ZXLD1350

30V 350mA LED DRIVER with AEC-Q100

ZXLD1350

Document number: DS33468 Rev. 8 - 2

18 of 22

www.diodes.com

March 2011

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A Product Line of

Diodes Incorporated

Application Information (Continued)

Example:

For VIN =12V, L=47µH, rL=0.64V, VLED=3.4V, Iavg =350mA and VD =0.36V

tON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622µs

tOFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21µs

This gives an operating frequency of 546kHz and a duty cycle of 0.34.

These and other equations are available as a spreadsheet calculator from the Diodes website.

Note that in practice, the duty cycle and operating frequency will deviate from the calculated values due to dynamic switching

delays, switch rise/fall times and losses in the external components.

Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to

equalize the undershoot and overshoot and improves temperature stability of the output current.

Diode Selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse

leakage at the maximum operating voltage and temperature. The recommended diode for use with this part is the ZLLS1000.

This has approximately ten times lower leakage than standard Schottky diodes, which are unsuitable for use above 85°C. It

also provides better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time.

The table below gives the typical characteristics for the ZLLS1000:

Diode

Forward Voltage

@ 100mA (mV)

Continuous

Current

(mA)

Reverse Leakage

@ 30V 85°C

(mA)

Package

ZLLS1000

310

1000

300

TSOT23

If alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a

continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of

the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device.

The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the

LX output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including

supply ripple, does not exceed the specified maximum value.

Reducing Output Ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor Cled across the LED(s) as

shown below:

ZXLD1350

LED

Cled

L1

LX

I

SENSE

D1

R

S

V

IN

V

IN