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LTC3875
APPLICATIONS INFORMATION
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
V
(
)
2
OUT
=
P
I
MAIN
MAX
V
IN
I
( )
2
MAX
V
IN
2
1
V
– V
INTVCC
MILLER
V
– V
(
IN
OUT
=
P
I
MAX
SYNC
V
IN
where δ is the temperature dependency of R
is the effective top driver resistance (approximately 2Ω at
V
= V
), V
is the drain potential and the change
GS
MILLER
IN
in drain potential in the particular application. V
is the data sheet specified typical gate threshold voltage
specified in the power MOSFET data sheet at the speci-
fied drain current. C
MILLER
using the gate charge curve from the MOSFET data sheet
and the technique described above. Both MOSFETs have
2
I
R losses while the topside N-channel equation includes
an additional term for transition losses, which peak at
the highest input voltage. For V
rent efficiency generally improves with larger MOSFETs,
while for V
> 20V, the transition losses rapidly increase
IN
to the point that the use of a higher R
lower C
actually provides higher efficiency. The
MILLER
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
DS(ON)
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
24
V
OUT
V
IN
V
– V
IN
OUT
V
IN
(
)
1+ δ
+
R
DS(ON)
(
)
(
)
R
C
•
DR
MILLER
1
+
• f
V
MILLER
)
2
(
)
1+ δ
R
DS(ON)
DS(ON)
is the calculated capacitance
< 20V, the high cur-
IN
device with
DS(ON)
vs temperature curve, but
For more information
An optional Schottky diode across the synchronous
MOSFET conducts during the dead time between the
conduction of the two large power MOSFETs. This pre-
vents the body diode of the bottom MOSFET from turning
on, storing charge during the dead time and requiring a
reverse-recovery period which could cost as much as sev-
eral percent in efficiency. A 2A to 8A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition loss due to their larger junction
capacitance.
Soft-Start and Tracking
The LTC3875 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 configured
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 1.14V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
, R
DR
Once the RUN pin voltage is above 1.22V, the channel pow-
ers up. A soft-start current of 1.25µA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
MILLER
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
fold-back is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defined to be the voltage range from 0V to 0.6V on the
TK/SS pin. The total soft-start time can be calculated as:
t
SOFTSTART
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode
up to TK/SS = 0.5V. Between TK/SS = 0.5V and 0.56V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.56V. The output ripple
is minimized during the 60mV forced continuous mode
window ensuring a clean PGOOD signal.
When the channel is configured to track another supply,
the feedback voltage of the other supply is duplicated by
www.linear.com/LTC3875
C
SS
= 0.6 •
1.25µA
3875fb
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