LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 19

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
enough so that it will not affect main loop compensation
and transient behavior.
To avoid noise coupling into the feedback voltages
(V
placed close to the V
V
should be routed together as a differential pair to the
remote output. For best accuracy, these traces to the
remote output and ground should be connected as close
as possible to the desired regulation point.
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 LTC3838 can be pro-
grammed from 200kHz to 2MHz by connecting a resistor
FB2
OUTSENSE1
and SGND pins. Remote output and ground traces
+ or V
FB2
), the resistor dividers should be
OUTSENSE1
V
R
OUTSENSE1
Figure 2. Differential Output Sensing Used to Correct Line Loss Variations
in a High Power Distributed System with a Shared Ground Plane
FB2
LTC3838
R
+
+
FB1
V
and V
OUTSENSE1
OUTSENSE1
–
M
M
T
B
–
, or
L
C
IN
+
–
from the RT pin to signal ground. The value of this resistor
can be chosen according to the following formula:
The overall controller system, including the clock PLL
and switching channels, has a synchronization range of
no less than ±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 the external clock for
maximum synchronization margin. Refer to the “Phase
and Frequency Synchronization” section for more details.
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. 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.
V
C
IN
OUT1
R
OTHER CURRENTS
T
SHARED GROUND
GROUND TRACE
POWER TRACE
[ ]
±V
±V
FLOWING IN
PARASITICS
PARASITICS
kΩ
DROP(PWR)
DROP(GND)
PLANE
I
LOAD
=
f kHz
41550
[
]
– 2.2
C
OUT2
I
LOAD
3838 F02
LTC3838
19
3838fa