Device description, A product line of diodes incorporated – Diodes ZXLD1356/ ZXLD1356Q User Manual
Page 6
ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
6 of 29
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
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 Figure 1 - Block diagram and Figure 2 operating waveforms)
0V
V
IN
200mV
230mV
0V
SENSE voltage
V
SENSE+
V
SENSE-
Toff
Ton
170mV
0V
5V
V
IN
0.15V
ADJ
0.15V
ADJ
I
OUTnom
I
OUTnom
+15%
I
OUTnom
-15%
V
ADJ
LX voltage
Coil current
Comparator
input voltage
Comparator
output
Figure 2. Theoretical 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(s)
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 200mV (measured on the I
SENSE
pin with respect
to V
IN
). The average output current I
OUTnom
is then defined by this voltage and R
S
according to:
I
OUTnom
= 200mV/R
S
Nominal ripple current is ±30mV/R
S