Application information (continued) – Diodes ZXLD1350 User Manual
Page 16
ZXLD1350
30V 350mA LED DRIVER with AEC-Q100
ZXLD1350
Document number: DS33468 Rev. 8 - 2
16 of 22
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
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
Taiyo Yuden
Keme