ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 15

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
C
The selection of C
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current
occurs when only one controller is operating. The control-
ler with the highest (V
in the formula below to determine the maximum RMS
capacitor current requirement. Increasing the output cur-
rent drawn from the other controller will actually decrease
the input RMS ripple current from its maximum value. The
out-of-phase technique typically reduces the input
capacitor’s RMS ripple current by a factor of 30% to 70%
when compared to a single phase power supply solution.
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle (V
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current of one channel must be
used. The maximum RMS capacitor current is given by:
This formula has a maximum at V
= I
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’
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 be
paralleled to meet size or height requirements in the
design. Due to the high operating frequency of the
LTC3827-1, ceramic capacitors can also be used for C
Always consult the manufacturer if there is any question.
IN
OUT
C
and C
IN
/2. This simple worst-case condition is commonly
Required I
OUT
Selection
IN
RMS
U
is simplified by the 2-phase architec-
OUT
≈
)(I
I
MAX
U
V
IN
OUT
[
) product needs to be used
( )(
V
OUT
IN
W
= 2V
V
IN
OUT
–
OUT
, where I
V
OUT
U
)/(V
)
IN
]
/ 1 2
). To
RMS
IN
.
The benefit of the LTC3827-1 2-phase operation 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 reduced overlap of
current pulses required 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. Also, the input protection fuse re-
sistance, battery resistance, and PC board trace resistance
losses are also reduced due to the reduced peak currents
in a 2-phase system. The overall benefit of a multiphase
design will only be fully realized when the source imped-
ance of the power supply/battery is included in the effi-
ciency testing. The sources of the top MOSFETs should be
placed within 1cm of each other and share a common
C
sirable voltage and current resonances at V
A small (0.1μF to 1μF) bypass capacitor between the chip
V
suggested. A 10Ω resistor placed between C
V
The selection of C
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The
output ripple (ΔV
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
IN
IN
IN
Δ
(s). Separating the sources and C
pin provides further isolation between the two channels.
pin and ground, placed close to the LTC3827-1, is also
V
OUT
RIPPLE
≈
I
RIPPLE
increases with input voltage.
OUT
RIPPLE
OUT
⎛
⎜
⎝
ESR
) is approximated by:
is driven by the effective series
is the ripple current in the induc-
+
8
fC
1
OUT
IN
LTC3827-1
⎞
⎟
⎠
may produce unde-
OUT
IN
is the output
IN
(C1) and the
.
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