LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 13

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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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 8 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
APPLICATIONS INFORMATION
DR
P
P
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
=
=
( )
⎡
⎢
⎣
V
MILLER
V
V
V
V
OUT
IN
IN
IN
INTVCC
–
V
2
IN
(
V
⎛
⎝ ⎜
I
actually provides higher effi ciency. The
MAX
OUT
I
MAX
2
IN
1
2 2
–
R losses while the topside N-channel
)
(
> 20V the transition losses rapidly
DS(ON)
V
2
I
THMIN
⎞
⎠ ⎟
MAX
(
( )(
1 δ
R
+
DR
)
2
vs Temperature curve, but
)
+
( )
R
I
+
C
DS ON
V
MILLER
TH
δ
(
1
M M IN
R
D D S ON
)
+
⎤
⎥
⎦
(
)
( )
•
DS(ON)
f
THMIN
)
DS(ON)
IN
device
< 20V
is the
and
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.
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
This formula has a maximum at V
= I
used for design because even signifi cant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3835-1, ceramic capacitors
can also be used for C
if there is any question.
The selection of C
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
IN
OUT
C Required I
ΔV
and C
IN
OUT
/2. This simple worst-case condition is commonly
RIPPLE
OUT
≈
I
RIPPLE
increases with input voltage.
Selection
OUT
RIPPLE
RMS
OUT
⎛
⎜
⎝
) is approximated by:
ESR
IN
≈
. Always consult the manufacturer
is driven by the effective series
is the ripple current in the induc-
I
MAX
V
+
IN
8
fC
⎡
⎣
(
1
OUT
V
IN
OUT
OUT
LTC3835-1
= 2V
⎞
⎟
⎠
)/(V
)(
OUT
V
OUT
IN
IN
is the output
). To prevent
–
, where I
V
OUT
IN
13
. A 1A
)
38351fc
⎤ ⎤
⎦
RMS
1 2 /