LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 15

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Multiple MOSFETs can be used in parallel to lower R
and meet the current and thermal requirements if desired.
The LTC3703 contains large low impedance drivers ca-
pable 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 (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
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
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:
This formula has a maximum at V
= I
monly 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
temperature than required. Several capacitors may also be
placed in parallel to meet size or height requirements in the
design.
I
O(MAX)
CIN RMS
(
/2. This simple worst-case condition is com-
)
≅
I
O MAX
(
U
)
V
V
OUT
IN
U
⎛
⎜
⎝
V
V
OUT
IN
IN
W
= 2V
–
⎟ 1
⎞
⎠
1 2
OUT
/
, where I
U
OUT
DS(ON)
/V
RMS
IN
Because tantalum and OS-CON capacitors are not avail-
able 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 ca-
pacitance values decrease even more when used at the
rated voltage. X5R and X7R type ceramics are recom-
mended for their lower voltage and temperature coeffi-
cients. 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,
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 approxi-
mately:
where R
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
required to minimize voltage ripple. The output ripple
(∆V
%
∆
OUT
V
I
RMS ALUM
OUT
) is approximately equal to:
ESR
,
≤ ∆
is the ESR of the aluminum capacitor and C is
I ESR
L
⎛
⎜
⎝
≈
OUT
1 8
is primarily determined by the ESR
+
+
(
8
fC
fCR
1
1
OUT
ESR
⎞
⎟
⎠
)
2
•
100
LTC3703
%
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