D' =

=1-D

0.58

L >

D

D'

-1

0.29

L >

D

D'

-1

LM3501

SNVS230B – MAY 2004 – REVISED MAY 2005

www.ti.com

In the event that the primary LED network is disconnected from the LM3501-21, the output voltage will increase

and be limited to 20.5V (typ.). There is a 1V hysteresis associated with this circuitry which will cause the output

to fluctuate between 20.5V and 19.5V (typ.) if the primary network is disconnected. In the event that the network

is reconnected regulation will begin at the appropriate output voltage. The 20.5V limit allows the use of 25V 1 µF

ceramic output capacitors.

RELIABILITY AND THERMAL SHUTDOWN

The maximum continuous pin current for the 8 pin thin micro SMD package is 535 mA. When driving the device

section for average switch current). To preserve the long term reliability of the device the average switch current

should not exceed 535 mA.

The LM3501 has an internal thermal shutdown function to protect the die from excessive temperatures. The

thermal shutdown trip point is typically 150°C. There is a hysteresis of typically 35°C so the die temperature must

decrease to approximately 115°C before the LM3501 will return to normal operation.

INDUCTOR SELECTION

The inductor used with the LM3501 must have a saturation current greater than the cycle by cycle peak inductor

current (see Typical Peak Inductor Currents table below). Choosing inductors with low DCR decreases power

losses and increases efficiency.

The minimum inductor value required for the LM3501-16 can be calculated using the following equation:

(4)

The minimum inductor value required for the LM3501-21 can be calculated using the following equation:

(5)

the NMOS power switch found in the Typical Performance Characteristics 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 recommendation for

use with most applications. Bench-level verification of circuit performance is required in these special cases,

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

(6)

LED Current

VIN

# LEDs

15

20

30

40

50

60

(V)

(in series)

mA

mA

mA

mA

mA

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

(1)

16

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