English
Language : 

LTC3859_15 Datasheet, PDF (23/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller
LTC3859
APPLICATIONS INFORMATION
The peak-to-peak drive levels are set by the INTVCC voltage.
This voltage is typically 5.4V during start-up (see EXTVCC
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. Pay close
attention to the BVDSS 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 RDS(ON), Miller capacitance CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in VDS. This result is
then multiplied by the ratio of the application applied VDS
to the gate charge curve specified VDS. 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 = VOUT
VIN
Buck Sync Switch Duty Cycle = VIN − VOUT
VIN
Boost Main Switch Duty Cycle = VOUT − VIN
VOUT
Boost Sync Switch Duty Cycle = VIN
VOUT
The MOSFET power dissipations at maximum output
current are given by:
( ) ( ) PMAIN_BUCK
=
VOUT
VIN
IOUT(MAX )
2
1+ δ RDS(ON) +
(VIN
)2
⎛
⎝⎜
IOUT(MAX
2
)
⎞
⎠⎟
(RDR
)(CMILLER
)
•
⎡
⎢
⎣
VINTVCC
1
−
VTHMIN
+
1
VTHMIN
⎤
⎥(f)
⎦
( ) ( ) PSYNC _BUCK
=
VIN
− VOUT
VIN
IOUT(MAX )
2
1+ δ
RDS(ON)
( ) ( ) PMAIN_BOOST =
VOUT − VIN
VIN2
VOUT
2
IOUT(MAX) •
( ) 1+ δ
RDS(ON)
+
⎛
⎜
⎝
V
2
OUT
VIN
⎞
⎟
⎠
⎛
⎝⎜
IOUT(MAX
2
)
⎞
⎠⎟
•
( )( ) RDR
CMILLER
•
⎡
⎢
⎣
VINTVCC
1
−
VTHMIN
+
1
VTHMIN
⎤
⎥(f)
⎦
( ) ( ) PSYNC _BOOST
=
VIN
VOUT
IOUT(MAX )
2
1+ δ RDS(ON)
where z is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTHMIN is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I2R 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 VIN < 20V (high VIN for the boost) the high
current efficiency generally improves with larger MOSFETs,
while for VIN > 20V (low VIN for the boost) the transition
losses rapidly increase to the point that the use of a higher
RDS(ON) device with lower CMILLER actually provides higher
efficiency. 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 RDS(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 efficiency at high VIN. A 1A to 3A Schottky is generally
3859fa
23