Device description – Diodes ZXLD1350 User Manual
Page 5
ZXLD1350
30V 350mA LED DRIVER with AEC-Q100
ZXLD1350
Document number: DS33468 Rev. 8 - 2
5 of 22
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Device Description
The device, in conjunction with the coil (L1) and current sense resistor (R
S
), forms a self-oscillating continuous-mode buck
converter.
Device operation (Refer to block diagram and Figure 1 - 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
ADJ
) appears directly at the (+) input of the comparator.
When input voltage V
IN
is first applied, the initial current in L1 and RS is zero and there is no 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
IN
to ground, via R
S
, L1 and the LED(s). The current rises at a rate determined
by V
IN
and L1 to produce a voltage ramp (V
SENSE
) across R
S
. The supply referred voltage V
SENSE
is 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
ADJ
), the
comparator output switches 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
ADJ
.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V
IN
. The current decays at a rate
determined by the LED and diode forward voltages to produce a falling voltage at the input of the comparator. When this
voltage returns to V
ADJ
, the comparator output switches high again. This cycle of events repeats, with the comparator input
ramping between limits of V
ADJ
± 15%.
Switching thresholds
With V
ADJ
=V
REF
, the ratios of R1, R2 and R3, define an average V
SENSE
switching threshold of 100mV (measured on the
I
SENSE
pin with respect to V
IN
). The average output current I
OUTnom
is then defined by this voltage and Rs according to:
I
OUTnom
=100mV/R
S
Nominal ripple current is ±15mV/R
S
Adjusting output current
The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting
resistor (200k 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
SENSE
switching threshold and adjust the output current. The filter is third order,
comprising three sections, each with a cut-off frequency of nominally 4kHz.
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
15mA and switch leakage is below 1mA.