ZXLD1321

Issue 1 - January 2008

18

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© Zetex Semiconductors plc 2008

Additional notes which apply to all operational modes

Note with all these circuits 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

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 reasonably 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 M_LOAD 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 ISENSE 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. ISENSE is a high impedance

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

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.