Zxld1352, Device description – Diodes ZXLD1352 User Manual
Page 5
ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
5 of 21
December 2010
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Device Description
The device, in conjunction with the coil (L1) and current sense resistor (RS), 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 R
S
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 for noise suppression 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 PWM signals to adjust the output current. The filter is first order, comprising one section with a cut-off frequency of
nominally 600kHz.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The ADJ pin 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 20mA and switch leakage is below 1mA.