The minimum inductor value required for the LM3501-16 can

be calculated using the following equation:

The minimum inductor value required for the LM3501-21 can

be calculated using the following equation:

For both equations above, L is in µH, V

of the chip in Volts, R

power switch found in the Typical Performance Characteris-

tics section in ohms and D is the duty cycle of the switching

regulator. The above equation is only valid for D greater than

or equal to 0.5. For applications where the minimum duty

cycle is less than 0.5, a 22 µH inductor is the typical recom-

mendation for use with most applications. Bench-level veri-

fication of circuit performance is required in these special

cases, however. The duty cycle, D, is given by the following

equation:

where V

Typical Peak Inductor Current (mA)(Note 10)

VIN

(V)

# LEDs

(in

series)

LED Current

15

mA

20

mA

30

mA

40

mA

50

mA

60

mA

2.7

2

82

100

134

160

204

234

3

118

138

190

244

294

352

4

142

174

244

322

X

X

5

191

232

319

413

X

X

3.3

2

76

90

116

136

172

198

3

110

126

168

210

250

290

4

132

158

212

270

320

X

5

183

216

288

365

446

X

4.2

2

64

76

96

116

142

162

3

102

116

148

180

210

246

4

122

146

186

232

272

318

5

179

206

263

324

388

456

L = 22 µH, 160 m

Ω DCR max. Coilcraft DT1608C-223

= 25˚C

The typical cycle-by-cycle peak inductor current can be cal-

culated from the following equation:

where I

frequency, L is the inductance and

η is the converter effi-

ciency of the total driven load. A good typical number to use

for

η is 0.8. The value of η can vary with load and duty cycle.

The average inductor current, which is also the average V

SW

pin current, is given by the following equation:

The maximum output current capability of the LM3501 can

be estimated with the following equation:

where I

include but are not limited to:

Coilcraft DT1608C series

Coilcraft DO1608C series

TDK VLP4612 series

TDK VLP5610 series

TDK VLF4012A series

CAPACITOR SELECTION

Choose low ESR ceramic capacitors for the output to mini-

mize output voltage ripple. Multilayer X7R or X5R type ce-

ramic capacitors are the best choice. For most applications,

a 1 µF ceramic output capacitor is sufficient.

Local bypassing for the input is needed on the LM3501.

Multilayer X7R or X5R ceramic capacitors with low ESR are

a good choice for this as well. A 1 µF ceramic capacitor is

sufficient for most applications. However, for some applica-

tions at least a 4.7 µF ceramic capacitor may be required for

proper startup of the LM3501. Using capacitors with low

ESR decreases input voltage ripple. For additional bypass-

ing, a 100 nF ceramic capacitor can be used to shunt high

frequency ripple on the input. Some recommended capaci-

tors include but are not limited to:

TDK C2012X7R1C105K

Taiyo-Yuden EMK212BJ105 G

LAYOUT CONSIDERATIONS

The input bypass capacitor C

be placed close to the device and connect between the V

IN

and GND pins. This will reduce copper trace resistance

which effects the input voltage ripple of the IC. For additional

input voltage filtering, a 100 nF bypass capacitor can be

placed in parallel with C

to ground. The output capacitor, C

close to the LM3501 and connected directly between the

V

C

directly effects output voltage ripple and efficiency. The cur-

rent setting resistor, R

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