LTC3703
applicaTions inForMaTion
turethanrequired.Severalcapacitorsmayalsobeplacedin
parallel to meet size or height requirements in the design.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
vs temperature curve, and
DS(ON)
typically varies from 0.005/°C to 0.01/°C depending on
the particular MOSFET used.
BecausetantalumandOS-CONcapacitorsarenotavailable
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
onthepowerMOSFETs.Aluminumelectrolyticshavemuch
higher bulk capacitance, however, they have higher ESR
and lower RMS current ratings.
Multiple MOSFETs can be used in parallel to lower R
DS(ON)
and meet the current and thermal requirements if desired.
TheLTC3703containslargelowimpedancedriverscapable
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
downthedriversbyaddingsmallgateresistors(5Ωorless)
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.
A good approach is to use a combination of aluminum
electrolyticsforbulkcapacitanceandceramicsforlowESR
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:
Input Capacitor Selection
1
%IRMS,ALUM
≈
•100%
In continuous mode, the drain current of the top MOSFET
2
1+ (8fCRESR
)
is approximately a square wave of duty cycle V /V
OUT 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:
where R
is the ESR of the aluminum capacitor and C
ESR
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.
1/2
–1
VOUT
V
IN
ICIN(RMS) ≅ IO(MAX)
V
V
OUT
IN
Output Capacitor Selection
The selection of C
is primarily determined by the ESR
This formula has a maximum at V = 2V , where I =
RMS
O(MAX)
OUT
IN
OUT
required to minimize voltage ripple. The output ripple
I
/2. This simple worst-case condition is commonly
(∆V ) is approximately equal to:
usedfordesignbecauseevensignificantdeviationsdonot
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
capacitorortochooseacapacitorratedatahighertempera-
OUT
1
∆VOUT ≤ ∆IL ESR+
8fCOUT
3703fc
15
Linear Systems [ Linear Systems ]
