NJU6050 Просмотр технического описания (PDF) - Japan Radio Corporation

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NJU6050

High Efficiency White LED Driver

Japan Radio Corporation  

NJU6050

(2) Inductor Selection

The following equation gives the optimum inductor value:

VOUT-VIN(MIN)+VD

L=

ICL

tOFF

Formula (2)

VD

: Schottky diode Voltage

ICL

:Switch current limit

The power supply voltage VIN may fluctuate in battery-powered applications. For this reason, the minimum

voltage should be applied to the VIN in Formula (2).

The NJU6050 has about 100ns of delay time (TDELAY), which is defined as the period from the reach of the

current limit 350mA to the MOS-switch-off. The TDELAY may cause an overshoot-inductor-current, which is called

the peak current IL,PEAK, and calculated by Formula (3). Therefore, it is recommended that an inductor with a

rating twice of the IL,PEAK and a low DCR (DC resistance) be used for high efficiency.

IL,PEAK = ICL +

VIN(MAX)+VDS

L

tDELAY

Formula (3)

VDS

VIN(MAX)

: Drain-Source voltage of MOS switch (=ILIMIT*RON)

: Maximum of VIN Voltage

(3) 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.

(4) 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 NJU6050 application because of not

only the low ESR but its small package. A ceramic capacitor is also recommended for the input

decoupling-capacitor C1, and should be placed as close to the NJU6050 as possible.

(5) Open-circuit Protection

The zener diode D2 works as an open-circuit protection. When the LED is disconnected from the circuit, the

feedback voltage on the FB terminal is pulled down to 0V, which makes the switching frequency maximum.

Without the diode D2, the SW voltage exceeds the absolute maximum rating, and this voltage stress may cause a

permanent damage to the IC.

The zener voltage must be greater than the maximum forward-voltage of the LED string.

Ver.2004-02-25

-3-

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