LT4180
13
4180fb
For more information www.linear.com/4180
applicaTions inForMaTion
Using Guard Rings
The LT4180 includes a total of four track/holds in the
Virtual Remote Sense path. For best accuracy, all leakage
sources on the CHOLD pins should be minimized.
At very low dither frequencies, the circuit board layout
may include guard rings which should be tied to their
respective guard ring drivers.
To better understand the purpose of guard rings, a simplified
model of hold capacitor leakage (with and without guard
rings) is shown in Figure 11. Without guard rings, a large
difference voltage may exist between the hold capacitor
(Pin 1) node and adjacent conductors (Pin 2) producing
substantial leakage current through the leakage resistance
(R
LKG
). By adding a guard ring driver with approximately
the same voltage as the voltage on the hold capacitor node,
the difference voltage across R
LKG1
is reduced substantially
thereby reducing leakage current on the hold capacitor.
Figure 12. Clock Interface for Synchronization
OSCSYNC
4180 F12
LT4180
REGULATOR
Figure 11. Simplified Leakage Models
(with and without Guard Rings)
4180 F11
1 2
R
LKG
WITH
GUARD RING
WITHOUT
GUARD RING
1 2
R
LKG1
R
LKG2
Spread Spectrum Operation
Virtual Remote Sense functionality relies on sampling
techniques. Because switching power supplies are com-
monly used, the LT4180 uses a variety of techniques to
minimize potential interference (in the form of beat notes
which may occur between the dither frequency and power
supply switching frequency). Besides several types of
internal filtering, and the option for Virtual Remote Sense/
power supply synchronization, the LT4180 also provides
spread spectrum operation.
By enabling spread spectrum operation, low modula-
tion index pseudo-random phasing is applied to
Virtual Remote Sense timing. This has the effect of
converting any remaining narrow-band interference into
broadband noise, reducing its effect.
Increasing Voltage Correction Range
Correction range may be slightly improved by regulating
INTV
CC
to 5V. This may be done by placing an LDO between
V
IN
and INTV
CC
. Contact Linear Technology Applications
for more information.
Synchronization
Linear and switching power supplies and regulators may
be used with the LT4180. In most applications regulator
interference should be negligible. For those applications
where accurate control of interference spectrum is de-
sirable, an oscillator output has been provided so that
switching supplies may be synchronized to the LT4180
(Figure 12). The OSC pin was designed so that it may di-
rectly
connect to most regulators, or drive opto-isolators
(for isolated power supplies).
