LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 18

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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LTC3858-1
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:
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current effi ciency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
APPLICATIONS INFORMATION
18
MILLER
DR
Main Switch Duty Cycle
P
P
Synchronous S
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
, can be approximated from the gate charge curve
MILLER
=
=
( )
⎡
⎢
⎣
V
MILLER
V
V
V
V
OUT
IN
DS(ON)
IN
IN
INTVCC
–
is equal to the increase in gate charge
V
2
IN
(
V
⎛
⎝ ⎜
I
actually provides higher effi ciency. The
MAX
OUT
w w itch Duty Cycle
I
, Miller capacitance, C
MAX
2
IN
1
–
2 2
R losses while the topside N-channel
)
(
> 20V the transition losses rapidly
V
2
I
⎞
⎠ ⎟
THMIN
MAX
(
( )(
1 δ
R
+
DR
=
)
2
V
)
+
(
V
R
OUT
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
•
DS(ON)
f
−
V
THMIN
IN
)
DS(ON)
V
OUT
IN
, input
device
< 20V
is the
and
DS
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 D1 and D2 shown in Figure 10
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 effi ciency 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 continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
C
IN
IN
Re
and C
quired I
OUT
Selection
OUT
RMS
)(I
IN
is simplifi ed by the 2-phase architec-
OUT
≈
I
MAX
DS(ON)
V
) product needs to be used in the
IN
⎡
⎣
(
V
vs Temperature curve, but
OUT
OUT
)(
V
)/(V
IN
–
IN
V
). To prevent
OUT
)
IN
⎤ ⎤
⎦
1 2 /
. A 1A
38581f
(1)