ZLED7x30 Datasheet

16

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

+

+

+

∆

=

To ensure stable output current, use a 1% accuracy resistor with adequate power tolerance and a good temperature

characteristic for Rs.

Table 3.1

Recommended Values for Sense Resistor Rs

Nominal Average Output Current (mA)

Value for RS

(Ω)

1200 (maximum for ZLED7030)

0.082

1000 (maximum for ZLED7330)

0.1

750 (maximum for ZLED7530)

0.13

667

0.15

350 (maximum for ZLED7730)

0.27

333

0.3

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 recommended 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 resisto

r in Ω

rL

L1 coil resistance in Ω

RLX

LX switch resistance in Ω

VD

D1 diode forward voltage at the required load current in V