SWITCHED-MODE DUAL VOLTAGE REGULATOR

Rev D1, Page 10/12

this internal resistance can determine the maximum

available output current (equation 4).

The EMI (electromagnetic interference) caused by the

coil should be taken into account. Toroidal core coils

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

RF chokes in the range of a few tens to a few hundreds

µH are suitable for modest EMI requirements.∗

Additional interference may be caused by decaying of

the voltage at VHL when the coil current drops to zero

(Fig. 6). Parasitic capacitances at VHL form an oscil-

lating circuit with the coil. This undesirable oscillating

circuit can be damped to an uncritical magnitude by

installing a resistor (> 10 kΩ) parallel to the coil.

lematic.

Due to the series regulators, the ripple of

the intermediate voltage VH does not affect the out-

put voltages VCC and VCCA. Therefore a low capac-

itance level without special demands on the internal

resistance is sufficient. A combination of electrolytic

and ceramic capacitor (e.g. 4.7 µF/100 nF) is recom-

mended. Tantalum capacitors are also possible when

they are allowed to operate at AC amplitudes like the

residual ripple of voltage VH.

The stability of the series regulators is guaranteed for

lected. The suppression of interference voltage is im-

proved by small capacitor series resistors. The combi-

nation of tantalum and ceramic capacitors is also rec-

ommended in this case. If one of the two outputs re-

mains open, its capacitor can be omitted.

To avoid feedback of interference from supply volt-

age VB onto output voltages VCC and VCCA, provide

blocking directly at pin VB. A combination of tantalum

and ceramic capacitors is also recommended in this

case (several µF/100 nF).

PRINTED CIRCUIT BOARD LAYOUT

The GND path from the switching regulator and from

each series regulator should be strictly separated to

avoid cross couplings. The neutral point of all GND

conductors is the GND connection at the iC-WD. It is

possible and not critical, however, to route the GND of

be very close to the pin VH however. To keep down the

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 blocking capacitors of supply voltage VB are to be

placed as close as possible to pins VB and GND. The

capacitors for the outputs VCC and VCCA should be

placed directly by the load and not directly by the iC

to also block interferences which are coupled via the

wiring to the load. A ground plane should be cut out

circuit conductor between VB, the shunt RVB, and VBR

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 Thermal Pad (optional with the SO8) should be

connected to an appropriate copper area on the PCB.

It has proven to be advantageous to use thermal vias

directly underneath the iC to transfer the power dissi-

pation to a different layer, e.g. a ground plane.

TOKO series 10RF459-. . . (SMD shielded)