LTC1539 LINER [Linear Technology], LTC1539 Datasheet - Page 24

• Manufacturer Part Number: LTC1539
• Description: Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1538-AUX/LTC1539
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 out-
put 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 gen-
erated 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 comprising 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 pro-
24
U
INFORMATION
U
W
U
duced is picked up by the current comparator input filter
circuit(s), as well as by the voltage feedback circuit(s). The
current comparator’s filter capacitor placed across the
sense pins attenuates the radiated 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 component by using an input capaci-
tance filter to SGND. The capacitors in both case serve to
integrate the induced current, reducing the susceptibility
to both the “loop” radiated magnetic fields and the trans-
former or inductor leakage fields.
The capacitor on INTV
high transient currents to the bottom gates and to re-
charge the boost capacitor. This capacitor should be a
4.7 F tantalum capacitor placed as close as possible to the
INTV
MOSFET gates exceeds 1A. The power ground pin of the
IC, connected to this capacitor, should connect directly to
the lower plates of the output capacitors to minimize the
AC ripple on the INTV
The previous instructions will yield a PC layout which has
three separate ground regions returning separately to the
bottom plates of the output capacitors: a signal ground, a
MOSFET gate/INTV
input capacitors, Schottky diode and synchronous
MOSFET. In practice, this may produce a long power
ground path from the input and output capacitors. A long,
low resistance path between the input and output capaci-
tor power grounds will not upset the operation of the
switching controllers as long as the signal and power
grounds from the IC pins does not “tap in” along this path.
CC
and PGND pins of the IC. Peak current driving the
CC
CC
CC
ground and the ground from the
IC power supply.
acts as a reservoir to supply the