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LTC1436A

LTC1436-PLL-A/LTC1437A

APPLICATIONS INFORMATION

OPERATING FREQUENCY (kHz)

300

250

200

150

100

200

300

400

1436 F02

500

VPLLLPF = 0V

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:

CpF

Frequency kHz

OSC

()=

()













137 10

)

–

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. When using Figure 2 for

synchronizable applications, choose COSC correspond-

ing to a frequency approximately 30% below your center

frequency. (See Phase-Locked Loop and Frequency Syn-

chronization.)

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.

OPERATING FREQUENCY (kHz)

100

150

200

1436 F03

250

300

VOUT = 5V

VOUT = 3.3V

VOUT ≤ 2.5V

Figure 3. Recommended Inductor Values

∆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 = 0.4 (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. 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 (TGS pin open),

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.

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