LTC1735CF datasheet(12/32 Pages) LINER

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LTC1735

1735fc

The internal oscillator runs at its nominal frequency (fO)

when the FCB pin is pulled high to INTVCC or connected to

ground. Clocking the FCB pin above and below 0.8V will

cause the internal oscillator to injection lock to an external

clock signal applied to the FCB pin with a frequency

between 0.9fO and 1.3fO. The clock high level must exceed

1.3V for at least 0.3

µs and the clock low level must be less

than 0.3V for at least 0.3

µs. The top MOSFET turn-on will

synchronize with the rising edge of the clock.

Attempting to synchronize to too high an external fre-

quency (above 1.3fO) can result in inadequate slope com-

pensation and possible loop instability. If this condition

exists simply lower the value of COSC so fEXT = fO according

to Figure 2.

APPLICATIO S I FOR ATIO

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because of

MOSFET gate charge losses. In addition to this basic trade

off, the effect of inductor value on ripple current and low

current operation must also be considered.

The inductor value has a direct effect on ripple current. The

inductor ripple current

∆IL decreases with higher induc-

tance or frequency and increases with higher VIN or VOUT:

∆I

OUT













()( )

–

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL= 0.3 to 0.4(IMAX). Remember,

the maximum

∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low current

operation. The transition to low current operation begins

when the inductor current reaches zero while the bottom

MOSFET is on. Burst Mode operation begins when the

average inductor current required results in a peak current

below 25% of the current limit determined by RSENSE.

Lower inductor values (higher

∆IL) will cause this to occur

at higher load currents, which can cause a dip in efficiency

in the upper range of low current operation. In Burst Mode

operation, lower inductance values will cause the burst

frequency to decrease.

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot af-

ford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool M

µ® cores. Actual core loss is independent of

core size for a fixed inductor value, but it is very dependent

on inductance selected. As inductance increases, core

losses decrease. Unfortunately, increased inductance re-

quires more turns of wire and therefore copper losses will

increase.

Figure 2. Timing Capacitor Value

OPERATING FREQUENCY (kHZ)

100

200

300

400

500

600

1735 F02

100.0

87.5

75.0

62.5

50.0

37.5

25.0

12.5

When synchronized to an external clock, Burst Mode

operation is disabled but the inductor current is not

allowed to reverse. The 25% minimum inductor current

clamp present in Burst Mode operation is removed,

providing constant frequency discontinuous operation

over the widest possible output current range. In this

mode the synchronous MOSFET is forced on once every

10 clock cycles to recharge the bootstrap capacitor. This

minimizes audible noise while maintaining reasonably

high efficiency.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

Kool M

µ is a registered trademark of Magnetics, Inc.

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