ZLED7015 Datasheet

© 2016 Integrated Device Technology, Inc.

12

April 20, 2016

S

S

FB

OUT

R

V

R

V

I

3

.

0

=

=

2

Circuit Description

The ZLED7015 is a constant-current boost converter that can also function as a constant voltage driver for LED

applications. The boost converter topology features an internal 35V power switch and feedback circuit to control

the output to the LED string. Optimal performance is achieved by operating in continuous conduction mode within

the application’s load current range; i.e., the current through inductor L1 is always above 0A. The output current is

set by an external resistor and the device supports DC linear or PWM dimming. Additional device features include

selectable open-load/over-voltage protection, over-temperature protection, under-voltage lockout, and an

automatic soft-start function to minimize the inrush current during startup.

2.1

EN Pin, VP Pin, and Soft-Start Function

The EN pin can be used to enable or disable the ZLED7015. When the voltage on the EN pin rises above the ON

threshold voltage VENon specified in Table 1.3, the ZLED7015 begins its “soft-start.” When the voltage at EN falls

below the OFF threshold voltage VENoff, the ZLED7015 shuts down. There is a hysteresis between VENon and VENoff

to prevent intermittent operation.

The ZLED7015’s internal “soft-start” function ensures a smooth device start-up by preventing excessive in-rush

current and providing a controlled rise in the output current. When the soft-start function is initiated, the internal

circuitry clamps the internal switch current threshold to zero during a 1millisecond delay. The soft-start function

then gradually increases the internal switch current threshold over a 4-5 msec period. See Figure 1.11 for an

illustration of typical waveforms during start-up.

Important: The EN pin requires termination. If the EN pin is not used to control operation, connect it to the

positive power supply through a 100k

Ω resistor. Do not allow the EN pin to float.

The VP pin is connected to the ZLED7015’s internal power regulator and requires an external bypass capacitor,

CVP, to ensure correct device operation.

2.2

Output Current Control

The LED current is determined by the voltage across the external sense resistor RS, which is fed back to the input

FB. Internally, this is compared with the internal feedback reference voltage, VFB (see typical application shown on

page 2), and the duty cycle of the internal power switch is adjusted to reduce or increase the output current IOUT.

Selection of RS is discussed in section 2.2.1, and other external components are discussed in section 3. Dimming

can be controlled by superimposing a DC or filtered pulse-width modulated (PWM) signal on the feedback voltage

from RS. Refer to sections 2.2.2 and 2.2.3, respectively, for more details.

2.2.1

Output Current and RS

The ZLED7015 continually adjusts the output current, IOUT, in order to maintain the voltage level at the FB pin

equal to the internal feedback reference voltage, VFB. Equation (1) shows the basic relationship between IOUT, VFB,

and RS for the basic application shown in Figure 2.1.

(1)

Where

IOUT = Average output current through the LED(s) in amperes

VFB = Internal feedback reference voltage (see Table 1.3, parameter 1.3.7)