ZXLD1356QDACTC datasheet(5/28 Pages) DIODES

5 / 28 page

ZXLD1356Q

Document number: DS37077 Rev. 2 - 2

5 of 28

www.diodes.com

October 2015

ZXLD1356Q

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 Figure 1 Block Diagram and Figure 2 Operating Waveforms)

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 (VADJ) appears directly

at the (+) input of the comparator.

When input voltage VIN 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 output is high. This turns MN on and switches the LX pin low, causing current to flow

from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN and L1 to produce a voltage ramp (VSENSE) across

RS. The supply referred voltage VSENSE 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 (VADJ), 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 VADJ.

When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. 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 VADJ, the comparator output

switches high again. This cycle of events repeats, with the comparator input ramping between limits of VADJ ± 15%.

Switching Thresholds

With VADJ = VREF, the ratios of R1, R2 and R3 define an average VSENSE switching threshold of 200mV (measured on the ISENSE pin with respect

to VIN). The average output current IOUTnom is then defined by this voltage and RS according to:

IOUTnom = 200mV/RS

Nominal ripple current is ±30mV/RS

VIN

200mV

230mV

SENSE voltage

VSENSE+

VSENSE-

Toff

Ton

170mV

VIN

0.15VADJ

IOUTnom

IOUTnom +15%

IOUTnom -15%

VADJ

LX voltage

Coil current

Comparator

input voltage

Comparator

output

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