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LTC3703

3703f

Multiple MOSFETs can be used in parallel to lower RDS(ON)

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 (5

Ω 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 VOUT/VIN

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:

CIN RMS

O MAX

OUT

()

(

)

–

≅









This formula has a maximum at VIN = 2VOUT, where IRMS

= IO(MAX)/2. This simple worst-case condition is com-

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.

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:

()

•%

fCR

RMS ALUM

ESR

≈

100

where RESR 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 COUT is primarily determined by the ESR

required to minimize voltage ripple. The output ripple

(

∆VOUT) is approximately equal to:

∆≤ ∆









I ESR

OUT

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