LTC3851
14
3851fb
The peak-to-peak drive levels are set by the INTV
CC
voltage.
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
IN
< 5V); then, sub-logic level threshold MOSFETs
(V
GS(TH)
< 3V) should be used. Pay close attention to the
BV
DSS
specifi cation for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R
DS(ON)
, Miller capacitance, C
MILLER
, input
voltage and maximum output current. Miller capacitance,
C
MILLER
, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C
MILLER
is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
fl at divided by the specifi ed change in V
DS
. This result is
then multiplied by the ratio of the application applied V
DS
to the gate charge curve specifi ed V
DS
. When the IC is
operating in continuous mode, the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle =
V
OUT
V
IN
Synchronous Switch Duty Cycle =
V
IN
–V
OUT
V
IN
The MOSFET power dissipations at maximum output
current are given by:
P
MAIN
=
V
OUT
V
IN
I
MAX
()
2
1+δ
()
R
DS(ON)
+
V
IN
()
2
I
MAX
2
⎛
⎝
⎜
⎞
⎠
⎟
R
DR
()
C
MILLER
()
•
1
V
INTVCC
–V
TH(MIN)
+
1
V
TH(MIN)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
(f)
P
SYNC
=
V
IN
–V
OUT
V
IN
I
MAX
()
2
1+δ
()
R
DS(ON)
where δ is the temperature dependency of R
DS(ON)
and
R
DR
(approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V
TH(MIN)
is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V,
the high current effi ciency generally improves with larger
MOSFETs, while for V
IN
> 20V, the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
MILLER
actually provides higher effi ciency. The
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
DS(ON)
vs Temperature curve, but
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
IN
. A 1A to 3A Schottky is generally
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 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 1A 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
APPLICATIONS INFORMATION
