Zxld1352, Application information – Diodes ZXLD1352 User Manual
Page 14
ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
14 of 21
December 2010
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information
(Continued)
Shutdown Mode
Taking the ADJ pin to a voltage below 0.2V will turn off the output and supply current will fall to a low standby level of 20µA
nominal.
Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above V
REF
will increase output current above the
100% nominal average value. (See graphs for details).
Soft-start
An external capacitor from the ADJ pin to ground will provide 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. 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 1µF 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