LTC3415 datasheet(14/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

To avoid noise coupling into VOSNS+, the resistor divider

should be placed near the VOSNS+ and VOSNS– pins and

physically close to the LTC3613. The remote output and

ground traces should be routed together as a differential

pair to the remote output. These traces should be termi-

nated as close as physically possible to the remote output

point that is to be accurately regulated through remote

differential sensing.

Switching Frequency Programming

The choice of operating frequency is a trade-off between

efficiency and component size. Lowering the operating fre-

quency improves efficiency by reducing MOSFET switching

losses but requires larger inductance and/or capacitance

to maintain low output ripple voltage. Conversely, raising

the operating frequency degrades efficiency but reduces

component size.

The switching frequency of the LTC3613 can be pro-

grammed from 200kHz to 1MHz by connecting a resistor

from the RT pin to signal ground. The value of this resistor

is given by the following empirical formula:

RT kΩ

[]=

41550

fkHz

[]

–2.2

RFB1

RFB2

CIN

VIN

COUT1

COUT2

3613 F02

ILOAD

OTHER CURRENTS

FLOWING IN

SHARED GROUND

PLANE

POWER TRACE

PARASITICS

±VDROP(PWR)

–

GROUND TRACE

PARASITICS

±VDROP(GND)

ILOAD

LTC3613

VOSNS+

PGND

PVIN

VOSNS–

Figure 2. Differential Output Sensing Used to Correct Line Loss Variations

in a High Power Distributed System with a Shared Ground Plane

Not counting resistor tolerances, the switching fre-

quency could still have a ±10% deviation from the ideal

programmed value. The internal PLL has a synchroniza-

tion range of ±30% around this programmed frequency.

Therefore, during external clock synchronization be sure

that the external clock frequency is within this ±30% range

of the RT programmed frequency. It is advisable that the

RT programmed frequency be equal to the external clock

for maximum synchronization margin. Refer to Phase and

Frequency Synchronization for further details.

Inductor Selection

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use of

smaller inductor and capacitor values. A higher frequency

generally results in lower efficiency because of MOSFET

gate charge losses and top MOSFET transition losses.

In addition to this basic trade-off, the effect of inductor

value on ripple current and low current operation must

also be considered.

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