zxld1366 Zetex Semiconductors plc., zxld1366 Datasheet - Page 6

• Manufacturer Part Number: zxld1366
• Description: High Accuracy 1a, 60v Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

Device description
The device, in conjunction with the coil (L1) and current sense resistor (R
oscillating continuous-mode buck converter.
Device operation (refer to Figure 1 - Block diagram and Figure 2 Operating waveforms)
Operation can be best understood by assuming that the ADJ pin of the device is unconnected and
the voltage on this pin (V
When input voltage V
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from V
and L1 to produce a voltage ramp (V
forced across internal resistor R1 by the current sense circuit and produces a proportional current
in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of
the comparator. When this reaches the threshold voltage (V
low and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of V
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V
decays at a rate determined by the LED(s) and diode forward voltages to produce a falling voltage
at the input of the comparator. When this voltage returns to V
high again. This cycle of events repeats, with the comparator input ramping between limits of
V
Switching thresholds
With V
200mV (measured on the I
then defined by this voltage and R
I
Nominal ripple current is ±30mV/R
Adjusting output current
The device contains a low pass filter between the ADJ pin and the threshold comparator and an
internal current limiting resistor (50k nom) between ADJ and the internal reference voltage. This
allows the ADJ pin to be overdriven with either DC or pulse signals to change the V
switching threshold and adjust the output current.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit
falls below the threshold (0.2V nom.), the internal regulator and the output switch are turned off.
The voltage reference remains powered during shutdown to provide the bias current for the
shutdown circuit. Quiescent supply current during shutdown is nominally 60 A and switch
leakage is below 5 A.
Issue 2 - September 2008
© Diodes Incorporated 2008
OUTnom
ADJ
± 15%.
ADJ
= 200mV/R
= V
IN
REF
to ground, via R
, the ratios of R1, R2 and R3 define an average V
S
ADJ
IN
.
ADJ
is first applied, the initial current in L1 and R
SENSE
) appears directly at the (+) input of the comparator.
S
, L1 and the LED(s). The current rises at a rate determined by V
pin with respect to V
S
S
according to:
SENSE
) across R
6
IN
S
. The supply referred voltage V
). The average output current I
ADJ
ADJ
), the comparator output switches
, the comparator output switches
SENSE
S
is zero and there is no
switching threshold of
ZXLD1366
www.diodes.com
S
www.zetex.com
), forms a self-
IN
. The current
OUTnom
SENSE
SENSE
IN
is
is