ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 18

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
LTC3787
Power MOSFET Selection
Two external power MOSFETs must be selected for each
controller in the LTC3787: one N-channel MOSFET for the
bottom (main) switch, and one N-channel MOSFET for the
top (synchronous) switch.
The peak-to-peak gate drive levels are set by the INTV
voltage. This voltage is typically 5.4V during start-up
(see EXTV
threshold MOSFETs must be used in most applications.
Pay close attention to the BV
MOSFETs as well; many of the logic level MOSFETs are
limited to 30V or less.
Selection criteria for the power MOSFETs include the
on-resistance R
voltage and maximum output current. Miller capacitance,
C
usually provided on the MOSFET manufacturer’s data
sheet. C
along the horizontal axis while the curve is approximately
flat divided by the specified change in VDS. This result
is then multiplied by the ratio of the application applied
VDS to the gate charge curve specified VDS. When the IC
is operating in continuous mode, the duty cycles for the
top and bottom MOSFETs are given by:
If the maximum output current is I
nel takes one half of the total output current, the MOSFET
18
MILLER
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
, can be approximated from the gate charge curve
MILLER
CC
pin connection). Consequently, logic-level
DS(ON)
is equal to the increase in gate charge
, Miller capacitance C
V
OUT
DSS
V
OUT(MAX)
OUT
− V
specification for the
IN
V
V
OUT
IN
and each chan-
MILLER
, input
CC
power dissipations in each channel at maximum output
current are given by:
where d is the temperature dependency of R
proximately 1Ω) 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
equation includes an additional term for transition losses,
which are highest at low input voltages. For high V
high current efficiency generally improves with larger
MOSFETs, while for low V
increase to the point that the use of a higher R
with lower C
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+ d) is generally given for a MOSFET in the
form of a normalized R
d = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
P
P
MAIN
SYNC
=
=
(V
V
MILLER
• R
• C
V
OUT
OUT
IN
DS(ON)
MILLER
•
− V
V
actually provides higher efficiency. The



2
I
2
OUT(MAX)
IN
IN
R losses while the bottom N-channel
+ k • V
• f
)V
DS(ON)
2
OUT
IN
3
the transition losses rapidly
OUT
•
vs Temperature curve, but






2
I
OUT(MAX)
• 1+ δ
•
(
I
OUT(MAX)
2
2 • V
)
IN
• R



DS(ON)
2
DS(ON)
DS(ON)
• 1+ δ
(
device
IN
(ap-
)
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the