SS6896 datasheet(9/12 Pages) SSC

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Rev.2.01 6/06/2003

SS6896

Application Information

Inductor Selection

A 15

µH inductor is recommended for most SS6896

applications. Although small size and high efficiency

are major concerns, the inductor should have low

core losses at 1.4MHz and low DCR (copper wire

resistance).

Capacitor Selection

The small size of ceramic capacitors makes them

ideal for SS6896 applications. X5R and X7R types

are

recommended

because

they

retain

their

capacitance over wider ranges of

voltage and

temperature than other types, such as Y5V or Z5U.

A 4.7

µF input capacitor and a 1µF output capacitor

are sufficient for most SS6896 applications.

Diode Selection

Schottky diodes, with their low forward voltage drop

and fast reverse recovery, are the ideal choices for

SS6896 applications. The forward voltage drop of a

Schottky diode represents the conduction losses in

the diode, while the diode capacitance (CT or CD)

represents the switching losses. For diode selection,

both forward voltage drop and diode capacitance

need to be considered. Schottky diodes with higher

current ratings usually have lower forward voltage

drop and larger diode capacitance, which can cause

significant switching losses at the 1.4MHz switching

frequency of the SS6896. A Schottky diode rated at

100mA to 200mA is sufficient for most SS6896

applications.

LED Current Control

LED current is controlled by a feedback resistor (R1

in Fig. 1). The feedback reference is 1.23V. The

LED current is 1.23V/R1. In order to have accurate

LED current, precision resistors are preferred (1%

recommended). The formula for R1 selection is

shown below.

R1 = 1.23V/ILED

(1)

Open-Circuit Protection

In the cases of output open-circuit, when the LEDs

are disconnected from the circuit or the LEDs fail,

the feedback voltage will be zero. The SS6896 will

then switch to a high duty cycle resulting in a high

output voltage, which may cause the SW pin

voltage to exceed its maximum 30V rating. A zener

diode can be used at the output to limit the voltage

on the SW pin (Fig. 20). The zener voltage should

be larger than the maximum forward voltage of the

LED string. The current rating of the zener should

be larger than 0.1mA.

Dimming Control

There are three different types of dimming control

circuits as follows:

1. Using a PWM signal

PWM

brightness

control

provides

the

widest

dimming range by pulsing the LEDs on and off using

the control signal. The LEDs operate at either zero

or full current, The average LED current changes

with the duty cycle of the PWM signal. Typically, a

1kHz to 10kHz PWM signal is used. PWM dimming

with the SS6896 can be accomplished two different

ways (see Fig. 21). The

SHDN pin can be driven

directly or a resistor can be added to drive the FB

pin. If the

SHDN pin is used, increasing the duty

cycle will increase the LED brightness. If the FB pin

is used, increasing the duty cycle will decrease the

brightness. Using this method, the LEDs are dimmed

using FB and turned off completely using

SHDN .

2. Using a DC Voltage

For some applications, the preferred method of

brightness control uses a variable DC voltage to

adjust the LED current. The dimming control using a

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