ZLED7015 Datasheet

© 2016 Integrated Device Technology, Inc.

17

April 20, 2016

3

Application Circuit Design

The following sections cover selection of the external components shown in the typical application on page 1.

3.1

External Component – RS

Recommendation: Use precision resistors (±1% or better tolerance) for the RS resistor to ensure accurate control

of the LED current.

See section 2.2.1 for the equation for selecting the value of RS, which sets the nominal current output.

3.2

External Component – Inductor L1

Designing the circuit so that the current through inductor L1 is always above 0V (i.e., continuous mode) typically

gives the best performance due to improved load regulation and reduced output ripple. Select an inductor that has

a saturation current and a current rating greater than the mean input current.

The inductor value selection requires trade-offs between unwanted ripple current and parasitic effects. A larger

value inductor reduces inductor ripple current, resulting in less output ripple voltage; however, higher values also

increase parasitic resistance, which can degrade performance. For most applications, a 10µH inductor with a

saturation current >2.5A is adequate. See section 7.4 for layout restrictions.

3.3

External Components – Input Decoupling Capacitors C1 and CIN

The input capacitors C1 and CIN minimize the input voltage noise and ripple. Recommendation: use a 22µF or

larger low-ESR electrolytic capacitor for CIN in parallel with a 1µF ceramic capacitor rated at greater than the input

voltage plus a safety margin for C1.

3.4

External Component –Output Capacitors C2 and COUT

The output capacitors C2 and COUT minimize the output voltage ripple. Recommendation: use a 22µF or larger

low-ESR electrolytic capacitor for COUT in parallel with a 1µF ceramic capacitor rated at greater than the output

voltage plus a safety margin for C2.

3.5

External Component – Diode D1

For the diode D1, select a high-speed, low-capacitance Schottky diode with low reverse leakage at the maximum

operating voltage and temperature to ensure maximum efficiency and performance.

Important: Choose diodes with a continuous current rating higher than the maximum output load current and a

peak current rating above the peak coil current. When operating above 85°C, the reverse leakage of the diode

must be addressed because it can cause excessive power dissipation in the diode, especially when the output

voltage is relatively high. Its reverse breakdown voltage must be greater than the over-voltage protection level

VOVP (see section 4.2).

Note: Silicon diodes have higher forward voltage and higher voltage overshoot before they start conducting, which

can increase the peak voltage on the LX output. Ensure that the total voltage appearing on the LX pin, including

supply ripple, is within the specified range (see Table 1.1).

3.6

Additional External Components

For the VDD input, connect resistor RVDD to the positive power supply and connect ceramic capacitor CVDD to

ground. Recommendations: use 1µF for CVDD; use 300

Ω for R

VDD with input voltages

≥ 8V, use 50Ω for R

VDD with

input voltages < 8V.

For the VP pin, connect a 10µF ceramic bypass capacitor to ground (CVP).

If the EN pin is not used, connect a 100k

Ω resistor to the positive power supply (R

EN). Do not allow the EN pin to

float.