iC-WD A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 10/12
this internal resistance can determine the maximum To avoid feedback of interference from supply volt-
available output current (equation 4).
age VB onto output voltages VCC and VCCA, provide
blocking directly at pin VB. A combination of tantalum
The EMI (electromagnetic interference) caused by the and ceramic capacitors is also recommended in this
coil should be taken into account. Toroidal core coils case (several µF/100 nF).
have little noise radiation but are expensive and difficult
to install. Bar cores are reasonably priced and easy
to handle but emit higher radiation. Reasonably priced
PRINTED CIRCUIT BOARD LAYOUT
The GND path from the switching regulator and from
RF chokes in the range of a few tens to a few hundreds
each series regulator should be strictly separated to
µH are suitable for modest EMI requirements.∗
avoid cross couplings. The neutral point of all GND
conductors is the GND connection at the iC-WD. It is
Additional interference may be caused by decaying of
possible and not critical, however, to route the GND of
the voltage at VHL when the coil current drops to zero
the supply VB and the base point of capacitor CVH to-
(Fig. 6). Parasitic capacitances at VHL form an oscil-
gether to the neutral point. The capacitor CVH should
lating circuit with the coil. This undesirable oscillating
be very close to the pin VH however. To keep down the
circuit can be damped to an uncritical magnitude by
installing a resistor (> 10 kΩ) parallel to the coil.
decay at the open end of the coil (pin VHL), the capac-
itance of this connection should be low, that means the
connection should be short.
The selection of the backup capacitor CVH is unprob-
lematic. Due to the series regulators, the ripple of
The blocking capacitors of supply voltage VB are to be
the intermediate voltage VH does not affect the out-
placed as close as possible to pins VB and GND. The
put voltages VCC and VCCA. Therefore a low capac-
capacitors for the outputs VCC and VCCA should be
itance level without special demands on the internal
placed directly by the load and not directly by the iC
resistance is sufficient. A combination of electrolytic
to also block interferences which are coupled via the
and ceramic capacitor (e.g. 4.7 µF/100 nF) is recom-
wiring to the load. A ground plane should be cut out
mended. Tantalum capacitors are also possible when
underneath the wiring of CVCC and CVCCA. The printed
they are allowed to operate at AC amplitudes like the
circuit conductor between VB, the shunt RVB, and VBR
residual ripple of voltage VH.
should have a low impedance, since voltage drops in
the supply path change the effective size of the shunt
and reduce the maximum cut-off current.
The stability of the series regulators is guaranteed for
the entire load range when the values for CVCC and
CVCCA given in the electrical characteristics are se-
lected. The suppression of interference voltage is im- The Thermal Pad (optional with the SO8) should be
proved by small capacitor series resistors. The combi- connected to an appropriate copper area on the PCB.
nation of tantalum and ceramic capacitors is also rec- It has proven to be advantageous to use thermal vias
ommended in this case. If one of the two outputs re- directly underneath the iC to transfer the power dissi-
mains open, its capacitor can be omitted.
pation to a different layer, e.g. a ground plane.
∗ e.g.: Siemens Matsushita B78108-S1224-J (220 µH/250 mA, axial leads), TDK series NLC565050T-. . . (SMD),
TOKO series 10RF459-. . . (SMD shielded)
ICHAUS [ IC-HAUS GMBH ]
