Datasheet

LTC3859A
24
3859af
applicaTions inForMaTion
efficiency. The synchronous MOSFET losses for the buck
controllers are greatest at high input voltage when the top
switch duty factor is low or during a short-circuit when the
synchronous switch is on close to 100% of the period. The
synchronous MOSFET losses for the boost controller are
greatest when the input voltage approaches the output volt-
age or during an overvoltage event when the synchronous
switch is on 100% of the period.
The term (1+ z) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs Temperature curve, but
z = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes D4, D5, and D6 shown in
Figure 13 conduct during the dead-time between the
conduction of the two power MOSFETs. This prevents
the body diode of the synchronous MOSFET from turning
on, storing charge during the dead-time and requiring a
reverse recovery period that could cost as much as 3%
in efficiency at high V
IN
. A 1A to 3A Schottky is generally
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 =
I
OUT(MAX)
• V
OUT
− V
IN(MIN)
( )
C
OUT
• V
OUT
• f
V
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.
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.