LTC3703
15
3703fc
applicaTions inForMaTion
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve, and
typically varies from 0.005/°C to 0.01/°C depending on
the particular MOSFET used.
Multiple MOSFETs can be used in parallel to lower R
DS(ON)
and meet the current and thermal requirements if desired.
The LTC3703 contains large low impedance drivers capable
of driving large gate capacitances without significantly
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 (5Ω or less)
to reduce noise and EMI caused by the fast transitions.
Schottky Diode Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power MOS-
FETs. 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 junc-
tion 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
CIN(RMS)
≅I
O(MAX)
V
OUT
V
IN
V
IN
V
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
capacitor or to choose a capacitor rated at a higher tempera-
ture 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 available
in voltages above 30V, for regulators with input supplies
above 30V, choice of input capacitor type is limited to
ceramics or aluminum electrolytics. Ceramic capacitors
have the advantage of very low ESR and can handle high
RMS current, however ceramics with high voltage ratings
(>50V) are not available with more than a few microfarads
of capacitance. Furthermore, 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 recommended for their lower
voltage and temperature coefficients. Another consider
-
ation
when using ceramics is their high Q which if not
properly damped, may result in excessive voltage stress
on the power MOSFETs. Aluminum electrolytics have much
higher bulk capacitance, however, 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
RMS,ALUM
≈
1+(8fCR
ESR
)
2
• 100%
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
OUT
≤ ∆I
L
ESR+
1
8fC
OUT
