LTC3703-5
15
37035fa
Multiple MOSFETs can be used in parallel to lower R
DS(ON)
and meet the current and thermal requirements if desired.
The LTC3703-5 contains large low impedance drivers
capable of driving large gate capacitances without signifi-
cantly slowing transition times. In fact, when driving
MOSFETs with very low gate charge, it is sometimes
helpful to slow down the drivers by adding small gate
resistors (10
Ω
or less) to reduce noise and EMI caused by
the fast transitions.
Schottky Diode Selection
The Schottky diode D1 shown in the circuit on the first
page of this data sheet conducts during the dead time
between the conduction of the power MOSFETs. This
prevents the body diode of the bottom MOSFET from
turning on and storing charge during the dead time and
requiring a reverse recovery period that could cost as
much as 1% to 2% in efficiency. A 1A Schottky diode is
generally a good size for 3A to 5A regulators. Larger
diodes result in additional losses due to their larger
junction capacitance. The diode can be omitted if the
efficiency loss can be tolerated.
Input Capacitor Selection
In continuous mode, the drain current of the top MOSFET
is approximately a square wave of duty cycle V
OUT
/V
IN
which must be supplied by the input capacitor. To prevent
large input transients, a low ESR input capacitor sized for
the maximum RMS current is given by:
I
I
V
V
V
V
CIN RMS
O MAX
OUT
IN
IN
OUT
(
)
(
)
/
–
≅
⎛
⎝⎜
⎞
⎠⎟
1
1 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
O(MAX)
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that the ripple current ratings from
capacitor manufacturers are often based on only 2000 hours
of life. This makes it advisable to further derate the capaci-
tor or to choose a capacitor rated at a higher temperature
than required. Several capacitors may also be placed in
parallel to meet size or height requirements in the design.
Because tantalum and OS-CON capacitors are not avail-
able in voltages above 30V, ceramics or aluminum
electrolytics must be used for regulators with input sup-
plies above 30V. Ceramic capacitors have the advantage of
very low ESR and can handle high RMS current, but
ceramics with high voltage ratings (>50V) are not available
with more than a few microfarads of capacitance. Further-
more, ceramics have high voltage coefficients which means
that the capacitance values decrease even more when used
at the rated voltage. X5R and X7R type ceramics are rec-
ommended for their lower voltage and temperature coef-
ficients. Another consideration when using ceramics is
their high Q which, if not properly damped, may result in
excessive voltage stress on the power MOSFETs. Alumi-
num electrolytics have much higher bulk capacitance, but
they have higher ESR and lower RMS current ratings.
A good approach is to use a combination of aluminum
electrolytics for bulk capacitance and ceramics for low
ESR and RMS current. If the RMS current cannot be
handled by the aluminum capacitors alone, when used
together, the percentage of RMS current that will be
supplied by the aluminum capacitor is reduced to
approximately:
%
(
)
•
%
,
I
fCR
RMS ALUM
ESR
≈
+
1
1
8
100
2
where R
ESR
is the ESR of the aluminum capacitor and C is
the overall capacitance of the ceramic capacitors. Using an
aluminum electrolytic with a ceramic also helps damp the
high Q of the ceramic, minimizing ringing.
Output Capacitor Selection
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple. The output ripple
(
∆
V
OUT
) is approximately equal to:
∆
≤ ∆
+
⎛
⎝⎜
⎞
⎠⎟
V
I ESR
fC
OUT
L
OUT
1
8
Since
∆
I
L
increases with input voltage, the output ripple is
highest at maximum input voltage. ESR also has a signifi-
cant effect on the load transient response. Fast load
transitions at the output will appear as voltage across the
ESR of C
OUT
until the feedback loop in the LTC3703-5 can
change the inductor current to match the new load current
APPLICATIO S I FOR ATIO
W
U
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