Diodes ZXLD1322 - Manual (Page 15)

ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
15 of 17
www.diodes.com
April 2010
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A Product Line of
Diodes Incorporated
Additional Notes
Note that the ON time is set by the time it takes the coil to reach the peak current. This peak value is reduced by the control
loop to give the desired LED power, so the ON time can vary over a wide range. The minimum coil current can be zero
(discontinuous operation) or finite (continuous operation) depending on the supply voltage, LED current and the LED
voltage. The OFF time is set by an internal timer and is nominally 1.2
μs at 100% LED power (V
ADJ
= 500mV), increasing to
about 8 μs at 10% LED power (V
ADJ
= 50mV). The longer OFF time and variable peak current enables the circuit to dim the
LED whilst maintaining continuous switching, rather than "skipping" or stalling and continuous running is better for reducing
electrical noise and also for eliminating audible noise from the coil core.
Layout Considerations
As with all switching DC to DC converters, the currents can be large. Using small inductors with a reasonable high supply
voltage will cause currents to change quickly. High dI/dt can cause inductively-coupled spikes into adjacent tracks. At the
transition from of the ON phase to the OFF phase and back, where the power transistor switches, the voltage at the collector
rises and falls quickly. High dV/dt can cause capacitively coupled spikes into adjacent tracks, especially if they have a high
impedance. For this reason, all tracks on the PCB should be thick, to minimise drops, and short to keep all the components
coupled tightly together.
A double-sided board should be used with a ground plane to screen the tracks and provide a good ground return for the
various functions and the rear exposed pad on the package should have an appropriately-sized land with good ground
connections, both to reduce electrical noise due to ground drops and to improve thermal conductivity.
The input decoupling capacitor C1 should be very close to the chip pins and the LED sense resistor R3 should have Kelvin
tracks to M_VIN and VIN to achieve LED current measurement accuracy, as the PCB tracks will have comparable
resistance to the 100m
Ω resistor, so taking sense tracks to the current monitor which are not connected close to the ends of
R3 will cause a measurement error.
The peak current sense resistor R1 should have short tracks to the ground at the bottom end and Kelvin tracks to I
SENSE
at
the top end. This resistor might need to be only 25m Ω and PCB track resistance becomes comparable if the tracks are not
very short. I
SENSE
is a high impedance input, so a thin track from this pin directly to the top of R
SENSE
resistor R1 will still
give an accurate measurement.
The ADJ pin should have short tracks, as this is a fairly low-level signal controlling the power of the system. As it needs to
be less than 28mV for shutdown, a close ground connection is needed for the pull-down device, as any ground drops could
raise the potential. In particular, if a bipolar transistor is used as a pull-down device, this will have an appreciable V
SAT
,
which could perhaps be half the shutdown potential.
The bottom of the thermistor must be coupled very closely to ground, as the TADJ pin varies the LED current from 100% to
0% for a voltage change of only 25mV, so any noise on the bottom of the thermistor will seriously affect the accuracy of the
Thermal Protection circuit.
Ordering Information
Device
Reel Size
(mm)
Reel Width
(mm)
Quantity
per reel
Device Mark
ZXLD1322DCCTC 33.02 12 3000 1322