STLD20CP1 datasheet(11/19 Pages) STMICROELECTRONICS

11 / 19 page

STLD20CP1

Application information

11/19

current reaches 5%. The eleventh pulse sets the LED current back to ILED max. Figure 8

shows a timing diagram for single-wire pulse dimming.

Analog dimming can be achieved by connecting a resistor network to the FB pin (Figure 9).

6.3

Peak inductor current limitation and soft start function

An integrated current sensor will sense the peak drain current of the switch P_SW in order

to keep the inductor current below its saturation level. Since the peak drain current exceeds

the fixed limit IPK by the drain current reference, the comparator turns off the switch P_SW.

During start up, this peak drain current limitation behaves inherently as a soft-start function.

6.4

Charge pump

The charge pump includes soft-start circuitry to limit inrush current at turn-on or when the

device changes operation from x1 to x2. During the soft-start time, the output current is set

to 10% of the maximum set by RTH or RFL (depending on the state of the SEL pin), until

VFB_CP reaches regulation. This function is implemented to avoid battery stress.

At start-up, the pin OUT_CP goes to VIN, working in an x1 condition. Following this phase, if

VFB_CP >150 mV, the LED current is regulated and the device continues to operate in a 1x

condition. Otherwise, if VFB_CP <150 mV, the device changes to a 2x operating condition

bringing VOUT_CP>VIN. allowing LED current regulation.

The device will change back to 1x mode when the input voltage rises above VOUT_CP by

approximately 50 mV.

6.5

Current on power LED (flash)

The current that level flowing through the power LED in both modes (torch and flash) can be

selected using the following formula:

IFLASH = K (0.6/RFL), ITORCH = K (0.6/RTH)

where K is a value of approximately 1220.

Figure 10 shows the relationship between the TF signal control and flash duration time for

one camera shot, while Figure 11 represents multiple shots.

6.6

Over-voltage and short-circuit protection

In case of open-circuit failure of the power LED, the output voltage is limited to about 5.5 V

by gating the charge pump on/off.

If the power LED experiences a short-circuit or if the output charge pump is grounded, the

charge pump is turned off.

6.7

Start-up procedure

Before inserting the driving preferences with the digital pins it is mandatory to turn on the

device with correct supply voltage with all the digital pins set to GND.

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