Datasheet

LTC3890-3
18
38903f
For more information www.linear.com/3890-3
applicaTions inForMaTion
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3890-3: 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
CC
voltage.
This voltage is typically 5.1V during start-up (see EXTV
CC
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. Pay close
attention to the BV
DSS
specification for the MOSFETs as well.
Selection criteria for the power MOSFETs include the
on-resistance, R
DS(ON)
, Miller capacitance, C
MILLER
, input
voltage and maximum output current. Miller capacitance,
C
MILLER
, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C
MILLER
is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in V
DS
. This result is
then multiplied by the ratio of the application applied V
DS
to the Gate charge curve specified V
DS
. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle =
V
OUT
V
IN
Synchronous Switch Duty Cycle =
V
IN
− V
OUT
V
IN
The MOSFET power dissipations at maximum output
current are given by:
P
MAIN
=
V
OUT
V
IN
I
MAX
( )
2
1+ δ
( )
R
DS(ON)
+
V
IN
( )
2
I
MAX
2






R
DR
( )
C
MILLER
( )
•
1
V
INTVCC
– V
THMIN
+
1
V
THMIN






f
( )
P
SYNC
=
V
IN
– V
OUT
V
IN
I
MAX
( )
2
1+ δ
( )
R
DS(ON)
where δ is the temperature dependency of R
DS(ON)
and
R
DR
(approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V
THMIN
is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V
the high current efficiency generally improves with larger
MOSFETs, while for V
IN
> 20V 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 top switch duty factor is low or during
a 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
DS(ON)
vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.