Datasheet

LTC3835-1
13
38351fc
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
IR
V
I
MAIN
OUT
IN
MAX DS ON
IN
MAX
=
()
+
()
+
()
2
2
1 δ
()
22
11
⎛
⎝
⎜
⎞
⎠
⎟
()( )
+
RC
VVV
DR MILLER
INTVCC THMIN TH
•
–
MMIN
SYNC
IN OUT
IN
MAX
f
P
VV
V
IIR
⎡
⎣
⎢
⎤
⎦
⎥
()
=
()
+
()
–
2
δ
DDS ON()
where δ is the temperature dependency of R
DS(ON)
and
R
DR
(approximately 2Ω) 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.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs Temperature curve, but
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
could cost as much as 3% in effi ciency at high V
IN
. A 1A
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
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
OUT
)/(V
IN
). To prevent
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:
C RequiredI
IN RMS
≈
()( )
⎡
⎣
I
V
VVV
MAX
IN
OUT IN OUT
–
⎤⎤
⎦
12/
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
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
IN
. Always consult the manufacturer
if there is any question.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
OUT
) is approximated by:
ΔVI ESR
fC
OUT RIPPLE
OUT
≈+
⎛
⎝
⎜
⎞
⎠
⎟
1
8
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increases with input voltage.