LTC1435ACS datasheet(8/20 Pages) LINER

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LTC1435A

APPLICATIONS INFORMATION

The value of COSC is calculated from the desired operating

frequency:

CpF

OSC ()

–













1.37(10 )

Frequency (kHz)

A graph for selecting COSC vs frequency is given in Figure

2. As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency (see Efficiency

Considerations). The maximum recommended switching

frequency is 400kHz.

greater core losses. A reasonable starting point for setting

ripple current is

∆IL=0.4(IMAX).Remember,themaximum

∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low current opera-

tion. The transition to low current operation begins when

the inductor current reaches zero while the bottom MOSFET

is on. 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.

The Figure 3 graph gives a range of recommended induc-

tor values vs operating frequency and VOUT.

OPERATING FREQUENCY (kHz)

300

250

200

150

100

200

300

400

1435A F02

500

Figure 2. Timing Capacitor Value

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

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

= ()( )









1–

Accepting larger values of

∆IL allows the use of low induc-

tances, but results in higher output voltage ripple and

Kool M

µ is a registered trademark of Magnetics, Inc.

For low duty cycle, high frequency applications where the

required minimum on-time,

ON MIN

OUT

IN MAX

()

= ()( )

is less than 350ns, there may be further restrictions on the

inductance to ensure proper operation. See Minimum On-

Time Considerations section for more details.

Inductor Core Selection

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

selected. High efficiency converters generally cannot afford

the core loss found in low cost powdered iron cores, forc-

ing the use of more expensive ferrite, molypermalloy or Kool

µ® cores. Actual core loss is independent of core size for

OPERATING FREQUENCY (kHz)

100

150

200

1435A F03

250

300

VOUT = 5.0V

VOUT = 3.3V

VOUT ≤ 2.5V

Figure 3. Recommended Inductor Values

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