LTC3890
applicaTions inForMaTion
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.
The drains of the top MOSFETs should be placed within
1cmofeachotherandshareacommonC (s). Separating
IN
the drains and C may produce undesirable voltage and
IN
current resonances at V .
IN
A small (0.1µF to 1µF) bypass capacitor between the chip
V pin and ground, placed close to the LTC3890, is also
IN
suggested. A 10Ω resistor placed between C (C1) and
IN
Incontinuousmode,thesourcecurrentofthetopMOSFET
is a square wave of duty cycle (V )/(V ). To prevent
the V pin provides further isolation between the two
IN
channels.
OUT
IN
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:
The selection of C
is driven by the effective series
OUT
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
IMAX
1/2
output ripple (∆V ) is approximated by:
OUT
CIN Required IRMS
≈
V
V – V
IN
OUT
(
OUT ) (
)
(1)
V
IN
1
∆VOUT ≈ ∆I ESR+
L
This formula has a maximum at V = 2V , where I
8 • f • COUT
IN
OUT
RMS
= I /2. This simple worst-case condition is commonly
OUT
where f is the operating frequency, C
is the output
OUT
usedfordesignbecauseevensignificantdeviationsdonot
offermuchrelief.Notethatcapacitormanufacturers’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
capacitance and ∆I is the ripple current in the inductor.
L
The output ripple is highest at maximum input voltage
since ∆I increases with input voltage.
L
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:
can also be used for C . Always consult the manufacturer
IN
if there is any question.
The benefit of the LTC3890 2-phase operation can be cal-
culatedbyusingEquation1forthehigherpowercontroller
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.
RB
RA
VOUT = 0.8V 1+
To improve the frequency response, a feedforward ca-
pacitor, C , may be used. Great care should be taken to
FF
route the V line away from noise sources, such as the
FB
inductor or the SW line.
V
OUT
R
C
FF
1/2 LTC3890
V
B
FB
R
A
3890 F05
Figure 5. Setting Output Voltage
3890fb
19
Linear Systems [ Linear Systems ]
