ZLED7020 datasheet(14/28 Pages) ETC2

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ZLED7020

High Current 40V LED Driver with Internal Switch

Data Sheet

August 12, 2011

the prior written consent of the copyright owner. The information furnished in this publication is subject to changes without notice.

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2.4

Control of Output Current via a PWM Signal on the ADJ Pin

An external pulse width modulation (PWM) control signal input on the ADJ pin can be used for brightness or gated

on/off control of the output current by driving the output current to a value below the nominal average current

determined by RS. See Figure 3.3 for a typical application circuit. The PWM or gated on/off control signal can be

within the range of 0 to 5 V. The logic high level must be higher than 1.2V and the logic low level must be below

0.2V. It must be capable of driving the ADJ pin’s input impedance RADJ (approximately 500k ; internal pull-up

resistor to VREF).

2.5

Control of Output Current via a Microcontroller Signal on the ADJ Pin

An external control signal from the open drain output of a microcontroller can provide on/off or PWM brightness

control by driving the ADJ pin. See Figure 3.4 for a typical application circuit. Diode D2 and resistor R1 shown in

Figure 3.4 suppress any negative high-amplitude spikes on the ADJ input due to the drain-source capacitance of

the FET in the microcontroller’s output. Negative spikes on the ADJ input could cause output current errors or

unintended ZLED7x20 operation. The signal input to the ADJ pin must be capable of driving the ADJ pin’s input

impedance RADJ (approximately 500k ; internal pull-up resistor to VREF).

2.6

Shutdown Mode

If the ADJ pin voltage VADJ is ≤ VADJoff (0.2V ± 0.05V), the supply current and output on the LX pin are quiescent at

a low standby level (IINQoff = 120µA nominal). Raising the ADJ pin voltage so that VADJ ≥ VADJion (0.25V ± 0.05V)

will switch the output back to full operational mode.

2.7

ZLED7x20 Protection Features

2.7.1

Thermal Shut-down Protection

The ZLED7x20 automatically protects itself from damage due to over-temperature conditions. If the ZLED7x20’s

temperature exceeds the thermal shutdown threshold (TSD = 150°C, typical), the ZLED7x20 will shutdown. To

avoid erratic ZLED7x20 operation, a 20K hysteresis (TSD-HYS) is applied that prevents it from returning to operation

until its temperature falls below the hysteresis threshold (TSD - TSD-HYS). Also refer to section 3.2 for additional

thermal considerations.

2.7.2

LED Open Load Protection

As a step-down converter, the ZLED7x20 has inherent open-load circuit protection. Since the L1 inductor is

connected in series with the LED string, the current flow is interrupted if the load is open and the LX output of the

ZLED7x20 will not be damaged. This provides an advantage over other products such as boost converters, for

which the internal switch can be damaged by back EMF forcing the drain above its breakdown voltage.

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