LTC3858 Linear Technology, LTC3858 Datasheet - Page 19

• Manufacturer Part Number: LTC3858
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

applicaTions inForMaTion
exception is if low input voltage is expected (V
then, sub-logic level threshold MOSFETs (V
should be used. Pay close attention to the BV
fication 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
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
flat divided by the specified change in V
then multiplied by the ratio of the application applied V
to the gate charge curve specified 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:
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
MILLER
DR
Main Switch Duty Cycle
Synchronous S
P
P
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
, can be approximated from the gate charge curve
MILLER
=
=
( )



V
V
V
V
V
OUT
IN
DS(ON)
IN
IN
INTVCC
is equal to the increase in gate charge
–
V
2
IN
(
V

 
I
MAX
OUT
w w itch Duty Cycle
I
, Miller capacitance, C
MAX
1
2 2
–
)
V
(
2
I
THMIN

 
MAX
(
( )(
1 δ
R
+
DR
=
)
2
V
)
+
(
V
OUT
R
1 δ
IN
C
V
+
DS ON
MILLER
TH
(
1
)
DS
M M IN
=
R
)
. When the IC is
D D S ON
DS
V
+
IN



)
(
. This result is
( )
MILLER
•
f
−
GS(TH)
V
THMIN
)
IN
DS(ON)
V
DSS
OUT
IN
< 4V);
speci-
, input
is the
< 3V)
and
DS
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
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
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes D3 and D4 shown in Figure 12
conduct during the dead-time between the conduction of
the two power MOSFETs. This prevents the body diode of
the bottom 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 V
to 3A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition losses
due to their larger junction capacitance.
C
The selection of C
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can be
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (V
formula shown in Equation 1 to determine the maximum
RMS capacitor current requirement. Increasing the out-
put current drawn from the other controller will actually
decrease the input RMS ripple current from its maximum
value. The out-of-phase technique typically reduces the
input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
IN
and C
OUT
MILLER
Selection
OUT
)(I
IN
actually provides higher efficiency. The
is simplified by the 2-phase architec-
OUT
2
IN
R losses while the topside N-channel
> 20V the transition losses rapidly
DS(ON)
) product needs to be used in the
vs Temperature curve, but
www.DataSheet4U.com
LTC3858
DS(ON)
IN
IN

< 20V
device
. A 1A
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