ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 16

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LTC3850-2
APPLICATIONS INFORMATION
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
16
MILLER
DR
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
P
P
SYNC
MAIN
(approximately 2Ω) is the effective driver resistance
, can be approximated from the gate charge curve
MILLER
=
=
( )
⎡
⎢
⎢
⎣
V
V
V
V
MILLER
IN
V
OUT
INTVCC
IN
IN
–
2
V
DS(ON)
is equal to the increase in gate charge
IN
⎛
⎝ ⎜
(
V
I
I
OUT
MAX
actually provides higher effi ciency. The
MAX
–
2 2
1
2
IN
R losses while the topside N-channel
V
, Miller capacitance C
(
)
TH MIN
> 20V the transition losses rapidly
⎞
⎠ ⎟
I
2
MAX
(
(
(
R
1 δ
DR
+
) )
V
2
)
V
OUT
)(
+
(
)
IN
1 δ R
R
C
V
+
DS ON
MILLER
T T H MIN
(
)
1
(
DS
V
DS ON
)
IN
. When the IC is
DS
+
)
)
(
– V
V
•
⎤
⎥
⎥
⎦
. This result is
IN
•
TH(MIN)
DS(ON)
MILLER
)
OUT
f
OSC
DS(ON)
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 conduct during the dead time
between the conduction of the two power MOSFETs. These
prevent the body diodes of the bottom MOSFETs from turn-
ing 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
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.
Soft-Start and Tracking
The LTC3850-2 has the ability to either soft-start by itself
with a capacitor or track the output of another channel or
external supply. When one particular channel is confi gured
to soft-start by itself, a capacitor should be connected to
its TK/SS pin. This channel is in the shutdown state if its
RUN pin voltage is below 1.2V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.2V, the channel pow-
ers up. A soft-start current of 1.3μA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defi ned to be the voltage range from 0V to 0.8V on the
TK/SS pin. The total soft-start time can be calculated as:
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
t
SOFTSTART
= 0.8 •
IN
1.3µA
C
DS(ON)
. A 1A to 3A Schottky is generally
SS
vs Temperature curve, but
38502f