Datasheet

LTC3827
17
3827ff
APPLICATIONS INFORMATION
selected. As inductance increases, core losses go down.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for each
controller in the LTC3827: one N-channel MOSFET for the
top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
CC
voltage.
This voltage is typically 5V during start-up (see EXTV
CC
Pin Connection). Consequently, logic-level
threshold
MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
IN
< 5V);
then, sub-logic level threshold MOSFETs (V
GS(TH)
< 3V)
should be used. Pay close attention to the BV
DSS
speci-
fi 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+ 
()
R
DS(ON)
+
V
IN
()
2
I
MAX
2






R
DR
()
C
MILLER
()
•
1
V
INTVCC
–V
THMIN
+
1
V
THMIN






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 2) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V
THMIN
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
a short-circuit when the synchronous switch is on close
to 100% of the period.