LTC1438/LTC1439
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APPLICATIONS INFORMATION
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3. Are the SENSE
–
and SENSE
+
leads routed together with
minimum PC trace spacing? The filter capacitors be-
tween SENSE
+
1 (SENSE
+
2) and SENSE
–
1 (SENSE
–
2)
should be as close as possible to the LTC1438/LTC1439.
4. Do the (+) plates of C
IN
connect to the drains of the
topside MOSFETs as closely as possible? This capacitor
provides the AC current to the MOSFETs.
5. Is the INTV
CC
decoupling capacitor connected closely
between
INTV
CC
and the power ground pin? This capaci-
tor carries the MOSFET driver peak currents.
6. Keep the switching nodes, SW1 (SW2), away from
sensitive small-signal nodes. Ideally the switch nodes
should be placed at the furthest point from the LTC1438/
LTC1439.
7. Use a low impedance source such as a logic gate to drive
the PLLIN pin and keep the lead as short as possible.
PC Board Layout Suggestions
Switching power supply printed circuit layouts are cer-
tainly among the most difficult analog circuits to design.
The following suggestions will help to get a reasonably
close solution on the first try.
The output circuits, including the external switching
MOSFETs, inductor, secondary windings, sense resistor,
input capacitors and output capacitors all have very large
voltage and/or current levels associated with them. These
components and the radiated fields (electrostatic and/or
electromagnetic) must be kept away from the very sensi-
tive control circuitry and loop compensation components
required for a current mode switching regulator.
The electrostatic or capacitive coupling problems can be
reduced by increasing the distance from the radiator,
typically a very large or very fast moving voltage signal.
The signal points that cause problems generally include:
the “switch” node, any secondary flyback winding voltage
and any nodes which also move with these nodes. The
switch, MOSFET gate and boost nodes move between V
IN
and PGND each cycle with less than a 100ns transition
time. The secondary flyback winding output has an AC
signal component of –V
IN
times the turns ratio of the
transformer, and also has a similar <100ns transition
time. The feedback control input signals need to have less
than a few millivolts of noise in order for the regulator to
perform properly. A rough calculation shows that 80dB of
isolation at 2MHz is required from the switch node for low
noise switcher operation. The situation is worse by a factor
of the turns ratio for the secondary flyback winding. Keep
these switch node related PC traces small and away from
the “quiet” side of the IC (not just above and below each
other on the opposite side of the board).
The electromagnetic or current loop induced feedback
problems can be minimized by keeping the high AC
current (transmitter) paths and the feedback circuit (re-
ceiver) path small and/or short. Maxwell’s equations are at
work here, trying to disrupt our clean flow of current and
voltage information from the output back to the controller
input. It is crucial to understand and minimize the suscep-
tibility of the control input stage as well as the more
obvious reduction of radiation from the high current
output stage(s). An inductive transmitter depends upon
the frequency, current amplitude and the size of the
current loop to determine the radiation characteristic of
the generated field. The current levels are set in the output
stage once the input voltage, output voltage and inductor
value(s) have been selected. The frequency is set by the
output stage transition times. The only parameter over
which we have some control is the size of the antenna we
create on the PC board, i.e., the loop. A loop is formed with
the input capacitance, the top MOSFET, the Schottky diode
and the path from the Schottky diode’s ground connection
and the input capacitor’s ground connection. A second
path is formed when a secondary winding is used com-
prising the secondary output capacitor, the secondary
winding and the rectifier diode or switching MOSFET (in
the case of a synchronous approach). These “loops”
should be kept as small and tightly packed as possible in
order to minimize their “far field” radiation effects. The
radiated field produced is picked up by the current com-
parator input filter circuit(s), as well as by the voltage
feedback circuit(s). The current comparator’s filter ca-
pacitor placed across the sense pins attenuates the radi-
ated current signal. It is important to place this capacitor
immediately adjacent to the IC sense pins. The voltage
sensing input(s) minimizes the inductive pickup compo-
nent by using an input capacitance filter to SGND. The
capacitors in both case serve to integrate the induced
