LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 19

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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Equation 1 has a maximum at V
= I
used for design because even signifi cant 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 LTC3858-1, ceramic capacitors
can also be used for C
if there is any question.
The benefi t of the 2-phase operation can be calculated
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 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 sources of the top MOSFETs should be placed within
1cm of each other and share a common C
the sources and C
current resonances at V
A small (0.1μF to 1μF) bypass capacitor between the chip
V
also suggested. A 10Ω resistor placed between C
and the V
two channels.
The selection of C
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
APPLICATIONS INFORMATION
IN
OUT
Δ
pin and ground, placed close to the LTC3858-1, is
V
OUT
/2. This simple worst-case condition is commonly
IN
≈
Δ
pin provides further isolation between the
I ESR
L
⎛
⎜
⎝
OUT
IN
OUT
may produce undesirable voltage and
) is approximated by:
IN
+
. Always consult the manufacturer
is driven by the effective series
IN
8 • •
.
f C
1
OUT
IN
= 2V
⎞
⎟
⎠
OUT
IN
(s). Separating
, where I
IN
(C1)
RMS
Soft-Start (SS Pins)
The start-up of each V
the respective SS pin. When the voltage on the SS pin
is less than the internal 0.8V reference, the LTC3858-1
regulates the V
instead of 0.8V. The SS pin can be used to program an
external soft-start function.
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 7. An internal
1μA current source charges the capacitor, providing a
where f is the operating frequency, C
capacitance and ΔI
The output ripple is highest at maximum input voltage
since ΔI
Setting Output Voltage
The LTC3858-1 output voltages are each set by an exter-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 6. 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.
V
OUT
Figure 7. Using the SS Pin to Program Soft-Start
L
=
FF
increases with input voltage.
0 8
, may be used. Great care should be taken to
FB
.
line away from noise sources, such as the
Figure 6. Setting Output Voltage
V
FB
1/2 LTC3858-1
⎛
⎝ ⎜
pin voltage to the voltage on the SS pin
1
+
L
is the ripple current in the inductor.
R
R
OUT
V
C
B
A
FB
SS
⎞
⎠ ⎟
is controlled by the voltage on
SS
SGND
1/2 LTC3858-1
V
OUT
R
R
38581 F05
B
A
38581 F06
LTC3858-1
C
OUT
FF
is the output
19
38581f