LM3500 datasheet(14/18 Pages) NSC

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Application Information (Continued)

Typical Peak Inductor Currents (mA)

(V)

# LEDs

(in

series)

LED Current

2.7

100

134

160

204

234

118

138

190

244

294

352

142

174

244

322

191

232

319

413

3.3

116

136

172

198

110

126

168

210

250

290

132

158

212

270

320

183

216

288

365

446

4.2

116

142

162

102

116

148

180

210

246

122

146

186

232

272

318

179

206

263

324

388

456

CIN =COUT =1µF

L = 22 µH, 160 m

Ω DCR max. Coilcraft DT1608C-223

2 and 3 LED applications: LM3500-16 or LM3500-21; LED VF = 3.77V at

20mA; TA = 25˚C

4 LED applications: LM3500-16 or LM3500-21; LED VF = 3.41V at 20mA; TA

= 25˚C

5 LED applications: LM3500-21 only; LED VF = 3.28V at 20mA; TA = 25˚C

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

culated from the following equation:

where I

OUT is the total load current, FSW is the switching

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

pin current, is given by the following equation:

The maximum output current capability of the LM3500 can

be estimated with the following equation:

where I

CL is the current limit. Some recommended inductors

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 1µF ceramic output capacitor is sufficient.

Local bypassing for the input is needed on the LM3500.

Multilayer X7R or X5R ceramic capacitors with low ESR are

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

sufficient for most applications. However, for some applica-

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

proper startup of the LM3500. Using capacitors with low

ESR decreases input voltage ripple. For additional bypass-

ing, a 100nF 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

IN, as shown in Figure 1, must

be placed close to the device and connect between the V

and GND pins. This will reduce copper trace resistance

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

input voltage filtering, a 100nF bypass capacitor can be

placed in parallel with C

IN to shunt any high frequency noise

to ground. The output capacitor, C

OUT, should also be placed

close to the LM3500 and connected directly between the

OUT and GND pins. Any copper trace connections for the

OUT capacitor can increase the series resistance, which

directly effects output voltage ripple and efficiency. The cur-

rent setting resistor, R

LED, should be kept close to the FB pin

to minimize copper trace connections that can inject noise

into the system. The ground connection for the current set-

ting resistor should connect directly to the GND pin. The

AGND pin should connect directly to the GND pin. Not

connecting the AGND pin directly, as close to the chip as

possible, may affect the performance of the LM3500 and

limit its current driving capability. Trace connections made to

the inductor should be minimized to reduce power dissipa-

tion, EMI radiation and increase overall efficiency. It is good

practice to keep the V

SW routing away from sensitive pins

such as the FB pin. Failure to do so may inject noise into the

FB pin and affect the regulation of the device. See Figure 2

and Figure 3 for an example of a good layout as used for the

LM3500 evaluation board.

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