LTC3859 Linear Technology, LTC3859 Datasheet - Page 22

• Manufacturer Part Number: LTC3859
• Description: Buck/Buck/Boost Synchronous Controller
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LTC3859
The peak-to-peak drive levels are set by the INTV
This voltage is typically 5.4V during start-up (see EXTV
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. Pay close
attention to the BV
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 manufacturers’ data
sheet. C
along the horizontal axis while the curve is approximately
fl at divided by the specifi ed change in V
then multiplied by the ratio of the application applied V
to the gate charge curve specifi ed V
operating in 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:
22
MILLER
Buck Main Switch Duty Cycle =
Buck Sync Switch Duty Cycle =
Boost Main Switch Duty Cycle
Boost Sync Switch Duty Cycle
P
(V
P
MAIN _ BUCK
SYNC _ BUCK
V
IN
INTVCC
)
, can be approximated from the gate charge curve
2
MILLER
I
OUT(MAX)
1
V
2
DS(ON)
is equal to the increase in gate charge
THMIN
V
V
V
DSS
OUT
IN
IN
V
, Miller capacitance C
(R
IN
specifi cation for the MOSFETs as
V
I
OUT(MAX)
DR
V
OUT
THMIN
)(C
1
I
MILLER
OUT(MAX)
(f)
2
V
V
V
V
V
IN
OUT
DS
V
IN
1
OUT
OUT
) •
IN
V
V
. When the IC is
DS
IN
OUT
V
2
OUT
. This result is
R
1
MILLER
V
DS(ON)
IN
CC
voltage.
R
, input
DS(ON)
CC
DS
where
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
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
current effi ciency generally improves with larger MOSFETs,
while for V
losses rapidly increase to the point that the use of a higher
R
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+ ) is generally given for a MOSFET in the
form of a normalized R
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
DS(ON)
= 0.005/°C can be used as an approximation for low
P
P
1
R
MAIN _ BOOST
SYNC _ BOOST
DR
device with lower C
is the temperature dependency of R
R
C
IN
DS(ON)
MILLER
> 20V (low V
IN
< 20V (high V
V
V
•
V
OUT
OUT
2
IN
IN
V
R losses while the main N-channel
V
DS(ON)
. A 1A to 3A Schottky is generally
2
V
INTVCC
OUT
IN
I
V
OUT(MAX)
IN
MILLER
IN
V
IN
for the boost) the transition
2
vs Temperature curve, but
1
I
IN
V
OUT(MAX)
V
OUT
actually provides higher
for the boost) the high
THMIN
2
2
www.DataSheet4U.com
I
1
OUT(MAX)
V
•
THMIN
THMIN
R
DS(ON)
DS(ON)
1
2
•
is the
(f)
and
3859f