LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 19

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

maximum RMS capacitor current requirement. Increas-
ing the output current 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
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 LTC3857, ceramic capacitors
can also be used for C
if there is any question.
The benefit of the LTC3857 2-phase operation can be cal-
culated by using Equation 1 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 resistance, 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 impedance of the
power supply/battery is included in the efficiency testing.
APPLICATIONS INFORMATION
C
IN
OUT
Required I
/2. This simple worst-case condition is commonly
RMS
≈
IN
I
MAX
V
. Always consult the manufacturer
IN
⎡ ⎣
(
V
OUT
IN
OUT
)
(
= 2V
V
)/(V
IN
–V
OUT
IN
OUT
). To prevent
, where I
)
⎤ ⎦
1/2
RMS
(1)
The drains of the top MOSFETs should be placed within
1cm of each other and share a common C
the drains and C
current resonances at V
A small (0.1μF to 1μF) bypass capacitor between the chip
V
suggested. A 10Ω resistor placed between C
the V
channels.
The selection of C
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (∆V
where f is the operating frequency, C
capacitance and ∆I
The output ripple is highest at maximum input voltage
since ∆I
Setting Output Voltage
The LTC3857 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 5. The regulated output voltage
is determined by:
To improve the frequency response, a feedforward ca-
pacitor, C
route the V
inductor or the SW line.
IN
ΔV
V
pin and ground, placed close to the LTC3857, is also
OUT
IN
OUT
L
pin provides further isolation between the two
=0.8V 1+
FF
increases with input voltage.
≈ΔI
, may be used. Great care should be taken to
FB
L
line away from noise sources, such as the
Figure 5. Setting Output Voltage
⎛
⎝ ⎜
⎛
⎝ ⎜
1/2 LTC3857
IN
ESR+
OUT
may produce undesirable voltage and
OUT
L
R
R
) is approximated by:
is the ripple current in the inductor.
B
A
V
FB
8•f•C
⎞
⎠ ⎟
is driven by the effective series
IN
.
1
OUT
V
OUT
R
R
3857 F05
⎞
⎠ ⎟
B
A
C
OUT
LTC3857
FF
IN
(s). Separating
is the output
IN
(C1) and
19
3857fc