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LTC3616

3616f

APPLICATIONS INFORMATION

Frequency Synchronization

The LTC3616’s internal oscillator can be synchronized to

an external frequency by applying a square wave clock

signal to the RT/SYNC pin. During synchronization, the top

switch turn-on is locked to the falling edge of the external

frequency source. The synchronization frequency range

is 300kHz to 4MHz. During synchronization all operation

modes can be selected.

It is recommended that the regulator is powered down

(RUN pin to ground) before removing the clock signal

on the RT/SYNC pin in order to reduce inductor current

ripple.

AC coupling should be used if the external clock genera-

tor cannot provide a continuous clock signal throughout

start-up, operation and shutdown of the LTC3616. The

size of capacitor CSYNC depends on parasitic capacitance

on the RT/SYNC pin and is typically in the range of 10pF

to 22pF.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

ΔIL increases with higher VIN and decreases

with higher inductance:

ΔIL =

VOUT

fSW •L

⎛

⎝⎜

⎞

⎠⎟

•1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Having a lower ripple current reduces the core losses

in the inductor, the ESR losses in the output capacitors

and the output voltage ripple. A reasonable starting point

for selecting the ripple current is

ΔIL = 0.3 • IOUT(MAX).

The largest ripple current occurs at the highest VIN. To

guarantee that the ripple current stays below a specified

maximum, the inductor value should be chosen according

to the following equation:

L =

VOUT

fSW • ΔIL(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ •1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

The inductor value will also have an effect on Burst Mode

operation. The transition to low current operation begins

when the peak inductor current falls below a level set by the

burst clamp. Lower inductor values result in higher ripple

current which causes this to occur at lower load currents.

This causes a dip in efficiency in the upper range of low cur-

rent operation. In Burst Mode operation, lower inductance

values will cause the burst frequency to increase.

Inductor Core Selection

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

selected. Actual core loss is independent of core size for

fixed inductor value, but it is very dependent on the induc-

tance selected. As the inductance increases, core losses de-

crease. Unfortunately, increased inductance requires more

turns of wire and therefore, copper losses will increase.

LTC3616

SVIN

VIN

RT/SYNC

LTC3616

SVIN

VIN

0.4V

RT/SYNC

SGND

LTC3616

SVIN

fOSC

2.25MHz

fOSC

1/TP

fOSC 1/RT

VIN

RT/SYNC

SGND

1.2V

0.3V

LTC3616

SVIN

fOSC

1/TP

VIN

CSYNC

RT/SYNC

SGND

3616 F02

Figure 2. Setting the Switching Frequency

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