ZLED7010 Datasheet

© 2016 Integrated Device Technology, Inc.

12

April 20, 2016

(

)

LX

L

S

AVG

LED

IN

ON

R

r

R

I

V

V

I

L

T

+

+

∗

−

−

∗

=

∆

(

)

L

S

AVG

D

LED

OFF

r

R

I

V

V

I

L

T

+

∗

+

+

∗

=

∆

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 on

page 2. As given in section 1.3, the minimum period for TON is 200ns; the minimum period for TOFF is also 200ns.

LX Switch OFF Time (TOFF in s)

(3)

LX Switch ON Time (TON in s)

(4)

*

With the ZLED7010, the current ripple ∆I is internally set to an appropriate value of 0.3 * IAVG.

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 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.

Where

L

Coil inductance in H

∆I

Coil peak-peak ripple current in A *

VLED

Total LED forward voltage in V

VD

Diode forward voltage at the

required load current in V

IAVG

Required average LED current in A

RS

External current sense resistance in Ω

rL

Coil resistance in Ω

VIN

Supply voltage in V

RLX

Switch resistance in Ω