AN3223

Selection of external components

Doc ID 17553 Rev 2

5/29

2

Selection of external components

2.1

Input and output capacitor selection

It is recommended to use ceramic capacitors with low ESR as input and output capacitors. It

is recommended to use 10 µF / 6.3 V as a minimum value for the input capacitor, and 10 µF

/ 6.3 V as the optimal value for the output capacitor to achieve a good stability of the device,

for a supply range varying from low input voltage (2.5 V) to the maximum ratings of output

power.

Note:

See recommended components in Table 1.

2.2

Inductor selection

The STCF04 device works with the switching algorithm ILIM-ZCOM. It charges the inductor

until the current crosses the threshold for the ILIM function and then it discharges the energy

in the inductor to the output until it reaches the zero current value. Therefore, it is

recommended to use a 1 µH inductor as the minimum value, which guarantees a proper

function with the used algorithm and speed of used components in the silicon design.

Note:

See recommended components in Table 1.

2.3

LED selection

All LEDs with a forward voltage range from 2.5 V to 4.5 V are compatible with the STCF04.

The forward voltage spread of any selected LED must, however, lay within this range (2.5 V

to 4.5 V). It is possible to set the level of the LED current in Flash mode and Torch mode by

setting the dimming registers. The maximum level of the LED current in Flash mode can be

set by changing the external Flash resistor.

Note:

See recommended components in Table 1.

2.4

NTC and RX resistor selection

The STCF04 requires a negative thermistor (NTC) for sensing the LED temperature, as well

refer to the typical application schematic in

Figure 2 for more details.

and this creates a voltage divider supplied by the external reference voltage connected to

the NTC.

If the temperature of the NTC-thermistor rises due to the heat dissipated by the LED, the

voltage on the NTC pin increases. When this voltage exceeds 0.56 V, the NTC_W bit in the

STATUS register is set to high, and the ATN pin is set to low to inform the microcontroller

that the LED is becoming hot. The NTC_W bit is cleared by reading the STATUS register.

If the voltage on the NTC pin rises further and exceeds 1.2 V, the NTC_H bit in the STATUS

register is set to high, and the ATN pin is set to low to inform the microcontroller that the LED

is too hot and the device automatically goes to Ready mode to avoid damaging the LED.