ZLED7x30 Datasheet

13

April 20, 2016

Rs

V

1

.

0

K

I

X

dim

OUT

∗

=

The dimmed average output current is given by

(2)

Where

KX = Current ratio to initial current (see section 1.3).

The input conditions on the DIM1 and DIM2 pins set the number of current levels and the current ratio relative to the

initial average current for the dimming switch (DS) sequences as shown in Table 2.1.

Table 2.1

Dimming Configuration Options

Dimming Mode

DIM1

DIM2

Dimming Ratio K

No dimming

Floating

Floating

(100%)

1

Floating

GND

100%

 1

st DS  50%  2nd DS  20%  3rd DS  100% …

2

GND

Floating

100%

 1

st DS  60%  2nd DS  30%  3rd DS  100% …

3

GND

GND

100%

 1

st DS  30%  2nd DS  100% …

If a normal switch is detected or if DIM1 and DIM2 are both floating, the output current goes back to the initial state of

100% nominal average output current. Since ZLED7x30 needs to count the time for more than 2 seconds after the

switch is off during a normal switch, one capacitor (C1) equal to or greater than 220

μF is required to keep the chip

working in low quiescent current mode during this part of the off-time.

2.4

ZLED7x30 Protection Features

2.4.1

Thermal Shut-down Protection

The ZLED7x30 automatically protects itself from damage due to over-temperature conditions. If the ZLED7x30’s

temperature exceeds the thermal shutdown threshold (TSD = 150°C, typical), the ZLED7x30 will shutdown. To avoid

erratic ZLED7x30 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.

2.4.2

LED Open-Load Protection

As a step-down converter, the ZLED7x30 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

ZLED7x30 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.