LTC3854 LINER [Linear Technology], LTC3854 Datasheet - Page 12

• Manufacturer Part Number: LTC3854
• Description: Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3854
applicaTions inForMaTion
Inductor Core Selection
Once the value for L is determined, the type of inductor
must be selected. High efficiency converters generally
cannot afford the core loss found in low cost powdered
iron cores, forcing the use of more expensive ferrite or
molypermalloy cores. Actual core loss is independent of
core size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies; allowing design goals to
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3854 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak drive levels are set by the INTV
This voltage is 5V during start-up. Consequently, logic-
level threshold MOSFETs can be used in most applications.
The only exception is if low input voltage is expected (V
< 5V); then, sub-logic level threshold MOSFETs (V
< 3V) should be used. Pay close attention to the BV
specification for the MOSFETs as well; most 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 manufacturers’ data
sheet. C
along the horizontal axis while the curve is approximately
flat divided by the specified change in V

MILLER
, can be approximated from the gate charge curve
MILLER
DS(ON)
is equal to the increase in gate charge
, Miller capacitance C
DS
. This result is
MILLER
CC
voltage.
, input
GS(TH)
DSS
IN
then multiplied by the ratio of the applied V
charge curve specified V
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. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current efficiency generally improves with larger
MOSFETs, while for 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 top switch duty factor is low or during
short-circuit 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
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
DR
Main Switch Duty Cycle =
P
P
Synchronous Switch Duty Cycle =
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
=
=
( )




V
V
MILLER
V
V
V
OUT
IN
IN
IN
INTVCC
− V
V
2
IN
(
I

 
MAX
OUT
actually provides higher efficiency. The
I
MAX
2
− V
IN
2
R losses while the topside N-channel
1
)
(
DS(ON)
> 20V the transition losses rapidly
2
I
TH(MIN)

 
MAX
DS
(
(
1+ δ
R
. When the IC is operating in
DR
)
V
2
vs Temperature curve, but
V
)
OUT
(
)
IN
+
1+ δ
R
(
C
DS(ON)
V
MILLER
= D
TH(MIN)
)
1
R
V
IN
DS(ON)
+
)
− V
V
DS
•




TH(MIN)
IN
DS(ON)
(f)
OUT
DS(ON)
to the gate
IN
= 1− D
device
< 20V,
is the
3854fa
and