LTC3728L LINER [Linear Technology], LTC3728L Datasheet - Page 16

• Manufacturer Part Number: LTC3728L
• Description: Dual, 550kHz, 2-Phase Synchronous Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3728L/LTC3728LX
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 benefit of the LTC3728L/LTC3728LX multiphase clock-
ing 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 resis-
tance 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 two top MOSFETS
should be placed within 1cm of each other and share a
common C
duce undesirable voltage and current resonances at V
The selection of C
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple ( V
Where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. With I
ripple will typically be less than 50mV at the maximum V
assuming:
16
C
and C
OUT
V
OUT
L
= ripple current in the inductor. The output ripple
Recommended ESR < 2 R
OUT
IN
> 1/(8fR
(s). Separating the drains and C
I ESR
L
OUT
U
SENSE
OUT
is driven by the required effective
8
) is determined by:
U
fC
)
1
OUT
L
= 0.3I
OUT
SENSE
W
= output capacitance,
OUT(MAX)
IN
L
the output
U
increases
may pro-
IN
IN
.
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 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 dis-
charging 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 Nichicon, United 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 current
handling and load step requirements. Aluminum electro-
lytic, 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 capaci-
tor 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 applica-
tions providing that consideration is given to ripple cur-
rent 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 maximiz-
ing performance and minimizing overall cost. Other ca-
pacitor types include Nichicon PL series, Panasonic SP,
TH
pin OPTI-LOOP compensation
3728lxfa