regardless of the output capacitors selected.
Manufacturers such as Nichicon, Nippon Chemi-Con 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 ESR, RMS cur-
rent handling and load step requirements. Aluminum
electrolytic, dry tantalum and special polymer capaci-
tors 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 capaci-
tors 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
C
and C
assuming:
OUT
Recommended ESR < 2 R
OUT
> 1/(8fR
SENSE
)
SENSE
TH
3728l1fc
pin