ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 19

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

RMS capacitor current requirement. Increasing the out-
put 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 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 LTC3890, ceramic capacitors
can also be used for C
if there is any question.
The benefi t of the LTC3890 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 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.
APPLICATIONS INFORMATION
C
IN
OUT
Required I
/2. This simple worst-case condition is commonly
RMS
≈
IN
I
MAX
. Always consult the manufacturer
V
IN
⎡
⎣
(
V
OUT
IN
OUT
)
= 2V
(
V
)/(V
IN
OUT
– V
IN
). To prevent
, where I
OUT
)
⎤
⎦
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 satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
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 LTC3890 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
V
pin and ground, placed close to the LTC3890, is also
OUT
V
IN
OUT
L
pin provides further isolation between the two
= 0.8V 1+
FF
increases with input voltage.
, may be used. Great care should be taken to
FB
line away from noise sources, such as the
Figure 5. Setting Output Voltage
I
L
1/2 LTC3890
IN
OUT
ESR +
may produce undesirable voltage and
OUT
L
R
R
) is approximated by:
is the ripple current in the inductor.
V
B
A
FB
is driven by the effective series
IN
8 • f • C
.
V
1
OUT
OUT
R
R
3890 F05
B
A
C
OUT
LTC3890
FF
IN
(s). Separating
is the output
IN
(C1) and
19
3890f