ZXLD1350

30V 350mA LED DRIVER with AEC-Q100

ZXLD1350

Document number: DS33468 Rev. 8 - 2

16 of 22

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March 2011

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Application Information (Continued)

Soft-start

The device has inbuilt soft-start action due to the delay through the PWM filter. An external capacitor from the ADJ pin to

ground will provide additional soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-on

threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. With no external

capacitor, the time taken for the output to reach 90% of its final value is approximately 500

μs. Adding capacitance increases

this delay by approximately 0.5ms/nF.

The graph below shows the variation of soft-start time for different values of capacitor.

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so the device will not be

damaged, unlike in many boost converters, where the back EMF may damage the internal switch by forcing the drain above

its breakdown voltage.

Capacitor selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply

source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and

smooth the current ripple on the input supply. A minimum value of 1

μF is acceptable if the input source is close to the device,

but higher values will improve performance at lower input voltages, especially when the source impedance is high. The input

capacitor should be placed as close as possible to the IC.

For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended.

Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.

A table of recommended manufacturers is provided below:

Manufacturer

Website

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Kemet

www.kemet.com

AVX

www.avxcorp.com