LTC3859_15 Datasheet, PDF (24/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
Boost C_IN, C_OUT Selection
The input ripple current in a boost converter is relatively
low (compared with the output ripple current), because
this current is continuous. The boost input capacitor C_IN
voltage rating should comfortably exceed the maximum
input voltage. Although ceramic capacitors can be relatively
tolerant of overvoltage conditions, aluminum electrolytic
capacitors are not. Be sure to characterize the input voltage
for any possible overvoltage transients that could apply
excess stress to the input capacitors.
The value of C_IN is a function of the source impedance, and
in general, the higher the source impedance, the higher the
required input capacitance. The required amount of input
capacitance is also greatly affected by the duty cycle. High
output current applications that also experience high duty
cycles can place great demands on the input supply, both
in terms of DC current and ripple current.
In a boost converter, the output has a discontinuous current,
so C_OUT must be capable of reducing the output voltage
ripple. The effects of ESR (equivalent series resistance) and
the bulk capacitance must be considered when choosing
the right capacitor for a given output ripple voltage. The
steady ripple due to charging and discharging the bulk
capacitance is given by:
( ) Ripple = IOUT(MAX) • VOUT − VIN(MIN) V
C_OUT • V_OUT • f
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
DV_ESR = I_L(MAX) • ESR
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
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Capacitors are now available with low ESR and high ripple
current ratings such as OS-CON and POSCAP.
Buck C_IN, C_OUT Selection
The selection of C_IN for the two buck controllers is simplified
by the 2-phase architecture and its impact on the worst-
case RMS current drawn through the input network (bat-
tery/fuse/capacitor). It can be shown that the worst-case
capacitor RMS current occurs when only one controller
is operating. The controller with the highest (V_OUT)(I_OUT)
product needs to be used in the formula shown in Equa-
tion (1) to determine the maximum RMS capacitor current
requirement. Increasing 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_OUT)/(V_IN). To prevent
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:
( )( ) ^C_IN
Required
I_RMS
≈ ^IMAX
V_IN
⎡⎣
V_OUT
V_IN − V_OUT ⎤⎦^1/2 ₍₁₎
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
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 LTC3859, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
The benefit of the LTC3859 2-phase operation can be calcu-
lated 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.
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