Datasheet

LTC3859
23
3859fa
APPLICATIONS INFORMATION
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
specifi cation 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
fl at divided by the specifi ed change in V
DS
. This result is
then multiplied by the ratio of the application applied V
DS
to the gate charge curve specifi ed 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 2) 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 effi ciency 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
effi ciency. The synchronous MOSFET losses for the buck
controllers 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
synchronous MOSFET losses for the boost controller are
greatest when the input voltage approaches the output volt-
age or during an overvoltage event when the synchronous
switch is on 100% of the period.
The term (1+ z) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs Temperature curve, but
z = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes D4, D5, and D6 shown in
Figure 13 conduct during the dead-time between the
conduction of the two power MOSFETs. This prevents
the body diode of the synchronous MOSFET from turning
on, storing charge during the dead-time and requiring a
reverse recovery period that could cost as much as 3%
in effi ciency at high V
IN
. A 1A to 3A Schottky is generally