LTC3788 Linear Technology, LTC3788 Datasheet - Page 19

• Manufacturer Part Number: LTC3788
• Description: Dual Output Synchronous Boost Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
operating in continuous mode, the duty cycles for the top
and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
where δ is the temperature dependency of R
R
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
equation includes an additional term for transition losses,
which are highest at low input voltages. For high V
high current effi ciency generally improves with larger
MOSFETs, while for low V
increase to the point that the use of a higher R
with lower C
The synchronous MOSFET losses are greatest at high
input voltage when the bottom switch duty factor is low
or during overvoltage when the synchronous switch is on
close to 100% of the period.
DR
Main Switch Duty Cycle
P
P
Synchronous
MAIN
S S YNC
(approximately 1Ω) is the effective driver resistance
=
=
•
(
•
V
V
MILLER
R
C
V
OUT
OUT
IN
D D S ON
MILLER
(
S S witch Duty Cycle
•
−
V
I
2
actually provides higher effi ciency.
OUT MAX
2
)
V V
R losses while the bottom N-channel
IN
+
IN
•
k V
)
(
f
•
IN
OUT
the transition losses rapidly
=
3
)
2
OUT
•
V
•
I
OUT
(
OUT MAX
1 δ
V
•
+
OUT
I
(
OUT MAX
−
=
)
V
V
•
IN
(
V
IN
R
)
V
OUT
2
DS(ON)
IN
DS ON
•
DS(ON)
)
(
(
1 δ
•
+
R
)
device
DR
IN
)
and
the
The term (1+ δ) is generally given for a MOSFET in the
form of a normalized R
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
C
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
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
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
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:
where C
The steady ripple due to the voltage drop across the ESR
is given by:
IN
ΔV
V
and C
RIPPLE
OUT
ESR
OUT
must be capable of reducing the output voltage
= I
OUT
L(MAX)
=
is the output fi lter capacitor.
Selection
I
OUT MAX
IN
(
• ESR
is a function of the source impedance,
C
OUT
DS(ON)
)
• (
V
•
OUT
V
vs Temperature curve, but
OUT
−
• f f
V
IN MIN
(
LTC3788
IN
)
voltage rating
)
V
19
3788f