ZXLD1356 datasheet(5/29 Pages) DIODES

5 / 29 page

ZXLD1356

Document number: DS33470 Rev. 3 - 2

5 of 29

www.diodes.com

December 2010

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 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 its 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%.

VIN

200mV

230mV

SENSE voltage

VSENSE+

VSENSE-

Toff

Ton

170mV

VIN

0.15V

ADJ

0.15VADJ

IOUTnom

IOUTnom +15%

IOUTnom -15%

VADJ

LX voltage

Coil current

Comparator

input voltage

Comparator

output

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