Datasheet

LTC3859A
23
3859af
applicaTions inForMaTion
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3859A: one N-channel MOSFET for the
top switch (main switch for the buck, synchronous for the
boost), and one N-channel MOSFET for the bottom switch
(main switch for the boost, synchronous for the buck).
The peak-to-peak drive levels are set by the INTV
CC
voltage.
This voltage is typically 5.4V 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; many of the logic level MOSFETs are limited to 30V
or less.
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:
Buck Main Switch Duty Cycle =
V
OUT
V
IN
Buck Sync Switch Duty Cycle =
V
IN
− V
OUT
V
IN
Boost Main Switch Duty Cycle =
V
OUT
− V
IN
V
OUT
Boost Sync Switch Duty Cycle =
V
IN
V
OUT
The MOSFET power dissipations at maximum output
current are given by:
P
MAIN _BUCK
=
V
OUT
V
IN
I
OUT(MAX)
( )
2
1+ δ
( )
R
DS(ON)
+
(V
IN
)
2
I
OUT(MAX)
2






(R
DR
)(C
MILLER
) •
1
V
INTVCC
− V
THMIN
+
1
V
THMIN






(f)
P
SYNC _BUCK
=
V
IN
− V
OUT
V
IN
I
OUT(MAX)
( )
2
1+ δ
( )
R
DS(ON)
P
MAIN _BOOST
=
V
OUT
− V
IN
( )
V
OUT
V
IN
2
I
OUT(MAX)
( )
2
•
1+ δ
( )
R
DS(ON)
+
V
2
OUT
V
IN








I
OUT(MAX)
2






•
R
DR
( )
C
MILLER
( )
•
1
V
INTVCC
− V
THMIN
+
1
V
THMIN






(f)
P
SYNC _BOOST
=
V
IN
V
OUT
I
OUT(MAX)
( )
2
1+ δ
( )
R
DS(ON)
where z is the temperature dependency of R
DS(ON)
and
RDR (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 main N-channel
equations for the buck and boost controllers include an
additional term for transition losses, which are highest at
high input voltages for the bucks and low input voltages for
the boost. For V
IN
< 20V (high V
IN
for the boost) the high
current efficiency generally improves with larger MOSFETs,
while for V
IN
> 20V (low V
IN
for the boost) 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