LTC3858 Linear Technology, LTC3858 Datasheet - Page 20

• Manufacturer Part Number: LTC3858
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

LTC3858
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:
Equation 1 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 LTC3858, ceramic capacitors
can also be used for C
if there is any question.
The benefit of 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 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 top MOSFETs should be placed within
1cm of each other and share a common C
ing the sources and C
and current resonances at V
A small (0.1µF to 1µF) bypass capacitor between the chip
V
suggested. A 10Ω resistor placed between C
0
applicaTions inForMaTion
C
IN
IN
OUT
pin and ground, placed close to the LTC3858, is also
Re
/2. This simple worst-case condition is commonly
quired I
RMS
≈
IN
IN
I
MAX
. Always consult the manufacturer
may produce undesirable voltage
V
IN


IN
(
V
.
OUT
IN
OUT
= 2V
)(
V
)/(V
IN
OUT
–
IN
IN
V
). To prevent
(s). Separat-
, where I
OUT
IN
(C1) and
)
 

1 2 /
RMS
(1)
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
capacitance and ∆I
The output ripple is highest at maximum input voltage
since ∆I
Setting Output Voltage
The LTC3858 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, 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.
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 regulates
the V
of 0.8V. The SS pin can be used to program an external
soft-start function.
∆
V
OUT
V
FB
IN
OUT
pin provides further isolation between the two
pin voltage to the voltage on the SS pin instead
O
L
is the operating frequency, C
=
increases with input voltage.
FF
≈
0 8
, may be used. Great care should be taken to
FB
.
∆
line away from noise sources, such as the
I ESR
Figure 6. Setting Output Voltage
V
L
1/2 LTC3858

 



OUT
1
OUT
+
L
is the ripple current in the inductor.
) is approximated by:
R
R
V
OUT
is driven by the effective series
+
B
A
FB

 
8 • •
is controlled by the voltage on
f C
V
1
OUT
R
R
OUT
3858 F06
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B
A



C
OUT
FF
is the output
3858fa