STLED25

Detailed description

Doc ID 022323 Rev 3

17/30

6

Detailed description

The STLED25 is a high efficiency white LED power supply which integrates a step-up

converter and 5 high-side current sources. The device is controlled by two enable inputs

called EN13 and EN45 according to the labels of the LED channels. EN13 enables LED

channels 1, 2, 3 and EN45 enables LED channels 4, 5. The device is protected against

shorted LED channels by short LED protection and against open LED channels by open

LED protection. The device is able to recognize which channel has failed, and switch off this

channel to maintain correct operation of the remaining channels. The STLED25 device is

125 mA total output current capable and works over a battery voltage range of 2.3 V to 5.5

V. The typical output voltage during operation corresponds to the voltage level equal to

high level of application efficiency (86%) with the typical battery voltage of 3.7 V. The device

is also able to work at high efficiency when not all the LED channels of the device are used.

6.1

Boost DC-DC converter operation

The step-up bridge with current mode control of the regulation regulates the output voltage

voltage headroom for the related current sources. The typical headroom of the current

sources is in the range of 135 mV. The STLED25 contains a current limiter, with a typical

value 600 mA, and soft-start control to limit the supply current from the battery. The method

of regulation during startup and normal operation is explained in

Section 6.4.

6.2

Current setting

Each LED channel regulates to the same current. The value of the LED current is set by

obtain the LED current. The multiplication factor is 390.

The LED current is calculated with the following formula:

Equation 1

where:

6.3

EN13 and EN45

EN13 and EN45 play the dual role of enable pin and PWM dimming. EN13 drives strings

D1, D2, D3 while EN45 drives D4 and D5. Both EN13 and EN45 are pulled down internally

by 100 k resistors.

The EN pins accept a signal between 100 Hz and 800 Hz, at duty cycles from 0.39% to

100%. The output average current maximum error is 10% at 0.39%, PWM duty cycle at 300

Hz.

390

R

V

I

SET

SET

LED

×

=