Datasheet

LTC3788-1
17
37881fc
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are given by:
P
MAIN
=
(V
OUT
− V
IN
)V
OUT
V
2
IN
•I
OUT(MAX)
2
•1+δ
()
•R
DS(ON)
+k•V
3
OUT
•
I
OUT(MAX)
V
IN
•R
DR
•C
MILLER
•f
P
SYNC
=
V
IN
V
OUT
•I
OUT(MAX)
2
•1+δ
()
•R
DS(ON)
where δ is the temperature dependency of R
DS(ON)
and
R
DR
(approximately 1) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. The constant k,
which accounts for the loss caused by reverse recovery
current, is inversely proportional to the gate drive current
and has an empirical value of 1.7.
Both MOSFETs have I
2
R losses while the bottom N-channel
equation includes an additional term for transition losses,
which are highest at low input voltages. For high V
IN
the
high current efficiency generally improves with larger
MOSFETs, while for low V
IN
the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
MILLER
actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the bottom switch duty factor is low or dur-
ing overvoltage when the synchronous switch is on close
to 100% of the period.
The term (1+ δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
C
IN
and 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 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 the 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 voltage due to charging and discharging the
bulk capacitance is given by:
V
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:
∆V
ESR
= I
L(MAX)
• ESR
The LTC3788-1 can also be configured as a 2-phase single
output converter where the outputs of the two channels
are connected together and both channels have the same
duty cycle. With 2-phase operation, the two channels of
the dual switching regulator are operated 180 degrees
out-of-phase. This effectively interleaves the output current
pulses, greatly reducing the output capacitor ripple current.
As a result, the ESR requirement of the capacitor can be
relaxed. Because the ripple current in the output capacitor
is a square wave, the ripple current requirements for the
output capacitor depend on the duty cycle, the number
of phases and the maximum output current. Figure 3 il-
lustrates the normalized output capacitor ripple current
as a function of duty cycle in a 2-phase configuration. To
choose a ripple current rating for the output capacitor,
first establish the duty cycle range based on the output
voltage and range of input voltage. Referring to Figure 3,
choose the worst-case high normalized ripple current as
a percentage of the maximum load current.