Page 6

February, 1997 Toko, Inc.

TK65025

standpoint for the TK65025, this is essentially guaran-

teed when using a single battery cell to power the

converter.

Now, plugging in worst case conditions, the inductor

value can be determined by simply transforming the

above equation in terms of “L”:

L

V

I MIN

()

MIN

()

2 f

MAX

()

where “V

voltage at about two-thirds of the peak diode current

value. The peak diode current is the same as the peak

input current, the peak switch current, and the peak

inductor current. The formula is:

I

V

f L

(4)

Some reiteration is implied because “L” is a function

of “V

of “L”. The best way into this loop is to first approximate

“V

new “V

Then, if necessary, reiterate.

When selecting the actual inductor, it is necessary to

make sure that the peak current rating of the inductor (i.e.,

the current which causes the core to saturate) is greater

than the maximum peak current that the inductor will

encounter. To determine the maximum peak current, use

Eq. (4) again, but this time plugging in maximum values for

“V

It may also be necessary when selecting the inductor to

check the rms current rating of the inductor. Whereas

peak current rating is determined by core saturation, rms

current rating is determined by wire size and power

dissipation in the wire resistance.

The inductor rms

current is given by:

I

L RMS

I

V

3

(5)

where “I

used to check against inductor peak current rating, and

the term in the numerator within the radical that is added

to the [on-time] duty ratio, “D”, is the off-time duty ratio.

Toko America, Inc. offers a wide range of inductor

values and sizes to accommodate varying power level

requirements. The following series of Toko inductors

work especially well with the TK65025: 10RF, 12RF, 3DF,

D73, and D75.

The 5CA series can be used for

isolated-output applications, although such design objec-

tives are not considered here.

Other Converter Components

In choosing a diode, parameters worthy of consider-

ation are: forward voltage, reverse leakage, and capaci-

tance. The biggest efficiency loss in the converter is due

to the diode forward voltage. A schottky diode is typically

chosen to minimize this loss. Possible choices for Schottky

diodes are: LL103A from ITT MELF case; 1N5017 from

Motorola (through hole case); MBR0530 from Motorola

(surface mount) or 15QS02L from Nihon EC (surface

mount).

Reverse leakage current is generally higher in schottkys

than in pin-junction diodes. If the converter spends a good

deal of the battery lifetime operating at very light load (i.e.,

the system under power is frequently in a standby mode),

then the reverse leakage current could become a substan-

tial fraction of the entire average load current, thus degrad-

ing battery life. So don’t dramatically oversize the schottky

diode if this is the case.

Diode capacitance isn’t likely to make much of an

undesirable contribution to switching loss at this relatively

low switching frequency. It can, however, increase the

snubber dissipation requirement.

The snubber (optional) is composed of a series RC

network from the switch pin to ground (or to the output or

input if preferred). Its function is to dampen the resonant

LC circuit which rings during the inductor current deadtime.

When the current flowing in the inductor through the output

diode decays to zero, the parasitic capacitance at the

switch pin from the switch, the diode, and the inductor

winding has energy which rings back into the inductor,

flowing back into the battery. If there is no snubbing, it is

feasible that the switch pin voltage could ring below

ground. Although the IC is well protected against latchup,

this ringing may be undesirable due to radiated noise. In

order to do an effective job, the snubber capacitor should

be large (e.g., 5~20 times) in comparison to the parasitic

capacitance. If it is unnecessarily large, then it dissipates

extra energy every time the converter switches.

The

resistor of the snubber should be chosen such that it drops

a substantial voltage as the ringing parasitic capacitance

attempts to pull the snubber capacitor along for the ride. If

the resistor is too small (e.g., zero), then the snubber

capacitance just adds to the ringing energy. If the resistor

is too large (e.g., infinite) then it effectively disengages the

snubber capacitor from fighting the ringing.

The output capacitor, the capacitor connected from the