ZLED7x20 Datasheet

© 2016 Integrated Device Technology, Inc.

18

April 20, 2016

)

R

r

R

(

*

I

V

V

I

*

L

t

LX

L

S

AVG

LED

IN

ON

+

+

−

−

∆

=

)

r

R

(

*

I

V

V

I

*

L

t

L

S

AVG

D

LED

OFF

+

+

+

∆

=

3.3.2

Inductor L1

The recommended range for the L1 inductor is 33

μH to 220μH. 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. If the application

requires a high supply voltage and low output current, inductance values at the high end of this range are recom-

mended to minimize errors due to switching delays, which can reduce efficiency and increase ripple on the output.

Also see section 3.4 for layout considerations for L1. Equations (3) and (4) can be used to calculate tON and tOFF.

On Time for LX Switch (tONmin>200ns):

(3)

Off Time for LX Switch (tOFFmin>200ns):

(4)

Where:

Symbol

Description

L

L1 coil inductance in H

ΔI

L1 coil peak-peak ripple current in A (internally set to 0.3

∗ I

AVG)

VIN

Supply voltage in V

VLED

Total forward voltage in V for LED string

IAVG

Nominal average LED current in A

Rs

External current sense resistor in Ω

rL

L1 coil resistance in Ω

RLX

LX switch r

esistance in Ω

VD

D1 diode forward voltage at the required load current in V

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 relationship of the summed

LED forward voltages n * VF to the input voltage VIN is a reasonable approximation. Because the input voltage is a

factor in the on time, variations in the input voltage affect the switching frequency and duty cycle.

To achieve optimum performance, duty cycles close to 0.5 at the nominal average supply voltage are preferable

for improving the temperature stability of the output current.