LTC3826 Linear Technology, LTC3826 Datasheet - Page 17

• Manufacturer Part Number: LTC3826
• Description: 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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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 LTC3826: 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
This voltage is typically 5V during start-up (see EXTV
Connection). Consequently, logic-level threshold MOSFETs
must be used in most applications. The only exception
is if low input voltage is expected (V
logic level threshold MOSFETs (V
used. Pay close attention to the BV
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
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
MILLER
, can be approximated from the gate charge curve
MILLER
DS(ON)
is equal to the increase in gate charge
, Miller capacitance C
GS(TH)
DSS
IN
DS
< 5V); then, sub-
DS
. When the IC is
< 3V) should be
specifi cation for
. This result is
MILLER
CC
voltage.
, input
CC
Pin
DS
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.
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
a short-circuit when the synchronous switch is on close
to 100% of the period.
DR
Main Switch Duty Cycle
P
P
Synchronous Switch Duty Cycle
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
=
=
( )
V
V
V
V
MILLER
IN
INTVCC
V
OUT
IN
IN
2
– V
V
IN
(
I
I
MAX
MAX
OUT
actually provides higher effi ciency. The
1
– V
2
2
IN
R losses while the topside N-channel
)
THMIN
(
> 20V the transition losses rapidly
2
I
(
MAX
R
(
1+
DR
=
)
+
)
2
V
)
(
(
V
V
OUT
C
R
1+
IN
THMIN
MILLER
DS(ON)
1
)
=
R
DS(ON)
LTC3826
V
+
)
( )
IN
f
•
–
V
DS(ON)
IN
THMIN
V
DS(ON)
OUT
IN
17
device
< 20V
is the
3826fc
and