Sept 11 - 07 Rev E

SP7121 Low Dropout LED Driver

© 2007 Sipex Corporation

Page 5

Example

For a 2kHz signal, the minimum duty cycle

for the PWM signal is

%Minimum Duty cycle =3%

Fault Operation

If one of the LEDs is shorted, the LED

channel will provide the nominal current

value, thereby increasing power dissipation.

If all channels are shorted, excessive power

dissipation may damage the device. If an

LED is open, the LED pin voltage will be

pulled up to Vcc, and the LED current will

be reduced to 3mA for the other channels.

The SP7121 doesn’t have protection from

With an

operational maximum current of 30mA per

channel, the device may be damaged if the

value is below 8k .

APPLICATION INFORMATION

Thermal Considerations

The

SP7121

does

not

have

overtemperature protection. Special care

needs to be taken to ensure that the driver’s

junction temperature stays below 125˚C.

The thermal resistance of the package is

160˚C per Watt. Under normal operation

conditions with about 30mA of current per

LED and a voltage drop across the part of

1V, the part will still only dissipate .06 Watts

-- giving a temperature rise of only 11.4˚C

Example of thermal calculations.

Where P is total power dissipation

Vd is the voltage drop across the SP7121

is the total output current

Where P is the total power dissipation

is the thermal resistance of the package

per Watt.

Above example

The part will be able will operate up to 85˚C

under these conditions.

BOARD LAYOUT AND GROUNDING

Although the SP7121 does not require an

input capacitor for normal operation, a 1nF

capacitor can be used to obtain better

overall circuit performance. To obtain the

best performance from the SP7121, a

printed circuit board with ground plane is

required. High quality, low series resistance

ceramic 1nF bypass capacitors should be

used at the Vcc and GND pins (pins 1 and

2). This capacitor must be located as close

to

the

pins

as

possible.

The

traces

connecting the pins and the capacitor must

be kept short and should be made as wide

as possible.