ADP1147-3.3/ADP1147-5

–8–

REV. 0

The formula used to calculate the continuous operating fre-

quency is:

f

=

1

−

4

× C

tual measured value of the output voltage. When in regulation

decreases as the input voltage decreases. The ADP1147 will

1.5 volts. This is to eliminate the possible occurrence of audible

switching prior to dropout.

Now that the operating frequency has been determined and the

can be computed. The inductor L should be chosen so it will

ripple current.

The following equation is used to determine the required value

for inductor L:

25 mV

=

or

25 mV

tor value of:

5

× R

The ESR requirements for the output storage capacitor can be

relaxed by increasing the inductor value, but efficiency due to

copper losses will be reduced. Conversely, the use of too low an

inductance may allow the inductor current to become discon-

tinuous, causing the device to enter the power savings mode

prematurely. As a result of this the power savings threshold is

lowered and the efficiency at lower current levels is severely

reduced.

Inductor Core Considerations

Now that the minimum inductance value for L has been deter-

mined, the inductor core selection can be made. High efficiency

converters generally cannot afford the core losses found in low

cost powdered iron cores. This forces the use of a more expen-

sive ferrite, molypermalloy, or Kool Mu

efficiency in Figure 1 reflects the use of a molypermalloy core.

The cost of the inductor can be cut in half by Using a Kool Mu

core type CTX 50-4 by Coiltronics, but the efficiency will be

approximately 1%–2% less. The actual core losses are not de-

pendent on the size of the core, but on the amount of induc-

tance. An increase in inductance will yield a decrease in the

amount of core loss. Although this appears to be desirable, more

inductance requires more turns of wire with added resistance

and greater copper losses.

Kool Mu is a registered trademark of Magnetics, Inc.

Using a ferrite cores in a design can produce very low core

losses, allowing the designer to focus on minimizing copper loss

and core saturation problems. Ferrite cores exhibit a condition

known as “Hard Saturation,” which results in an abrupt collapse

of the inductance when the peak design current is exceeded.

This causes the inductor ripple current to rise sharply, the out-

put ripple voltage to increase and the power savings mode of

operation to be erroneously activated. To prevent this from

occurring the core should never be allowed to saturate.

Molypermalloy (from Magnetics, Inc.) is a very good, low loss

core material for a toroids, but is more expensive than a ferrite

core. A reasonable compromise between price and performance,

from the same manufacturer is Kool Mu. Toroidal cores are

extremely desirable where efficient use of available space and

several layers of wire are required. They are available in various

surface mount configurations from Coiltronics Inc. and other

companies.

Power MOSFET Selection and Considerations

The ADP1147 requires the use of an external P-channel

MOSFET. The major parameters to be considered when select-

The minimum input voltage determines if the design requires a

logic level or a standard threshold MOSFET. In applications

where the input voltage is > 8 volts, a standard threshold

< 2.5 volts is recommended. Note: If a logic level MOSFET

is selected, the supply voltage to the ADP1147 must not

(e.g., <

± 8 volts for IRF7304).

sumption is made that when the ADP1147 is operating in the

continuous mode, either the Schottky Diode or the MOSFET

are always conducting the average load current. The following

formulas are used to determine the duty cycle of each of the

components.

=

Schottky Diode Duty Cycle

=

Power MOSFET can be determined by:

(refer to Efficiency section). MOSFETS usually specify the 1+

δ

δ can be

approximated to 0.007/

°C for most low voltage MOSFETs.

Output Diode Considerations

When selecting the output diode careful consideration should be

given to peak current and average power dissipation so the

maximum specifications for the diode are not exceeded.