ZLED7010

40V LED Driver with Temperature Compensation

Data Sheet

August 12, 2010

© 2010 Zentrum Mikroelektronik Dresden AG — Rev. 1.1

All rights reserved. The material contained herein may not be reproduced, adapted, merged, translated, stored, or used without the

prior written consent of the copyright owner. The information furnished in this publication is PRELIMINARY and subject to changes

without notice.

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3

Application Circuit Design

3.1.

External Component – Inductor L1

Select the inductor value for L1 as needed to ensure that switch on/off times are optimized across the load

current and supply voltage ranges. Select a coil that has a continuous current rating above the required

average output current to the LEDs and a saturation current exceeding the peak output current.

Recommendation: Use inductors in the range of 15μH to 220μH with saturation current greater than 1A for

700mA output current or saturation current greater than 500mA for 350mA output current. For higher supply

voltages with low output current, select higher values of inductance, which result in a smaller change in output

current across the supply voltage range (refer to the graphs in section 1.4). See section 8.8 for layout

restrictions.

Equations (3) and (4) illustrate calculating the timing for LX switching for the example application circuit shown

200ns

.



LX

L

S

AVG

LED

IN

ON

R

r

R

I

V

V

I

L

T

















Where

L

Coil inductance in H

∆I

Coil peak-peak ripple current in A *

Total LED forward voltage in V

Diode forward voltage at the

required load current in V

Required average LED current in A

External current sense resistance in Ω

Coil resistance in Ω

Supply voltage in V

Switch resistance in Ω



L

S

AVG

D

LED

OFF

r

R

I

V

V

I

L

T















(3)

(4)

The inductance value has an equivalent effect on Ton and Toff and therefore affects the switching frequency.

For the same reason the inductance has no influence on the duty cycle for which the relation of the summed

is a factor in the ON time, variations in the input voltage affect the switching frequency and duty cycle.