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NJU6048

High Efficiency White LED Driver

Japan Radio Corporation  

NJU6048

! FUNCTIONAL DESCRIPTONS

(1) LED Current Control and Resistor RLED Selection

The NJU6048 incorporates the LED current control to regulate the LED current (ILED), which is programmed

by the feedback resistor (RLED) connected between the FB and the VSS terminals. Typically, the reference voltage

VREF is internally regulated to 0.25V and is used as the positive input of the built-in comparator. Formula (1) is

used to choose the value of the RLED, as shown below.

RLED

= VREF

I LED

--- Formula (1)

VREF=0.25 (typical)

The ILED is the constant current programmed by the RLED. When the feedback voltage on the FB terminal

reaches above the reference voltage VREF (i.e., ILED is above the level programmed by RLED), the output capacitor

C2 delivers the ILED. Once the feedback voltage drops below the reference voltage (i.e., ILED drops below the level

programmed by the RLED), the MOS switch is turned on, then the current of the inductor L1 begins increasing.

When the switch current reaches 250mA, the MOS switch is turned off, then the L1 delivers current to the output

through the diode D1 as the inductor current drops. After that, the MOS switch is turned on again and the switch

current increases up to 250mA. This switching cycle continues until the ILED reaches the level programmed by the

RLED, then the ILED current is maintained constant.

(2) Over Voltage Protection

OVP is designed to prevent the damage of internal NMOS switch in case the increased impedance of the

LED load (including the LED opened). Once the device detects over voltage at the output, the internal NMOS

switch is kept off until the output voltage drops below 14V.

(3) Inductor Selection

A 10uH inductor is recommended for most application. The selected inductor must have a saturation current

that meets the maximum peak current of the converter. Another important inductor parameter is the DC resistance.

The lower DC resistance the device has higher efficiency.

(4) Diode Selection

A Schottky diode with a low forward-voltage-drop and a fast switching-speed is ideal for the D1. And the D1

must have a rating greater than the output voltage and output current in the system.

(5) Capacitor Selection

A low ESR (Equivalent Series Resistance) capacitor should be used as the output capacitor C2 to minimize

output ripples. A multi-layer ceramic capacitor is the best selection for the NJU6048 application because of not

only the low ESR but also small package. Application requires good line regulation ±1%(typ) should use output

capacitor larger than 1uF. A ceramic capacitor is also recommended for the input decoupling-capacitor C1, and

should be placed as close to the NJU6048 as possible. A 4.7uF is sufficient for most applications.

Ver.2005-06-09

-3-

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