SP7614 datasheet(5/9 Pages) SIPEX

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Date: 6/30/04

SP7611A/7612/7614 Low Dropout LED Driver for Any Color LED

APPLICATION INFORMATION

SETTING THE LED CURRENT

The current flowing into LEDs is approximately 200

times greater than the current ISET. The LED current is

controlled by ICONTROL and RSET according to the follow-

ing formula:

ILED = 200 X (VCONTROL - VCTRL) / RSET

For VCONTROL = 3V and a specified LED current, the RSET

value can be evaluated using the diagram shown in the

Typical Performance Characteristics section. For any

other option, ISET vs. VCTRL. The LED's brightness can

also be adjusted by driving ENABLE or the CTRL pin with

a PWM signal. The driving signal frequency should be

greater than 100Hz to avoid flickering, increasing to

more than 1MHz, if necessary.

LEDs are very sensitive to temperature. In most cases

the maximum allowed junction temperature is 100ºC.

This temperature is the result of adding to the ambient

temperature the over temperature due to power dissipa-

tion, is described by the following:

Tj = TA + ØjA x I xVF

where Tj is the LED junction temperature, TA is the

ambient temperature, ØjA is the junction to ambient

thermal resistance, I is the LED current and VF is the LED

forward voltage.

The SP7611AA to SP7614 are designed to reduce the

current through LEDs, when the temperature rises and

the cathode voltage increases, as can be seen from

typical the "LED Current vs. LED Cathode Voltage"

graph under the Typical Performance Characteristics

section.

The SP761X driver's low dropout architecture can sig-

nificantly improve the efficiency compared to using simple

ballast resistors.

The system efficiency, defined as the ratio between the

LEDs power and the input supplied power can be

calculated as follows:

Efficiency = (VIN - VCATHODE) / VIN

The lower the VCATHODE , the higher the system effi-

ciency. Efficiency can be further improved using a higher

VIN with more LEDs as shown in example 3.

APPLICATION NOTES

The ultra-low voltage drop across the SP761X series of

LED drivers, allow the devices to drive white, blue, and

other color LEDs in a wide range of input voltages. The

driver can be used in many applications. Although, only

the SP7611A is shown in all three examples, any of the

SP762X series of LED drivers can be used in the

applications presented in this document, due to their

similar operation.

Example 1: Drive low VF white or blue LEDs directly

from single cell Li-ion

When using white or blue low VF LEDs, and utilizing the

drivers low voltage drop, only 3.4V in VINis needed for the

full 20mA LED current. At 3.1V, there is still 5mA typical

current available for the LEDs. The single cell Li-ion is

utilized in most applications like cell phones or digital still

cameras. In most cases, the Li-ion battery voltage level

only goes down to 3.0V voltage level, and not down to the

full discharge level (2.7V) before requesting the charger.

- TDROP < 0.3V

- VF (at 20mA) < 3.1V (Low VF)

- VIN (at 20mA) < - VDROP +VF = 3.4V

- VIN (at 5mA typical) ~ 3.1V

Key Advantages

1) No boost circuit needed for the LCD or keyboard

backlight

2) Drivers directly connected to a Li-ion battery

3) No EMI, no switching noise, no boost efficiency lost,

no capacitor, and no inductor

VIN

LED1

GND

CTRL

ON/OFF

ENABLE

VCONTROL

RSET

SP7611

I SET

LED3

LED2

LED4

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