LTC3613
APPLICATIONS INFORMATION
+
C
IN
V
C
IN
PV
IN
–
POWER TRACE
PARASITICS
L
LTC3613
+
SW
V
DROP(PWR)
–
V
V
OSNS
I
OSNS
PGND
LOAD
C
OUT1
OUT2
I
LOAD
R
R
FB1
FB2
GROUND TRACE
PARASITICS
V
DROP(GND)
OTHER CURRENTS
FLOWING IN
SHARED GROUND
PLANE
3613 F02
Figure 2. Differential Output Sensing Used to Correct Line Loss Variations
in a High Power Distributed System with a Shared Ground Plane
+
To avoid noise coupling into V
, the resistor divider
Not counting resistor tolerances, the switching fre-
quency could still have a 107 deviation from the ideal
programmed value. The internal PLL has a synchroniza-
tion range of 307 around this programmed frequency.
Therefore, during external clock synchronization be sure
thattheexternalclockfrequencyiswithinthis 307range
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.
OSNS
+
–
should be placed near the V
and V
pins and
OSNS
OSNS
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
efficiencyandcomponentsize.Loweringtheoperatingfre-
quencyimprovesefficiencybyreducingMOSFETswitching
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.
Inductor Selection
The operating frequency and inductor selection are inter-
relatedinthathigheroperatingfrequenciesallowtheuseof
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.
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:
41550
R kꢂ =
–2.2
[
]
T
f kHz
[
]
3613fa
14
Linear Systems [ Linear Systems ]
