ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 14

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LTC3851-1
APPLICATIONS INFORMATION
The peak-to-peak drive levels are set by the INTV
This voltage is typically 5V during start-up. Consequently,
logic-level threshold MOSFETs must be used in most ap-
plications. The only exception is if low input voltage is ex-
pected (V
(V
BV
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:
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
14
MILLER
DR
GS(TH)
DSS
Main Switch Duty Cycle =
P
P
Synchronous
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
specifi cation for the MOSFETs as well; most of the
, can be approximated from the gate charge curve
MILLER
< 3V) should be used. Pay close attention to the
IN
=
=
< 5V); then, sub-logic level threshold MOSFETs
( )
⎡
⎢
⎢
⎣
V
V
V
V
V
OUT
IN
IN
DS(ON)
INTVCC
IN
–
V
is equal to the increase in gate charge
2
IN
S S witch Duty Cycle =
(
⎛
⎝ ⎜
V
I
MAX
I
OUT
MAX
, Miller capacitance, C
2 2
–
1
)
V
(
2
⎞
⎠ ⎟
I
TH MIN
MAX
(
( )(
1 δ
R
(
+
DR
)
V
2
)
V
)
OUT
(
R
1 1 +
C
IN
+
DS ON
MILLER
V
δ R
(
T T H MIN
)
DS
1
(
)
DS ON
V
. When the IC is
DS
+
)
IN
•
(
)
. This result is
– V
V
⎤
⎥
⎥
⎦
MILLER
TH(MIN)
IN
( )
)
f
DS(ON)
CC
OUT
voltage.
, input
is the
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 effi ciency 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
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 diode conducts during the dead time
between the conduction of the two power MOSFETs. This
prevents the body diode of the bottom MOSFET 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 size 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 LTC3851-1 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or external supply. When the LTC3851-1 is confi gured
to soft-start by itself, a capacitor should be connected to
the TK/SS pin. The LTC3851-1 is in the shutdown state if
the RUN pin voltage is below 1.25V. TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.25V, the LTC3851-1
powers up. A soft-start current of 1μA 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
MILLER
actually provides higher effi ciency. The
2
IN
R losses while the topside N-channel
IN
> 20V, the transition losses rapidly
DS(ON)
. A 1A to 3A Schottky is generally
vs Temperature curve, but
DS(ON)
IN
device
< 20V,
38511f