LM3501 datasheet(17/26 Pages) TI1

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ICL

KD'

IOUT

VIND

2LFSW

IOUT

KD'

IL(AVE)

IOUT

KD'

IPK

VIND

2LFSW

LM3501

www.ti.com

SNVS230B – MAY 2004 – REVISED MAY 2005

Table 1. Typical Peak Inductor Current (mA)(1) (continued)

LED Current

VIN

# LEDs

(V)

(in series)

4.2

116

142

162

102

116

148

180

210

246

122

146

186

232

272

318

179

206

263

324

388

456

The typical cycle-by-cycle peak inductor current can be calculated from the following equation:

(7)

where IOUT is the total load current, FSW is the switching frequency, L is the inductance and η is the converter

efficiency 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 VSW pin current, is given by the following

equation:

(8)

The maximum output current capability of the LM3501 can be estimated with the following equation:

(9)

where ICL 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 minimize output voltage ripple. Multilayer X7R or X5R type

ceramic 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

applications 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 bypassing, a 100 nF ceramic capacitor

can be used to shunt high frequency ripple on the input. Some recommended capacitors include but are not

limited to:

TDK C2012X7R1C105K

Taiyo-Yuden EMK212BJ105 G

LAYOUT CONSIDERATIONS

The input bypass capacitor CIN, as shown in Figure 3, must be placed close to the device and connect between

the VIN 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 CIN to shunt any

high frequency noise to ground. The output capacitor, COUT, should also be placed close to the LM3501 and

connected directly between the VOUT and GND pins. Any copper trace connections for the COUT capacitor can

increase the series resistance, which directly effects output voltage ripple and efficiency. The current setting

resistor, RLED, 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 setting resistor should connect directly to the GND pin. The

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