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LTC3703

3703f

The oscillator can also be synchronized to an external

clock applied to the MODE/SYNC pin with a frequency in

the range of 100kHz to 600kHz (refer to the MODE/SYNC

Pin section for more details). In this synchronized mode,

Pulse Skip Mode operation is disabled. The clock high

level must exceed 2V for at least 25ns. As shown in

Figure 8, the top MOSFET turn-on will follow the rising

edge of the external clock by a constant delay equal to one-

tenth of the cycle period.

ripple current occurs at the highest VIN. To guarantee that

ripple current does not exceed a specified maximum, the

inductor in buck mode should be chosen according to:

OUT

LMAX

OUT

IN MAX

≥

∆









()

–

The inductor also has an affect on low current operation

when Pulse Skip Mode operation is enabled. The fre-

quency begins to decrease when the output current drops

below the average inductor current at which the LTC3703

is operating at its tON(MIN) in discontinuous mode (see

Figure 6). Lower inductance increases the peak inductor

current that occurs in each minimum on-time pulse and

thus increases the output current at which the frequency

starts decreasing.

Power MOSFET Selection

The LTC3703 requires at least two external N-channel

power MOSFETs, one for the top (main) switch and one or

more for the bottom (synchronous) switch. The number,

type and “on” resistance of all MOSFETs selected take into

account the voltage step-down ratio as well as the actual

position (main or synchronous) in which the MOSFET will

be used. A much smaller and much lower input capaci-

tance MOSFET should be used for the top MOSFET in

applications that have an output voltage that is less than

1/3 of the input voltage. In applications where VIN >> VOUT,

the top MOSFETs’ “on” resistance is normally less impor-

tant for overall efficiency than its input capacitance at

operating frequencies above 300kHz. MOSFET manufac-

turers have designed special purpose devices that provide

reasonably low “on” resistance with significantly reduced

input capacitance for the main switch application in switch-

ing regulators.

Selection criteria for the power MOSFETs include the “on”

resistance RDS(ON), input capacitance, breakdown voltage

and maximum output current.

The most important parameter in high voltage applica-

tions is breakdown voltage BVDSS. Both the top and

bottom MOSFETs will see full input voltage plus any

additional ringing on the switch node across its drain-to-

source during its off-time and must be chosen with the

3703 F08

2V TO 10V

MODE/

SYNC

tMIN = 25ns

0.8T

0.1T

D = 40%

T = 1/fO

APPLICATIO S I FOR ATIO

Figure 8. MODE/SYNC Clock Input and Switching

Waveforms for Synchronous Operation

Inductor

The inductor in a typical LTC3703 circuit is chosen for a

specific ripple current and saturation current. Given an

input voltage range and an output voltage, the inductor

value and operating frequency directly determine the

ripple current. The inductor ripple current in the buck

mode is:

∆=









OUT

()( )

–

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Thus highest efficiency operation is obtained at low

frequency with small ripple current. To achieve this how-

ever, requires a large inductor.

A reasonable starting point is to choose a ripple current

between 20% and 40% of IO(MAX). Note that the largest

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