LTC3851 LINER [Linear Technology], LTC3851 Datasheet - Page 14

• Manufacturer Part Number: LTC3851
• Description: Synchronous Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

LTC3851
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 shown on the fi rst page
conducts 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
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
Soft-Start and Tracking
The LTC3851 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 is confi gured to
soft-start by itself, a capacitor should be connected to
the TK/SS pin. The LTC3851 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
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
IN
. A 1A to 3A Schottky is generally a good size due to
MILLER
actually provides higher effi ciency. The
2
IN
R losses while the topside N-channel
> 20V, the transition losses rapidly
DS(ON)
vs Temperature curve, but
DS(ON)
IN
device
< 20V,
3851f