ML4950

6

After the appropriate inductor value is chosen, it is

necessary to find the minimum inductor current rating

required. Peak inductor current is determined from the

following formula:

I

tV

L

LPEAK

ON MAX

IN MAX

MIN

()

()

(

)

=

™

(3)

In the single cell application previously described, a

maximum input voltage of 1.6V would give a peak current

of 383mA. When comparing various inductors, it is

important to keep in mind that suppliers use different

criteria to determine their ratings. Many use a conservative

current level, where inductance has dropped to 90% of its

normal level. In any case, it is a good idea to try inductors

of various current ratings with the ML4950 to determine

which inductor is the best choice. Check efficiency and

maximum output current, and if a current probe is

available, look at the inductor current to see if it looks like

the waveform shown in Figure 3. For additional

information, see Application Note 29.

Suitable inductors can be purchased from the following

suppliers:

Coilcraft

(847) 639-6400

Coiltronics

(561) 241-7876

Dale

(605) 665-9301

Sumida

(847) 956-0666

XFMRS, Inc. (317) 834-1066

OUTPUT CAPACITOR

The choice of output capacitor is also important, as it

controls the output ripple and optimizes the efficiency of

the circuit. Output ripple is influenced by three

parameters: capacitance, ESR, and ESL. The contribution

due to capacitance can be determined by looking at the

change in capacitor voltage required to store the energy

delivered by the inductor in a single charge-discharge

cycle, as determined by the following formula:

tV

LC

V

V

OUT

ON

IN

OUT

IN

=

™

™™ ™

-

22

2

16

(4)

For a 1.2V input, a 2.5V output, a 27µH inductor, and a

47µF capacitor, the expected output ripple due to

capacitor value is 11mV.

Capacitor Equivalent Series Resistance (ESR) and

Equivalent Series Inductance (ESL), also contribute to the

output ripple due to the inductor discharge current

waveform. Just after the NMOS transistor turns off, the

output current ramps quickly to match the peak inductor

current. This fast change in current through the output

capacitor’s ESL causes a high frequency (5ns) spike that

can be over 1V in magnitude. After the ESL spike settles,

the output voltage still has a ripple component equal to

the inductor discharge current times the ESR. This

component will have a sawtooth shape and a peak value

equal to the peak inductor current times the ESR. ESR also

losses during the discharge cycle.

An output capacitor with a capacitance of 100µF, an ESR

of less than 0.1

W, and an ESL of less than 5nH is a good

general purpose choice. Tantalum capacitors which meet

these requirements can be obtained from the following

suppliers:

AVX

(207) 282-5111

Sprague

(207) 324-4140

If ESL spikes are causing output noise problems, an EMI

filter can be added in series with the output.

INPUT CAPACITOR

Unless the input source is a very low impedance battery, it

will be necessary to decouple the input with a capacitor

with a value of between 47µF and 100µF. This prevents

input ripple from affecting the ML4950 control circuitry,

during the charge and discharge cycles of the inductor.

Again, a low ESR capacitor (such as tantalum) is

recommended.

SETTING THE OUTPUT VOLTAGE

The adjustable output can be set to any voltage between

2V and 3V by connecting a resistor divider to the SENSE

V

RR

R

+

02

12

2

.

16

(5)

equation:

RR

12

02

1

=™

-





.

(6)