LTC3835 Linear Technology, LTC3835 Datasheet - Page 14

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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LTC3835
APPLICATIONS INFORMATION
inductance 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 the
LTC3835: One N-channel MOSFET for the top (main)
switch, and one N-channel MOSFET for the bottom (syn-
chronous) 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
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
14
MILLER
Main Switch Duty Cycle
Synchronous Switc
, can be approximated from the gate charge curve
MILLER
DS(ON)
is equal to the increase in gate charge
, Miller capacitance C
h h Duty Cycle
=
V
V
OUT
IN
GS(TH)
=
DSS
IN
DS
V
IN
< 5V); then, sub-
. When the IC is
DS
specifi cation for
< 3V) should be
–
V
. This result is
IN
MILLER
V
OUT
CC
voltage.
, input
CC
Pin
DS
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.
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 D1 shown in Figure 6 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
to 3A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition losses
due to their larger junction capacitance.
DR
P
P
SYNC
MAIN
(approximately 2Ω) is the effective driver resistance
=
=
( )
⎡
⎢
⎣
V
V
V
V
MILLER
IN
V
INTVCC
IN
OUT
IN
2
–
V
IN
⎛
⎝ ⎜
V
(
I
I
OUT
MAX
actually provides higher effi ciency. The
MAX
1
–
2 2
2
IN
R losses while the topside N-channel
V
(
)
THMIN
DS(ON)
> 20V the transition losses rapidly
⎞
⎠ ⎟
I
2
MAX
(
(
R
1 δ
DR
+
)
2
+
vs Temperature curve, but
)(
(
)
V
1 δ
R
C
TH
+
DS ON
MILLER
1
M M IN
(
)
R
D D S ON
)
⎤
⎥
⎦
+
)
( )
(
f
•
DS(ON)
THMIN
)
DS(ON)
IN
IN
device
< 20V
. A 1A
is the
3835fc
and